Astronomy Cast &#187; Observing

Fraser: Welcome to AstronomyCast, our weekly facts-based journey through the Cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain; I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University – Edwardsville. Hi, Pamela. How are you doing?

Pamela: I’m doing well. How are you doing, Fraser?

Fraser: Good. I see you’re rested from your exotic cruise.

Pamela: I’m not sure I’d say rested. The thing about vacations is there’s so much to do that you just come back a different form of tired.

Fraser: I think your body is so used to travel that it can’t tell the difference between holidays and going to some astronomy conference.

Pamela: No, that’s entirely true.

Fraser: So for anyone who wonders, we’ve taken our Google plus experiment to the next level, and we’re now recording this episode as a Google plus “hang-out on air,” which means that it’s just Pamela…me and Pamela in this recording, but we’ve got anyone who wants can actually watch us record the episode on Google plus, and then when we’re done, we’ll open it up and let people join us, and we’ll ask questions, and we’re going to record the whole thing, and we’re going to put it on YouTube or something, and so, you know, hopefully, try and make the whole thing a little more interactive because one of the coolest things about doing these hang-outs on air, or the “hang-outs” is that we get to answer questions and meet with the fans. We’re trying to sort of take that to the next level. So in the future, if you miss this one, we’re going to try to move to a regular schedule now where you can know that at a certain time you’ll be able to join and watch us record AstronomyCast, or you can just wait until it pops up in your audio player just like normal. Nothing’s going to change, just more – more better.

Pamela: But this means people can subscribe to our YouTube channel as well as our itunes feed.

Fraser: Yeah, right. It’s going to be very confusing.

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Fraser: OK, well let’s get cracking. So our lives are ruled by calendars and the calendars are ruled by astronomy, so as we near the end of 2011 and get ready to ring in the New Year, let’s discover the astronomy underlying the days, weeks, months, and years that segment our lives. So… I was really worried when I was doing that intro because I was thinking you know this almost starts to sound like astronomy, like, our lives are ruled by the motions of the stars. You know, it’s very “astrology sounding,” so not astrology, but astronomy runs everything.

Pamela: Right, so I mean, if you think about it, there are so many different things — mostly related to agriculture, admittedly — that without knowing exactly what dates the Sun is where, you’re going to end up freezing your vegetables, or not harvesting your wheat on time. So at the end of the day, our nearest star, our sun, rules how we should set up our calendar if we want our calendar to make sense for agricultural purposes, and if you get agriculture wrong, everyone dies of starvation.

Fraser: Right so getting some kind of calendar set up and in place is critical. Some of the things that happened, such as the day and night cycle, are just hardwired right into our evolution, but other things clearly are human constructions. So when did calendars first start to happen?

Pamela: As near as we can tell, there’s always been some sort of a calendar system, but there haven’t always been sensible calendar systems, and the problem that we run into is the Moon doesn’t politely orbit the Earth in an integer number of times every year, and so the easiest way to set up a calendar is to set it up based on the lunar cycles, based on full moon to full moon, or new moon to new moon, but if you do that, your year ends up being about ten days too short and so there’s this problem of “Oh, (insert expletive of choice)! How do we keep our year cycled with the planting season?” So then you have to start inserting leap months, but even if you just come up with some mathematical equation to try and tie it strictly to the Sun, even the planet’s rotation about its own axis isn’t an integer number of days per year, so you still end up with this leap-cycle problem, so basically we’ve always had calendars and they’ve never worked.

Fraser: Always had calendars, and they’ve never worked [missing audio]. So then what are some of the calendars? I mea, what are some of the early ones that people started to use? Because I guess the point being that because they never worked, people have needed to come up with some invention or some solution to solve the problem that Mother Nature doesn’t…you know, didn’t nicely match up the lunar cycles to the solar cycle.

Pamela: So pretty much looking across all the different calendars, you want to look at…we find over and over and over that early calendars tended to be built on the 19-year cycle because the number of lunar cycles it takes to line back up so that you have a full moon with the Sun in place “X,” and you have a full moon again with the Sun in place “X” is about (within a few hours) nineteen years. So culture after culture after culture built a 19-year calendar that was based on mostly having 12 lunar months, but then every few years sticking in some sort of a leap month, so this is just one of those things that everyone seemed to settle down upon in some point in their calendar.

Fraser: And so can you give me some examples? I mean, were there some cultures that used that?

Pamela: Well, we see it, for instance, in the Chinese calendar. This is one of the cultures that continues to use this type of calendar today, where they look at the lunar cycle, but it’s not built purely off the lunar cycle. The Islamic calendar is built purely off the lunar calendar. The Hebrew calendar is sort of-mostly-kind of built off the lunar calendar. And they look at where the Sun is, they look at where the Moon is, and really they’re all kind of complicated — and crazy math.
?

Fraser: [missing audio]

Pamela: Well, I mean, really they just sort of have to do things along the lines of, “OK, so the Sun made it most of the way across this particular constellation, we have a new Moon, so since it didn’t actually make it out of the constellation, we’re going to make this a leap month.” Or with the Arabic or Islamic calendar – it’s based on the first sighting of the crescent Moon each month. And it has to be a physical sighting of it on the 29th, or they assume it’s there on the 30th, and so whether a given month in a given country is 29 or 30 days depends on whether or not somebody saw the moon on the 29th day of the month, and so you end up with all of these things built in that are…it’s kind of head-scratching to try and put it all together.

Fraser: So if you were going to try and develop a calendar, what are the problems, as you say, you know, the problems that needed to be solved? Let’s sort of iterate through them.

Pamela: The primary problem that needed to be solved is each of the different cultures — and this really is a cultural problem — wanted to find a way to have their religious holidays fall at roughly the same time in the solar cycle from year to year. So in the Christian church, it was a problem of trying to figure out how to get Easter consistently about the same time every spring. With the Jewish calendar it’s the problem of trying to get Rosh Hashanah at roughly the same time in the fall. The Arabic calendar…they gave up. They simply cycle through so that every year Ramadan falls in a completely different month compared than the calendar used by the Western world. But many of these other calendars were trying to solve the problem of, “how do we have key celebrations fall during the same seasons that often somehow relate to the holidays being celebrated?” Even our own transition in the Western world, going from the Julian calendar, which dates back from the early 300s — that calendar wasn’t perfect, and Easter was drifting and this was a problem, so they came up with the Gregorian calendar to try and solve the problem of Easter.

Fraser: And so you’ve got this disconnect, right? Between the Sun takes – it doesn’t even take 365 days – it takes somewhere between 365 and 366, the Moon takes 29-ish days to go around, right? So each one of these is some kind of mathematical problem, right?

Pamela: Right, and so with the Moon being 29-ish days, the problem was solved by having months that alternated in lunar-based calendars from 29 to 30 days, and a lunar calendar loses about ten days on the solar calendar every year, so if you throw in an extra month every three years, you can sort of stay on cycle, but that still means that you have that month-long swoosh back and forth for holidays like Easter, so when they tried to come up with a calendar that didn’t have as much movement in it, that was when they took the, “OK, for religious reasons we have a seven day week. OK, so we have a 365-day year, mostly, but it’s actually 365.256363. How do we make up for that?” Well, .25 means, well, the first good mathematical calendar, it meant that every four years you have a leap year, so we developed, initially, the first really good calendar was a 365-day year with a leap year every fourth year, and that made the average year 365.25 days, and so now you’re just missing that .006363 part of the year. Now, while that doesn’t sound like a lot, over a couple thousand years, it caused the calendar to drift enough that Easter was misplaced by about 10 days, or at least the part of the year that was a valid part of the year to put Easter in started to drift, and so they decided in the late 1500s, “Crud! We need to figure out how to fix the leap days so that the year is an even more accurate representation of that 365.256363.”

Fraser: And they had to do some pretty radical surgery to their calendar at that point, didn’t they?

Pamela: Well, it wasn’t that radical except in terms of they had to figure out how to get the two calendars aligned. So the change to the calculation went from being 365-day year with one leap day every four years to a 365-day year with a leap year every four years, unless the year was a multiple of 100 in which case it was a leap year only if it was a multiple of 400. So for instance 1900 wasn’t a leap year, but 2000 was a leap year, so mathematically it didn’t change that month, but the problem was they were off by those 10 days. And so they had a couple of options: they could either have a leap day every year for several years, or they could just suck it up and move the entire calendar, and that’s actually what they decided to do.

Fraser: And that’s what I meant, was they just said, “OK fine, you know, let’s just shift the whole thing ten days. Everybody agree? OK, let’s do it.” You can just imagine the coordination that was involved.

Pamela: The thing was not everyone agreed. This was something that came out of the Catholic church, and when they were sorting all of this out in the late 1500s, it wasn’t a Catholic world, and so when they made the jump, initially only the Catholic European countries made the jump. It took until, well, very recently, actually, before all the nations of the world had finally, mostly, kinda, sorta given in to using the Gregorian calendar.

Fraser: Are there still people that don’t use the Gregorian calendar?

Pamela: Well, so you have to look at, well, what are they using the calendar for? So you still have…the Chinese have the Chinese calendar, the Arabic world still has the Arabic calendar, and all of those different nations are slightly off from one another as well, but for the most part, we finally do have all of the major countries have, at least for financial purposes, adopted it. Turkey was one of the last countries to adopt it, as was China; China adopted it in 1929, and Turkey adopted it in 1926.

Fraser: Are there other motions of the…like of the Earth, like, I know the Earth’s axis kind of wobbles a little bit; it precesses. Would that over long terms as well have an impact on our calendars?

Pamela: Well, the precession isn’t so much of a problem as the fact that the length of the day is actually changing, so as our Moon slowly moves further and further away from the Earth, we’re getting longer and longer days, so we can look back in the historical record, and within the fossil record start getting down to days that were many hours shorter than the current day. So this is where we keep having to add in leap seconds now and then because, well, our rotation rate is changing.

Fraser: And it’s at a level that…I mean, I guess the modern scientific timekeeping devices are so accurate that they actually can do that. And so do they actually do that? Do they actually modify the length of the day every year?

Pamela: Well, they don’t modify the length of the day, but they have been working to try and keep the calendar tied to the Sun, but they forfeited that in 2012, and there’s actually recently an announcement saying that there would be no more leap seconds starting in 2012. The problem that we run into is: modern-day timepieces are accurate enough to notice, “Crud! The Sun didn’t line up with the stars on the exact moment it was supposed to relative to my perfectly precise Atomic clock. Let’s fix this.” But every time they add a leap second in — that wreaks havoc with operating systems the globe over, so trying to push that out to all the cell phones, all the laptops, all of the…every electronic device out there, they gave up. And this is actually starting many different people to try and say, “Well, maybe it’s time to reconsider our calendars yet again.” In fact, there was recently a call put out at an international meeting to change our calendar yet again. Keep the seven day week because people realize there’s just some things that aren’t going to change, and the seven-day week is one of them. But what if we redo the calendar in such a way that every year Christmas is on a Sunday, every year your birthday is on the same day of the week, and we simply re-jigger the year, and where the leap years fall so that we can have this perfectly lined-up perpetual calendar? And the justification that they do for this is, if you think about it, if you work in academics, or if you work in a business that has lots of holidays, just trying to figure out, “Oh, crud! This year Fourth of July falls on day “X.” What day do you give people off? Oh, crud! This year Christmas falls on a Sunday, so we have a different number of vacation days compared to last year.” Lots and lots of time goes into figuring out how to schedule work holidays, how to schedule a lot of different things, so maybe if we re-jigger our calendar so that holidays are always the same day of the month, so that the year always starts on the same day of the week, we can save time on having to re-jigger our work schedule every year.

Fraser: Well, of course, though, I mean, our Thanksgiving here in Canada falls in a completely different month than yours does in the States, so you can imagine it’s a whole other level of coordination and cooperation. Have there been other… I mean, more radical ideas for your calendars? Things that…I mean, do we need to have seven-day weeks, do we need to have…?

Pamela: Well, we don’t need to have seven-day weeks, although it seems to be the right length of period that people are actually willing to work it. If you think…would you want to really work more than five days at a time without getting time off?

Fraser: I’ve been known to.

Pamela: Yeah well, we’ve both been known to, but imagine if that was the expectation.

Fraser: Yeah, exactly. Yeah.

Pamela: But there have been people who’ve moved to say, “perhaps its time we moved to a decimal system, perhaps it’s time to get rid of this whole 12-month thing altogether,” and then the Mayans they took the approach of “we’re just going to number every day.” They have months and all of that stuff as well, but their “Long Count” calendar… they just simply number the days; that’s how they handle it.

Fraser: We can’t talk about calendars in 2012 without talking about the Mayan calendar. How did the Mayan calendar work, then?

Pamela: The thing that’s hard to wrap your head around is their way of looking at numbers wasn’t a base-10 system like we’re used to. Instead they did things in base-18 and in base-20, so their “long calendar” is actually made of looking at all of these different, crazy cycles that take thousands of years to get through, and they just count the days from the beginning of all of it until today, and so the beginning of all of it, we think – it’s always hard looking at archeological records…we think the beginning of everything was 3114 B.C., August 11, 3114 BC if you want to be specific, and it’s simply been counting forward ever since then. And it’s built on a system where they have days, so there’s a one day, and then they have a month-like period which is 20 days, they have a full circle which is 360 days, they have a (I’m going to mispronounce this)…they have a “k’atun,” which is the cycle of all of these days, which is then 7200 days, and all of these cycles come together into the “b’ak’tun” (which I know I mispronounced), which is the culmination of all of these days cycling through, and that longest cycle is 144,000 days long, so there’s 20 days in the first cycle, then there’s 18 cycles of 20 in the second cycle, there’s 20 cycles of the previous one and so it’s…the entire thing is 1 x 20 x 18 x 20 x 20 to get to their calendar.

Fraser: Right. And 144,000 days from when the calendar started happens to sync up, probably, with December 21, 2012.

Pamela: It’s actually 14 times that.

Fraser: 14 times a hundred and…OK

Pamela: Right, and so this is where their myth starts to come in because their myth is: on the 14th of these cycles starting is the day when it goes to the next “b’ak’tun” and so that’s…

Fraser: …the end of the cycle.

Pamela: Yeah.

Fraser: Yeah. Right. And, in our equivalent, that’s because they numbered every day right from the beginning right until now…you know, this would be day 123,692, or something like that, right? That would be the way they would describe a day. That would get very…that would take up a lot of paper, a lot of stone.

Pamela: Well, they actually…because they’re using an almost base-20 math system, it actually ends up being number-number-dot, number-number-dot, number-number-dot, number-number-dot, number-number, which is still a pain, but it starts to…it’s the equivalent of saying the 5th day of the 13th month in the 24th year of the 15th cycle of the 30th cycle of cycles.

Fraser: And so then when this calendar…and so this calendar theoretically runs to an end. Would they…I guess the point is they never planned to be around that long; they never really thought about it, like, it was just…would they just…the whole system would just start again the next day?

Pamela: This is one of those things that we just don’t have the records to tell us. I mean, it’s a cycle, so yes, it does just start over, but I don’t think they’d ever really plan for it to start over, but we don’t really know, and that’s the crazy thing is they don’t have any “world ending” lore tied to this; they don’t have any “everyone’s going to die” lore tied to this — it’s just the calendar, and so it’s…

Fraser: You think about the fact that how, like, computer scientists didn’t really think through the implications of the year 2000, and they only wrote their code 20 years before the end of the century. They never expected that their software would get used for 20 years, or 25 years, “Oh yeah, no, we’ll just put in, you know, ‘87, ’89” — that never really occurred to them. As you can imagine, again going back to the Mayans building this calendar, “Well, are we going to need this in 5000 years? Nah,” you know? It just never came up, so…

Pamela: It’s the millennium bug.

Fraser: It’s the millennium bug, yeah, exactly…so now we’re having to deal with the millennium bug. Thanks, Mayans.

Pamela: It’s just kind of funny that the software programmers and the Mayans only have a 12-year difference in their failure to think through their calendars.

Fraser: To think through the long duration of that…yeah, that’s good. So then, what things would change our calendars? Would there be events? Would there be things that will happen in the far future, maybe, that would change our calendar dramatically?

Pamela: So, we do have slight changes in the equinox positions that do occur, not due to the precession of the pole, but because our entire orbit – it’s not circular, and so as our orbit slowly rotates in combination with the precession of the pole, we end up with changes in equinox, we end up with slight changes in the solstice dates and the spacings of those, and so the slight things add up over time to, again, leap seconds here and there which will eventually, given enough millennia, turn into leap days. So it’s just a matter of our planet isn’t fixed in space; its axis is turning, its date of perihelion is changing, its date of date of aphelion is changing, and as all these things slowly change, they affect our calendar.

Fraser: But that is something that’s going to happen over the course of…

Pamela: Millennium…

Fraser: But that’s still going to be a cycle, but it would be a, yeah, but it would be a bigger cycle. You just end up with the movement of our orbit sort of slowly rotating around the Sun. What about the fact that our rotation is slowing thanks to the Moon? Will we get to the point where days last a very long time?

Pamela: Well, the rate at which things are slowing is such that, yes, it will happen. Our Sun will probably destroy the Earth before we have to worry about it too much. So you can imagine over the remaining course of humanity we might see a five-hour change, but I think that’s the type of thing…there’s already human beings that quite happily run on 30-hour cycles as they work on shift work, so that’s the type of thing we can deal with over time.

Fraser: Evolution can deal with that. And then would there be something that would perhaps, you know, when the Sun turns into a red giant, and…?

Pamela: Yeah, we’re going to have to be on a different planet by then.

Fraser: …the planet’s center of gravity changes, we’ll spiral outward, that’ll make the years longer, right?

Pamela: Yes. Again, we’re likely to be on a different planet, or dead by then, so I’m not particularly worried about the calendar.

Fraser: Right. OK. Alright, alright…just checking. OK, cool! Well, thanks a lot for the calendar info, Pamela. And thanks to everybody who watched us as we did the recording.

This transcript is not an exact match to the audio file. It has been edited for clarity.

Ep. 234: Lunar Phases

admin — Tue, 11 Oct 2011 15:15:22 +0000

The Moon is a stark reminder that we actually live in a Universe filled with stars and planets and moons. The changing phases of the Moon show us the relative positions of the Earth, the Sun and the Moon as they interact with one another. Let’s learn about the different phases, the geometry of the whole system, and some of the interesting science wrapped up with our fascination of our only natural satellite.

Ep. 234: Lunar Phases

Show Notes

Google+: Fraser, Pamela
Astrogear
Surley Amy Astronomy Cast pendant
Galileo’s observations of the Moon – Rice U
Phases of the Moon demos — NOAO
Moon and Earth Phase Viewer
New Moon astrophoto — Universe Today
How Can You See the Sun and the Moon at the Same Time? — Universe Today
Understanding Waxing Crescent Moon Phases — EarthSky Blog
Moon Illusion — Universe Today
Eclipses, Episode #160
Waning Moon — Universe Today
Understanding the Gibbous Moon — EarthSky Blog
Quarter Moon — Universe Today
Video: Libration and phases of the Moon: One Year of the Moon in 2.5 Minutes — Universe Today
SuperMoon Illusion — Universe Today
Phases of Venus and Mercury
Annular eclipse for May 2012
The Moon at Perigee and Apogee — Inconstant Moon
Just to be Clear: The Moon Did Not Cause the Earthquake in Japan — Universe Today
Moon Phases 2011 — Universe Today
Moon Phases 2012 – Universe Today

Transcript: Lunar Phases

Fraser: Welcome to Astronomy Cast, our weekly facts-based journey through the Cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain; I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University – Edwardsville. Hi, Pamela. How are you doing?

Pamela: I’m doing well. How are you doing?

Fraser: Good. So once again, we’re recording this episode of Astronomy Cast as a Google hang-out, and so all of the…our eight closest friends who are listening to this episode — you can all wave, but keep your microphones silent. So…if you want to participate with us, probably the best thing to do is to go onto Google plus, add me and/or Pamela to your circle, and then you get the notifications on when we do them. Right now, they’re completely random, and I apologize for that, but that’s just sort of our schedule, so it’s sort of like if you happen to notice that we do the recording, and it fills up fast, and I apologize for that, and so if anyone from Google’s listening, let us get on to the Google hang-outs on-air — that would be awesome, and then we can broadcast it to a larger audience. So now, did you have any more…anything else to update this week?

Pamela: Um, no. I have absolutely nothing. It’s boring.

Fraser: You’re plugless?! What?! You?

Pamela: I…well, we need donations — we always need donations, but we have to restock our store, so go in there. You can buy lanyards, but you know, by the time people listen to this, we are going to be selling “Surlys,” so there may be “Surly Amys” available if you go in and check out Astrogear.org.

Fraser: These are cool little ceramic necklaces that have our logo on them among other things. OK, well let’s get rockin’ then. So the moon is a stark reminder that we actually live in a universe filled with stars and planets and moons. The changing phases of the moon show us the relative positions of the Earth, the Sun and the Moon as they interact with one another. Let’s learn about the different phases, the geometry of the whole system, and some of the interesting science wrapped up with our fascination of our only natural satellite. Did you like that? Was that a nice intro?

Pamela: You’re getting good!

Fraser: So I think that, you know, but I mean, when I look out and I see the Moon, and I see the phases, it’s…for me that’s the reminder that we live in the Universe and that we have this ball of rock orbiting around the Earth. So how did the early astronomers and philosophers and stuff try to come to grips with what they were seeing in orbiting the planet, or not even orbiting the planet, just in the sky?

Pamela: Yeah, it was a god. It was not actually attributed as the source of the tides until remarkably recently. That’s something that continues to confuse me is how did Galileo not realize, among everything else he realized, that the Moon is responsible for the tides? But it was seen as a god for a while… They realized that it was part of the Solar System and along with the planets and the Sun was originally put on an orbit going around and around the Earth, and it was a holy object and a celestial object, but they didn’t realize it was a rock until Galileo came along and that was actually kind of a complete change in paradigm. Before that Aristotilian philosophy had said that the Moon was a perfect sphere – it wasn’t a perfect color, but it was a perfect sphere, and when Galileo looked at it through a telescope, he realized there’s mountains. They didn’t have the concept of crater, but there were mountains, there were differences in coloration, he could see shadows, and that was when they finally realized 400 years ago: it’s a rock. And since then we’ve been trying to understand it from a geologic point of view, trying to understand it as another object a lot like the Earth in many ways.

Fraser: Right. So when we see the moon, when we see the phases, when we describe it as “phases,” what are we really seeing?

Pamela: We’re just seeing differences in geometry, basically, between us, the Sun and the Moon. As the Moon goes around and around the Earth, you can imagine there’s this line connecting the center of the Earth and the center of the Sun, and when the Moon is on that line between us and the Sun, all of the Sun’s light hits a side of the Moon we can’t see. Now, most of the time, the Moon isn’t actually on that line in particular. It’s above or below the line, such that it doesn’t come between us and the Sun. The Moon’s orbit is tilted relative to the Earth, and this a good thing, otherwise we’d get monthly lunar eclipses, and that would get really un-exciting after a while.

Fraser: Right, and I think the way to do this right, of course, is to go into a really dark room with like a tennis ball, and hold…and then turn a really bright light on or a flashlight on from one source, and then hold the tennis ball at arm’s length. Your head is the Earth, and that’s what we see, and then if you put the tennis ball right in between us and the…you know, you and the flashlight. You can’t see the illuminated side of the Moon, and that’s the new moon. Now, you could still have the tennis ball a little above or below the flashlight itself, so you could still actually see the flashlight, but you’re not going to be able to see the lit side of the tennis ball — and that’s the new moon.

Pamela: And the way it works is it’s actually a couple days’ orbit to either side of the new moon before we can start to clearly make out the crescent moon, and exactly how long depends on how good your eyes are. And holidays like Ramadan are actually tied to: when is it that you first see that crescent moon reappearing as the Moon comes out and starts to show its illuminated side again? And I know for me, in particular, my favorite views of the moon are these amazingly thin crescents that you can sometimes see in the twilight.

Fraser: And there’s some really neat astrophotos that I’ve seen as well, where photographers will catch the moon — you know, they’re trying to break the record for the newest moon that they’ve been able to image. They’ll try to image the moon hours or even minutes after it’s passed the new moon phase, and try to get the littlest sliver of sunlight.

Pamela: Right, and yeah, it’s really amazing, particularly when you can start to get it close to planets and things like that. There’s been a few cases where you’ve had the Moon right next to Venus, the Moon right next to Mercury in the sky, and one of the things I love is watching how often people get the crescent moon completely wrong in artwork because you need to think of the illuminated side of the Moon as chasing the Sun across the sky, so as the Moon gets closer and closer to the Sun, you end up with a thinner crescent and it’s curved so that the illuminated part is toward the Sun, and the non-illuminated part is away from the Sun. And as the Moon goes past the Sun, it switches to keep the illuminated side always closer. Now, this has the effect that as the crescent moon gets low on the horizon following a sunset…so you have the sunset first and then the Moon setting later, you should have basically horns poking up where the Moon is doing an imitation of a longhorn for all you UT alumni, and occasionally you’ll see crescent moons drawn so they’re perpendicular to the horizon, and that geometry just does not happen.

Fraser: That doesn’t happen. Right. OK, so let’s imagine that we’re going to sort of take one full circle – again, go to your imaginary dark room with your tennis ball held at arm’s length, and so you’re seeing this thin sliver of light on the edge of the tennis ball, and as you turn, you’re seeing that grow and grow and grow. Now, which way are you turning?

Pamela: So, the way I always remember it is you take your right hand, put it over your heart, and the direction of your fingertips — that’s the direction the Moon orbits, so it’s going from the right toward the left around your head if the North Pole is at the top of your head and the Sun is in front of you.

Fraser: So I’m turning left…is that right?

Pamela: Yes.

Fraser: In the room…OK, so I’m arm out, tennis ball, and I’m turning left, and so I’m seeing more and more light on the tennis ball; I’m seeing this wrap around and I guess that’s the indication…and that should have been the indication that the Moon is a sphere is that you’re seeing this crescent shape wrap around, this light on the Moon, that should have just been like, “Duh, everything’s a sphere, even the Earth. And they’re all orbiting one another, and the Sun’s probably a ball…” and you know, like, it’s funny that that didn’t sink in.

Pamela: Well, the Greeks were pretty good about understanding that the Moon is a sphere. It was the everyday people of Europe in the times of Columbus that weren’t so keen on the “round planet” thing going on. So it’s interesting how knowledge doesn’t always filter through and, um, yeah, yeah…so smart people did figure it out: it is a sphere based on the pattern of the shadows moving.

Fraser: And so now I’ve turned 90 degrees, and so you can imagine now that before my arm was stretched out pointing towards that flashlight. Now, I’ve turned left so that my, sort of, right shoulder is facing the light and I’m holding this tennis ball out, and now I guess I’m going to see half the ball illuminated?

Pamela: You have a first quarter moon, and the first quarter moon actually can do some really neat tricks. It’s a moon that you have a chance to see both during the day and during the night. It’s one that rises at noon, it’s high in the sky at six p.m., setting around midnight. This is a moon that people really like to have around for star parties, so a lot of groups will schedule their star parties specifically for first quarters, so they can show people the shadows that Galileo saw.

Fraser: Right, of course. I mean, the best time to look at the Moon with a telescope is this halfway point. You know, at a new moon you can’t see anything, at a full moon everything washes out, but when you have this quarter moon you have these nice, long shadows across the surface of the Moon and the craters are just highlighted, and you can really see them, so a lot of the times when you have this full moon, people are like “Oh, can we look at it with a telescope?” but that’s actually the worst time. It’s much better when it’s this quarter moon. Oh, and we actually got this…we did an article recently in Universe Today about this. People were wondering, “How can we see the Moon and the Sun at the same time?” — and this is it. I mean, if you are near the equinox, you’ve got these, sort of, night and day having roughly the same length of time, so you can absolutely have both the Sun and the Moon in the sky at the same time. So, it’s all geometry. Right, so now I’m holding this tennis ball, and I see it sort of half on and so now I’m going to keep turning, and so now my back is to the light, my…I’m holding the tennis ball, but the tennis ball’s not in my shadow, so I’m not actually blocking the light from the light to the ball, and so now I can see a full moon, so I can see the whole tennis ball that I can see is completely illuminated by this light.

Pamela: So, you’ve now watched the moon do what’s called “wax.” So “wax on, wax off”– the Moon does that. You’ve seen the Moon wax toward full, you now have a Moon that if you end up with a full moon precisely at the equinox, some really neat things can happen. So if you traveled to the Equator and it’s one of those special equinox days (September, March), you can have the Sun setting at 6 p.m. in the west at the exact same moment that that full moon is starting to peek itself up above the horizon in the east. This is a kind of magical thing to get to see. Even if you don’t live on the Equator, you still get to see the same effect. It’s just not quite as dramatic when you’re elsewhere on the planet. The full moon is the washed-out, hard-to-see-interesting-features Moon, but it’s still pretty impressive when it’s down low on the horizon, and this actually leads to “the Moon illusion.”

Right. “The Moon illusion” — this is where people always think the Moon looks way bigger when it’s close to the horizon. It’s the Moon is just rising, “Look how big the moon is!”. It’s hilarious if you go onto Twitter, and you do a search for Moon around the time of the full moon, you will see tweet after tweet, post after post, people going, “Why does the moon look so big? Look how big the moon looks!” and I’m often…I’ll just jump in and reply to people, I’m like: “It’s not actually big, it’s just an illusion, it’s a trick of your brain,” and I’ll link them to various articles that are happening, but the…and the way that you can test this out, right, is you hold your arms out at full you know at arm’s length, your nail on your pinky finger will cover up the Moon perfectly and then you try it again later when the Moon is really high up in the sky and you’ll see the same thing, so you’re clearly being tricked.

Pamela: And what’s kind of neat if you have a telephoto camera, you can actually magnify this illusion. Get so that there’s some dramatic building off distant on the horizon with the Moon rising right beside it. Well, the distance between you and the Moon hasn’t really changed, but the distance between you and that building has changed significantly enough that it appears really small. Now use that telephoto lens to zoom in on the building and the Moon will appear as big as the building — and this is just an effect of making the building the size of a fingernail so that it’s the same size as the Moon. It’s a great way to make a dramatic photo.

Fraser: And I’ve seen some great time-lapse photos that people have done where they capture the Moon every two minutes or so, and you get just circle, circle, circle, circle, circle, and you can see the transition of colors. The Moon is coming from the horizon up higher in the sky, and it’s getting through the atomospheric haze, and it’s changing its color from this deep red to yellow to white, but the size is exactly the same — it doesn’t change, and so you can really see clearly this is not the case. The Moon does not change in size at all, and yet if you go outside and look at the Moon, it will absolutely trick you every time — and you fall for it, too. Now, we mentioned that back when the Moon was a new moon, and now when the Moon is a full moon that, you know, the Moon is not blocking our view of the Sun, even though the Moon and the Sun are actually roughly the same size in the sky, and yet the shadow of the Earth is not falling on the Moon, so why when you get these…these…this geometry, why is this not happening? Why is the Moon not blocking every time, and why is the Moon not passing into our shadow every time?

Pamela: So we have this double-angle effect. The Earth is inclined relative to the Sun, and then the Moon’s orbit is inclined relative to the Earth and this adds up to have the Moon, most of the time, as much as more than 20 degrees above or below the center line that connects between the Earth and the Sun, and this difference in angle is sufficient to keep that little tiny moon from blocking that little, tiny sun in the sky. So the way to do this is to actually take a hula hoop and connect your tennis ball somehow (cut the hula hoop, drill a hole through the tennis ball), and take that hula hoop and tilt it slightly. And the act of tilting it…you can now see what the orbit does, where that tennis ball is most of the time above the line or below the line, but twice each month, it cuts across the line and we only end up with an eclipse at those two magical times, and it’s not magical, it’s physics, it’s geometry — at those two times of the year when the full moon just happens to occur near the time when the Moon is cutting across that line between the Earth and the Sun.

Fraser: Right, and we’ve mentioned before in our “Eclipses” episode that they often go in pairs — that you’ll get a solar eclipse and a lunar eclipse in…one after the other because the Moon is spending its time…it’s at the point in its orbit, or the point of its inclination where it is actually passing through the shadow, and then blocks the Sun on the, you know, half a month later. OK, so we’re at the point now where we’ve got our maximum brightness, the Moon is washed out, we’re not really seeing anything and then we’re turning, we’re continuing to turn, we’re turning left some more before we were waxing, so now the amount of Moon we’re seeing is starting to decrease again.

Pamela: So now we’re waxing off, or the correct term is “waning,” and a lot of people will mispronounce it as “wanning” so you can…

Fraser: “Wanning”…like you!

Pamela: I’m better now, it’s “waning.” I’ve learned, and…

Fraser: That’s all I’ll say. Just to…not to drag you through the mud, but in a previous episode, that is what you said, and I called you on it, and I did a bunch of research, and I was right, and anyway…who’s the astronomer now?! Anyway, let’s continue… I know, I’m the linguist…

Pamela: Yeah, well, this is what happens when you learn from books. Books don’t teach you how to pronounce things.

Fraser: Right. So the moon is waning and it is…we’re still turning left, the Moon is waning, the amount of light…so now we’re seeing almost like this crescent of darkness starting to appear on the Moon as it’s getting less and less, and the funny thing as well is you’ll still get, as I’ve said I’ve been watching the twitters recently, and people will still for about four days think that the moon looks full.

Pamela: Right, so as the Moon orbits past the position of true full moon, it takes us a while to catch on to the fact that this is now called the gibbous moon. This is any time the moon is less than full, you can have a waxing gibbous, you can have a waning gibbous…and it wanes its way towards what’s called third quarter.

Fraser: Right, so I’m continuing to turn left holding this tennis ball on the hula hoop at arm’s length with its slight tilt, and now, again, I’m seeing the Moon half-lit. The front part is lit from the light, the back part of it is in shadow because I’m seeing it from the side, I’m seeing it half lit, half in darkness, and it is a waning quarter moon now. It’s a last quarter moon? Is that right?

Pamela: Last quarter, third quarter — this is when you see the Moon in the morning. And I know one of the things that stumped me is you’re seeing half the Moon, and we call it a quarter moon, and that was profoundly disturbing! Well, it’s because it’s a 3-dimensional object, and so we’re seeing one quarter of a 3-dimensional sphere illuminated, so a full moon is a half moon.

Fraser: Right, we’re seeing one quarter illuminated; we’re not seeing the quarter that’s also illuminated, we’re seeing one quarter of it that’s dark, and we’re not seeing the other quarter of it that’s dark.

Pamela: Right.

Fraser: OK.

Pamela: So, it’s that silly geometry. Once again, if you want to learn geometry, the Moon offers you everything you ever didn’t know you needed to know.

Fraser: And even more… Right, so you’ve got… and then the Moon continues on in its orbit, day after day, and you get to the point where we approach it being a new moon again.

Pamela: Exactly, and the thing that is interesting about all of this is because the Moon’s orbit isn’t completely circular – it’s slightly elliptical, its speed actually varies as it goes around, sometimes it’s moving a little bit faster, sometimes it’s moving a little bit slower and this causes, since it’s rotating about its axis at a constant rate, this causes, sometimes its rotation gets a little ahead of its movement around the planet, sometimes it gets a little behind its movement around the planet, and this allows us to see a little bit more of the planet than we would get to see otherwise. And since its orbit is inclined up and down relative to the central line, we also get to see a little bit more in the north-south direction as well. So along the way, even though in general the Moon looks the same, if you take photo after photo after photo what you realize through the passing nights, is we’re actually getting to see a little bit extra of the Moon as we get to look over the top look under the bottom, look around to the east, look around to the west, and all these different motions together get referred to as the lunar librations.

Fraser: And there’s an astonishing video that we…we actually posted on Universe Today, so Nancy’s going to be doing our show notes, and she’s going to know exactly the video that she did where you see the Moon move through all of these phases and it just looks…it just looks amazing. You can see the Moon almost — I can’t even describe it, I’m using my hands here, but it looks like it’s sort of oscillating back and forth, it’s like it’s wobbling back and forth over this period. It’s one of the coolest videos you will ever see, so I highly recommend…look for…check our show notes, or do a Google search for lunar libration video, and its just astonishing! The other thing that’s really interesting to see is the fact that the Moon, as you said, it’s on an elliptical orbit, so the times that it’s very close and the times that it’s very far, actually will get out of sync with the full moons and the new moons, and so you will have full moons that are super-full, you know, these “super moons,” and then other times you’re going to have these times when the…even though it’s a full moon, it’s at the furthest point, you know, the apogee of its orbit, and so it looks a lot smaller, and it can be significant. So the Moon when it’s at the perogee and at full moon at the same time, it’s actually quite bright.

Pamela: And this is where we end up with annular vs. full solar eclipses is when you have the Moon, in the case of an eclipse, at new moon when it’s closest to the Earth, it’s much bigger and it’s able to block the Sun for longer; whereas, when you have the Moon at its greatest distance when it’s new moon, and you have a solar eclipse, this is when the Moon can’t even fully block the Sun, and you end up with what is called an annular eclipse. So there’s lots of different things to take into consideration, and one thing, though, that is a myth — there are people who are actually concerned when there’s a full moon with the Moon at its closest to the Earth that this can actually have major geological effects on the planet Earth, and there are people out there who tried to blame the earthquake and tsunami in Japan on a “super moon” that occurred a few days later. That’s just not something you actually have to worry about. The difference between these two things in terms of percent change, is sort of like if you’re in California and you jump east — how much closer are you to New York City at that point? It’s just not a lot to have to worry about.

Fraser: Yeah, again, you feel more gravity from, you know, I don’t know, a table in front of you than the Moon, so and the changes are not going to wrench the Earth’s surface apart, and it doesn’t matter! What does it have to do with the phase of the Moon? The Moon gets that close every month, and so whether it’s illuminated or not illuminated has no difference on the geologic impact on the Earth — so get that out of your heads.

Pamela: Right.

Fraser: So before we wrap this up there’s this one thing that’s kind of neat. So what we see playing out with the Moon going around the Earth, we also see with Venus going around the Sun; Venus goes through phases, too.

Pamela: Right, and so does Mercury; it’s just a lot harder to find Mercury, at least with a pair of binoculars — it tends to get lost in the twilight Sun. So one of the ways we’re able to figure out that Mercury and Venus go around the Sun and not around the Earth is from the phases that we’re able to see. If Mercury and Venus weren’t located where they are, we wouldn’t be able to see them go through essentially a full set of phases. So what happens is as Venus gets ready to pass behind the Sun, we can see it as almost full, or if you can ignore the glare of the Sun somehow, a full Venus. Now, as it comes back around towards us, it gets to a crescent phase as it passes above or below the Sun. We essentially have a new Venus phase if you could find it in the glare of the Sun. It works best if you’re in space and can block the Sun without having the atmosphere get illuminated in the process. And it was Galileo that was able to see Venus go through this full set of phases, and there’s something actually really awesome about seeing a crescent Venus. And you can really see the angular size — how much of your field of view and your eyepiece Venus takes up as it’s closest to you for the crescent phase, and then furthest away from you for the full phase, so you get to see this tall, skinny crescent Venus, and the much smaller full Venus in the greater distance.

Fraser: Yeah, yeah, and it’s actually brighter when it’s in the crescent phase than it is when it’s further away, right?

Pamela: Right.

Cool! Well, thanks a lot, Pamela. Well, so I hope you can all do this experiment in the room, show your kids, really let it sink in, and then never be confused by the phases of the moon again. That was awesome – thanks!

Sounds great! I’ll talk to you later.

This transcript is not an exact match to the audio file. It has been edited for clarity.

Ep. 227: The Big Dipper

admin — Mon, 25 Jul 2011 17:59:00 +0000

We wanted to spend a few shows talking about some of the most recognizable constellations in the night sky. We’ve chatted about Orion the Hunter, but now we’re going to talk about the Big Dipper, also known as Ursa Major, or the Great Bear – apologies to our southern hemisphere listeners.

Ep. 227: The Big Dipper

Transcript: The Big Dipper

Fraser: Welcome to Astronomy Cast, our weekly facts-based journey through the Cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain, I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University – Edwardsville. Hi, Pamela. How are you doing?

Pamela: I’m doing well. How are you doing, Fraser?

Fraser: Good, a little under the weather. We were going to record yesterday, and I was just totally out of it, but it was like a one-day cold, I’m not sure, but I’m feeling miles better, so…

Pamela: Oh, I’m so glad. It seems like this is the summer when everyone is getting sick.

Fraser: Well, the worst thing is when both parents are sick, you know, so me and my wife are sick and the kids are like, “why are you guys so lame?”

Pamela: It’s bad when your own kids say you’re lame.

Fraser: Exactly! “Come on, we want to do something.” “…uhhh…watch TV…leave us alone.” So we’ll be a lot better tomorrow, and definitely a lot better today. So…alright, well, we wanted to spend a few shows talking about some of the most recognizable constellations in the night sky. We’ve chatted about Orion, and now we’re going to talk about the Big Dipper, also known as Ursae Majoris, or the Great Bear: apologies to our Southern Hemisphere listeners. Alright, Pamela, so do you know where the Big Dipper is?

Pamela: I actually do.

Fraser: Yeah, me too. If you live in the Northern Hemisphere if you’ve ever looked in the sky, you’ve got to know where The Big Dipper is, but you know it’s one of those constellations – same with Orion – it is, on the surface, really recognizable, easy to find, and yet, as we’re about to get into, it’s got inner vagueness.

Pamela: Yes.

Fraser: So then can you give a little bit of history, or like, where should we start on this one?

Pamela: Well, I think the place to start is: there’s probably already people out there going, “The Big Dipper is not a constellation.”

Fraser: It’s an asterism — there! Ha! There! Gotcha! Yes, but it is part of Ursae Majoris, so…

Pamela: And the thing is many cultures, the main stars, the seven stars that we see as The Big Dipper are what are the constellation in just not Greek constellation sets, so if you were to instead look at this in Chinese or Japanese or Korean, it would be The Seven Stars, and that’s fine. It’s actually, in eastern Asian tradition, it’s The Northern Dipper, so our Big Dipper is a constellation if you just “switch your longitude” of looking up.

Fraser: What? What?

Pamela: Well, so, I’ve been watching TV, there’s “switch your latitude” commercials — I’m saying, “switch your longitude.” Go to eastern Asia. It was a bad joke; it was a really bad joke.

Fraser: I don’t have cable. I don’t watch commercials. So right, but the, I mean, the shape again, I mean, you know you look at — like I’m trying to think — Gemini, Virgo, you know, Sagittarius…when you look at Sagittarius and someone says you know whatever it’s supposed to be…it’s a teapot.

Pamela: Yeah, it’s a teapot.

Fraser: Right? You know, Gemini — you can kind of understand it’s like two lines, side by side; Leo there’s a backwards question mark, ok, maybe that’s a lion…

Pamela: But really, you’re guessing when you see all of those things, and…Big Dipper, yeah!

Fraser: No question — that is a dipper!

Pamela: Ladle, you could call it a ladle if you wanted to.

Fraser: Yeah, ladle, pot — absolutely what it is describing…

Pamela: And the other thing that it gets named as which — I think we live in the wrong culture to see it, but in Europe it gets referred to as the plow. And that if you actually think of the old-time, hand-pushed, make-your-life-suck plows — it really looks like one of those.

Fraser: So then, what sort of history-wise, where did this…where did it come from? How did it become what we call Ursa Major and The Big Dipper?

Pamela: So there you start getting into pre-history. This is one of those problems with Astronomy is people have been looking up since before they knew how to write things down, and a lot of the constellations and all the different cultures date back to pre-history and it’s kind of hard to figure out where they originally came from. With Greek mythology, Ursa Major is just one of the “bears” in the sky, and it’s related to Orion the Hunter in some places, and it’s cropped up in various ancient books, but you can’t actually say who was the first person to sit down and say what it was, but if you look in Homer’s Iliad, it’s in there as The Bear, which men also called the Wain, and Wain has also in some places gotten tied to Charlemagne, and Charles’s Wain, so that’s one that just crops up all over Europe in slightly different ways, but I can’t tell you exactly who the first person it was to actually figure it out.

Fraser: And the Ursa Major, the Great Bear in the Greek definitions of the constellations, it has other, as you said, it’s The Plow in Europe, but different civilizations have given it different constellations, different mythology.

Pamela: Exactly. Exactly. Now where this one crops up, in some ways it’s more interesting than talking about Ursa Major, is this is a constellation that has a star that if you look at it and you have really keen eyes and you’re in really still skies, you’ll start to pick out it has a little buddy. This is Mizar in the handle of The Big Dipper. So when you’re looking at the Big Dipper, you have the bowl, and then moving away from the bowl, you have the three stars that make up the handle, and the middle star of those three if you have really keen eyesight, you’ll see there’s a little buddy hanging out next to the really bright star, and that little buddy is Alcor, and this was an eye test to get into the elite military for a while, so while it’s hard to track down exactly where the constellation came from, I find it fascinating that this may have been one of the very first eye tests around.

Fraser: And it’s a great, easy thing to observe with a small telescope or even binoculars, you can see… Now is it actually like a binary star, or double star?

Pamela: Now when you say double star, that’s just two stars that appear close together. These aren’t stars that are orbiting one another or anything like that, but it does qualify as a widely-spaced double star. Now, the thing is these stars, if you keep looking at them better, they then split themselves apart again, making this a quadruple system because each of them are independently binary systems.

Fraser: And each independently are actually binaries, so they’ve got the stars orbiting each other.

Pamela: Right, so you have two stars that aren’t orbiting one another that are actually not two stars — that are actually four stars that appear as two stars until you get enough magnification and they split themselves.

Fraser: What does it take to see that? What kind of telescope?

Pamela: So, while they are both double systems, you can’t actually split both of them. It’s one of those unfortunate things where Alcor – it really… it’s a spectroscopic binary, so if you look at it with a big telescope attached to a spectrograph, watch it over time, you see the lines dancing apart from the two different stars, but with your standard backyard system, you’re not going to split it into two different stars. Mizar, on the other hand, all you really need is clear skies and a really good eyepiece on even a small backyard telescope, so I’d say pull out your handy dandy friendly 70 mm refractor and a 4 mm eyepiece, if your sky supports it, and you should be able to split those.

Fraser: Alright. So we’ve already picked out one of the stars, but there’s a bunch more very interesting stars in the constellation, so do you want to start with any of them?

Pamela: So if you start looking at Ursae Majoris, and you look at Alpha Ursae Majoris, this is the star in the upper right-hand corner of the pot if you have the handle shooting off to the left, so imagine it as a nice friendly ladle, someone’s holding and keeping stuff in with the handle to the left, pot to the right.

Fraser: Top front of the plow…

Pamela: Yes, exactly. So that is one of the brightest stars, but we’re used to thinking of constellations as politely being: alpha’s the brightest, beta’s the next, gamma’s the next, but with this particular constellation, they actually labeled things right to left and sort of didn’t worry about what was the brightest or not, so if you want to keep track of which is which, you start at the upper right-hand star and go around in a clockwise direction and you get alpha, beta, gamma, delta as you go around. Now, when you’re at the bottom of the pot, there’s a couple of really cool [missing audio] objects right next to Merak, the beta star, the bottom right-hand, the part that digs into the dirt of the plow, or the bottom right-hand side of the dipper — you have M108 and M97 just sitting there being stunningly beautiful in the field.o

Fraser: So yeah there’s a bunch of stars, they’re not individually as spectacular as the stars in Orion. In Orion you have Betelgeuse and Rigel and it’s a party, but in The Big Dipper, Ursa Major, really it’s about the objects that are clustered around the constellation itself . Many of the most famous Messier objects — ones that you all recognize looking at pictures from Hubble — are all located in this one constellation.

Pamela: And in fact the Hubble deep field is located near this particular object, so this is just like beautiful, glorious stuff all piled up in one place, just pick your telescope and you can decide how glorious you want it to be.

Fraser: So let’s run down the list…so what can we find in this constellation?

Pamela: OK, so looking at the base of the pot you have M97 and 109, and M97…unless you have a lot of magnification, doesn’t look all that glorious: it’s the Owl Nebula. I’ve tried observing it with a 30-inch and got an unimpressive blob of light on my CCD, but it’s still kind of cool, you can see color with a camera on a moderately-sized telescope and you can actually see with that moderate-sized telescope that it is a circular blob that has two eyeballs on it.

Fraser: Right and this is why it’s called the Owl Nebula. It looks like great big eyeballs — with your imagination and the Hubble Space Telescope.

Pamela: Or if you just integrate, and integrate, and integrate, and take a very long exposure…I didn’t do that.

Fraser: So what is it though?

Pamela: It’s a planetary nebula. It’s a star not too different from our Sun that at some point in its past puffed off its outer atmosphere, and that outer atmosphere is sitting there quite happily glowing in pretty colors.

Fraser: What’s causing these “owl eyes” in it, though?

Pamela: This is actually one of the reasons for building the Hubble Space Telescope is planetary nebulae are just plain confusing. We don’t know why they all look so amazingly different, and they have all of these different apparent structures, so something at some point interfered with the light getting to those parts of the nebula.

Fraser: But, I mean, you’ve got some of these situations like the Helix Nebula, and you’ve got these asymmetric outflows coming out of the stars – it’s spinning, right? And so you could have this situation where inside it’s just parts that are like darker material being sprayed out, or parts of it that are being cleared out?

Pamela: It’s impossible to come up with a nice, clean explanation for this one because it has this beautiful symmetric shell all the way around, then it basically looks like an hourglass that has a dark bit to the left and to the right, but then that shell around it, so it’s unclear what would cause it to not have a stripe all the way across the front. Why is it filled in in the center, but not filled in where the two eyeballs are? So this is where you end up with asymmetries that are just very, very hard to make sense of.

Fraser: Hm…OK, so that’s M97, the Owl Nebula, and that’s a rough one, so don’t expect that you’re going to see the owl eyes in your backyard telescope…but, look to Hubble for that one.

Pamela: So M108 is almost on top of it. They are very nearby in the sky and this is another one that isn’t going to look fabulously glorious in your average backyard telescope. It’s about nine arc minutes by 2 arc minutes, so it’s big but not that big and it’s going to appear pretty much as a fuzzy stripe across your field of view.

Fraser: Right. I mean it’s a spiral galaxy that we’re seeing edge-on.

Pamela: And it has amazing dust lanes, and you can actually see the dust lanes if you’re using a 12-inch telescope with good magnification in a dark site.

Fraser: Right. So it’s a really fantastic point of view from us to be able to see a galaxy that’s relatively close and see those dust lanes that normally we don’t see them when we see the galaxy face-on. Now we can really see that structure because we’re seeing it from the side, but again from a backyard telescope point of view, it’s not the greatest view.

Pamela: Right. So then now we have to jump over to the next star over, so now we’re at the bottom left-hand star, the Gamma Ursae Majoris and right next to it — again, thoroughly boring star, but right next to it is M109, which is a face-on spiral galaxy. Again, pretty small, pretty faint — this one’s about 11th magnitude, so you’re going to need a fairly large telescope to be able to see it, but if you can see it. It’s about 7 ½ by about 5 arc minutes in size and it has these glorious well-defined arms, and a bar across the center, so it’s not all that different from what our own galaxy might look like, so when you look at M109 and you think, “Hey, that’s what we look like to aliens…”

Fraser: Right, because the recent evidence is that the Milky Way seems to have these two spiral arms with this big bar in the middle. So it really is…I mean, a lot of the images that we see of what the Milky Way looks like is probably not what the Milky Way looks like.

Pamela: Right.

Fraser: It’s only in the last couple of years that they’ve really started to probe what our own galaxy looks like, and it’s more like this, and not those mini-armed galaxies.

Pamela: Yeah, that’s just one of those things that really…we keep wanting to look just like Andromeda, and we’re not. Andromeda is our big sister, and we’re a little bit smaller in we’re structured a little bit differently just like siblings don’t always look the same. Now, as you keep going around the “drinking gourd,” as it’s talked about in African American history, you have to go up the arm and then if you can imagine an equilateral triangle with the two end stars of the handle, so Mizar, that great double-double we talked about, and Alkaid, the last star in the handle – imagine them as the base of an equilateral triangle, and at that top point, that invisible to the eye point of that equilateral triangle, that’s where the Pinball Galaxy, M101, is located, and this is one of the most amazing things to image with a backyard telescope. It’s not that faint – it’s about 8th magnitude and it’s absolutely huge! It’s almost a ½ degree by ½ degree in size, and just sit on it for a few minute and all of these structures, all of these dust lanes…they start slowly coming out second by second by second in your CCD images, and it’s one of the most spectacular things to look at in the sky.

Fraser: Yeah, now without a CCD can you…I mean, it’s just a fuzzy bit, right?

Pamela: Actually, without a CCD it’s…in a telescope you can see it, but none of these things that I’ve been talking about are visible without binoculars or a telescope.

Fraser: No, but I mean like even with a backyard telescope, but without a CCD, right?

Pamela: Oh, without that?

Fraser: So just when you’re looking at it with your eyes, you’re kind of just seeing a fuzzy bit.

Pamela: One of the things you run into is because it’s so big, it doesn’t have a lot of surface brightness out in its arms, so when you look at it by eye through a telescope, all you’re going to see is the fuzzy core. And that’s OK; Andromeda’s the exact same way — all you see is the fuzzy core until you start exposing with your CCD or your camera.

Fraser: And what’s going on in this galaxy? I mean, I know it’s a pretty famous image, even the ones taken by Hubble.

Pamela: It’s just a standard run-of-the-mill-but-close-nearby-large-and-stunningly-beautiful spiral galaxy, so it has star formation, it has open clusters, it has globular clusters around it, but you’re not going to see those with your backyard telescope. It’s just everything a textbook says a spiral galaxy should be, and I think that’s one of the things that’s so interesting about it is it has those well-defined arms like you want to see in a grand design spiral. They’re not as prefect as you’d see in some of the barred spirals, but they’re still nice, well-defined arms. It’s tightly wound, you can see the blue color if you take a CCD exposure…it’s just textbook.

Fraser: And it’s big galaxy; it’s about twice as large as the Milky Way. Alright, so let’s move on…there’s pair of galaxies, right?

Pamela: Right. So as you start moving away from The Dipper, you can use it to navigate to all sorts of different things in different directions, and nearby are M81 and 82, which M81 is a nearby spiral galaxy, and it’s one of the nearby active, mean, eating, black hole-containing galaxies, so to find it on the sky what you want to do is go off the end of the pot and if you go straight up — again we have the handle to the left, the bowl to the right. If you go straight up you’re going to hit Draco, and if you go up hit Draco and go to the right, that’s where you’re going to find this pair: M81 and M82. Now, M82 – this is one that’s famous for all of these stunning Hubble images that show it as a colliding system, where you see basically what looks like a plus sign of nasty material in a certain way. It’s what’s called a star-bursting system, so when you look at it in the Hubble images, what you see is a fairly normal looking disk, but then it looks like there’s some sort of Phoenix springing from the top and the bottom of the image in the Hubble images. The material is actually quite red coming off the top and the bottom of the system. This is a fairly bright system, again, it’s magnitude 8.4, it’s moderate-sized 11 arc minutes by 4 arc minutes, and you can actually see that it has kind of a “dead bug” appearance…looks like a galaxy got squished on your eyepiece when you look at it through a large telescope on a dark, dark site. So find your friend who has a 20-inch Obsession and a really good eyepiece set, and this is one that you can actually start to see the crazy structure of.

Fraser: I don’t have one of those friends.

Pamela: Well, you need to go to more star parties.

Fraser: I guess, but uh, yeah, I mean, I think …so what’s going on with M82 then? I mean, it’s an active star-bursting galaxy, but what’s going on inside of it?

Pamela: Well, this is a system that, as near as we can tell, it’s probably gone through some sort of a recent collision and that’s what we’re witnessing is a starburst that was driven through something triggering the wild star formation…and tidal forces were involved distorting everything. It and M81 are close enough that they do affect one another, so it’s gone through at least one tidal encounter with M81, and this caused material to get funneled into the galaxy’s core. So here you have two systems nearby, they both have active galactic nuclei in their centers, the one of them came out much the worse for wear from the…it wasn’t a head-on collision, it was what’s called “galaxy harassment” when things get a little too close to one another and gravitationally muck one another up. M82 just came out much the worse for wear than M81.

Fraser: Yeah, I mean when you get a galaxy in this kind of situation. I know M82 has got like ten times the star formation as the Milky Way, so you know new stars are being formed at a furious rate.

Pamela: And the things is this is the type of system that when all of that wild star formation that’s currently going on comes to a stop, this is the type of system that without gas left behind, could fade into being a red spiral – one of those things that we’re now learning exist, but aren’t like anything you read about in textbooks.

Fraser: I think that one of The Big Dipper’s best uses is as like your “on ramp” to finding your constellations.

Pamela: That’s entirely true.

You know, like, from The Big Dipper, you could find your way to so many other constellations, and I think one of the most important things for it is finding the North Star, so why don’t we kind of wrap this up with using The Big Dipper to find the North Star.

Pamela: Well, and even if you can’t find the North Star, finding The Big Dipper and following it by itself will take you north for most parts of the planet, or most parts of the Northern Hemisphere at least. In African American slave songs, they talked about following the drinking gourd to find your way north to find your way to freedom. And if you want to actually find, and you have dark skies…you can’t find the north star in every city…if you want to find the North Star, you take the alpha and beta star, the sides of the gourd away from the handle, the sides of the ladle away from the handle and you follow them from the bottom of the saucepan up toward the top of the saucepan, and then go five times that distance and five times that distance will land you on a very faint, little, tiny — in this vastness of the sky — unassuming star that just happens to be located very, very close to the rotational axis of the celestial sphere.

Fraser: Right. So you start at the bottom corner, draw a line to the top corner, keep going about five times and you’ll be hitting the North Star, and then as I said it’s this ladder, right, because then you can see The Little Dipper sort of folding backwards off of the North Star and you know it’s more of a stretch to see it as The Little Dipper, but it’s there, and then from there you can see Cassiopeia and Perseus and all these things are all there, so it’s just fantastic.

Pamela: And there’s other things if you’ve ever gone to a planetarium show in the Northern Hemisphere, most of them at some point will say “arc to Arcturus,” and “spike to Spica,” and this is because you can follow the arc of The Big Dipper’s handle and arc off of it to this amazingly bright red star, and that’s Arcturus in the constellation Bootes… and then you can do a straight line off of that to “spike to Spica” which is Alpha Virginis, and so you can start to find yourself more and more constellations just by using this as a starting point, and one of my favorite in terms of that would just be funny if it was actually happening is there’s “leak to Leo,” which is if you imagine it as a slotted spoon that holds no soup, if you tried to hold soup, the soup would fall on the head of Leo the Lion.

Fraser: There you go. Alright, well that’s great, Pamela. Thank you very much, and we’ll talk to you next time.

Pamela: Sounds good. I’ll talk to you later.

This transcript is not an exact match to the audio file. It has been edited for clarity.

Ep. 224: Orion

admin — Tue, 28 Jun 2011 16:35:53 +0000

Most people know how to find two constellations: the Big Dipper, and Orion the Hunter. You can teach a small child to find Orion, and at the right time of year, they’ll find it in seconds. There’s so much going on in this spectacular constellation, from the star formation in the Orion Nebula to mighty red supergiant Betelgeuse, ready to explode. Let’s learn about the history and science of the constellation Orion.

Ep. 224: Orion

Show Notes

Astrogear
Orion in mythology — Windows to the Universe
Rigel (Beta Orionis) – EarthSky.org
Betelgeuse (Alpha Orionis) — Solstation
“Don’t Panic!” Belegeuse Won’t Explode in 2012 — Discovery Space
Bellatrix (Gamma Orionis)
Alnilam (Epsilon Orionis)
Orion’s Belt Stars — Universe Today
Video: Size comparison of planets and stars
Orion Nebula — SEDS
Tour the Orion Nebula — HubbleSite
Hubble’s view of M 43
Horsehead Nebula — NRAO
Bok Globules – Universe Today

Transcript: Orion

Fraser: Welcome to Astronomy Cast, our weekly facts-based journey through the Cosmos, where we help you understand not only what we know, but how we know what we know. I am Fraser Cain, publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University – Edwardsville. Hi Pamela, how are you doing?

Pamela: I’m doing well, Fraser. How are you doing?

Fraser: Doing great. Now, you wanted to do some plugging today…

Pamela: I did. We have a store full of t-shirts waiting for your summer apparel needs to be met, so go to Astrogear.com and get your Astronomy Cast t-shirts.

Fraser: Is it Astrogear.com, or .org?

Pamela: Both work…both work.

Fraser: OK. Alright. I didn’t know we had the “.com;” I knew we had the “.org.” Anyway, Astrogear.com: t-shirts, CDs, lanyards, posters…

Pamela: We have it all. Go get it now.

Fraser: Alright. Cool! Well, most people know how to find two constellations: the Big Dipper, and Orion the Hunter. You can teach a small child to find Orion, and at the right time of year, they’ll find it in seconds. There’s so much going on in this spectacular constellation from star formation in the Orion Nebula to the mighty red supergiant, Betelgeuse, ready to explode. Let’s learn about the history and science of Orion. Alright Pamela, so I guess, first, let’s pretend we’re talking to that small child and help them find Orion. Where and when should be looking, and what will we see?

Pamela: Well, as we record this in the Spring, it’s not there so much right now, but…

Fraser: Yeah, it’s kind of sinking away now.

Pamela: Right, so the reality is if you get up early in the morning you can see it. It’s currently a morning object, but the last slivers of the Dog Star actually will be…you can just tell that the dog star is starting to rise as the Sun is starting to rise in August, but until then, it will be rising before the Sun, so we’ll get at least Sirius’ dogs for a few more months — not Sirius’ dogs, Orion’s dogs for a few more months.

Fraser: But, winter’s the best time, right?

Pamela: Winter’s the best time. It’s an evening object. Look toward the equator. So if you’re in the Southern Hemisphere, look toward the northern skies; if you’re in the north, look toward the southern skies come winter, and it’s one of the most recognized sets of bright stars that people have used to make pictures throughout all the different cultures of the world.

Fraser: Yeah, and clearly obvious that it’s person-y, human-y-looking, right? I mean, no question, there’s the three stars of the belt, and then there’s the shoulder stars and the knee stars – is that what it’s described as?

Pamela: Well, yeah…

Fraser: And then there’s even more parts to it: there’s the sword…there’s all kinds of stuff. It’s all there.

Pamela: And what’s interesting is that we see it as person-y; other cultures see it as three sisters instead of three belt stars, and so they make up all sorts of different stories based around this set of bright stars that hangs out near the celestial equator, and it’s basically a giant box wearing a belt, and so parcel up that giant box however you want it. Now, in western lore it’s typically Orion the Hunter. Here in the northern hemisphere, the two stars you see generally pointing toward zenith are seen as the shoulders — and one of these is the bright-red Betelgeuse — and he’s seen as either holding up a sword or sometimes holding up a shield as he fends off the oncoming Taurus the Bull. So, it’s one of those constellations that people tend to turn all different sorts of things out of it. In fact, you can sometimes even see him in some of the drawings looking away from Taurus the Bull as Taurus comes up behind him.

Fraser: And so, what’s the history, then, of the constellation?

Pamela: It’s actually kind of as mixed-up as the pictures of the constellation are. It’s not one of the prominent stories in Greek lore, but the basics that most of the stories agree upon is Orion was a hunter, and he had a run-in with Scorpio, the giant scorpion, and after they both died, they got put into the heavens but on opposite sides of the sky, such that Scorpio is up high in the sky six months before Orion is up high in the sky.

Fraser: Now do you…is it one of those situations where they already had the story, and then they sort of saw that story mapped out in the sky, or do you think it went the other way?

Pamela: See, this is one of those things it’s very hard to know, especially these not-well-documented Greek lores because people have been lying on their backs, staring up at the stars, making up pictures as long as humanity has existed, and there’s some archeological – not archaeological, there’s some linguistic evidence that some of the star names are actually prehistoric, so before we had people to write things down, these stars were given their names, so it’s unclear. Now, some of the books that we look at that mention it include Homer’s Odyssey, and the constellation has got to predate Homer’s Odyssey, well there’s no “got to,” but it most likely predates Homer’s Odyssey, but the question is: how long have people’s grandmas and grandpas been telling that story? We just don’t know.

Fraser: So it’s hard to know whether they had the story, and then they mapped it back to the stars, or someone looked at the stars and saw the pictures, and then they came up with the story to explain it.

Pamela: Right.

Fraser: Yeah, that’s interesting, and so…but then from the scientific basis, what’s actually going on there? Obviously, it’s a collection of stars. Is it really collected the way we see it in the sky? Like, if you could look at it from a different perspective…

Pamela: No, in fact all those stars are at slightly different distances, such that the brightest star, Rigel, is hundreds of light years away, and the nearest star in the constellation is just 18 light years away. So, we have this vast disparity in the difference between the nearest and the brightest stars, and if you’re able to make a 3-dimensional map of this (and I’ve had various students do that as a class project), it actually shows this fabulous distance distribution even of the belt stars. So this is just a group of stars that appear lined-up, but that’s only because they happen to randomly be collected in 3-dimensional space in the same direction on the sky.

Fraser: And they happen to have a luminosity that balances out from our perspective.

Pamela: Exactly. And so this is where you end up with interesting things like Betelgeuse appears amazingly bright. It is amazingly bright, in fact it’s about 670 times the size of the Sun, so this is a giant, red, bright, huge star, and it’s about 640 light years away. Now, compare it — admittedly, it’s one of the brightest stars in the system, but compare it to one of the other fairly prominent stars in the system, so for instance, you have Bellatrix, this is the 22nd brightest star in the sky, and it’s only 240 light years away, and it’s only 7x the size of the Sun. Now, what you’re hearing, though, is this constant theme of “bigger than the Sun, bigger than the Sun, bigger than the Sun,” and, in fact, all the bright, bright stars that you notice when you look at Orion in general are as near as we can tell (and there’s a few we don’t have really good data on), but as near as we can tell, all of these stars are, in fact, bigger and younger than the Sun and some of the stars that we’re looking at — these are actually younger than the Earth. So, Alnilam (Epsilon Orionis is the better name for it if you want to pronounce it correctly)…it’s a 4 million-year-old star, which means the planet Earth, the planet we are standing on right now is older than this star, so someone hanging out watching the planet form who looked up at Orion would have seen it’s belt missing one of the stars.

Fraser: Well, now you’re jumping around a bit, so I’d like to be a little more organized here because, I mean, each one of these stars has an amazing story, plus there’s the Orion Nebula, and there’s other good stuff as well, so let’s just start up at the top. So the upper left-hand corner…

Pamela: Upper left-hand corner…

Fraser: That’s Betelgeuse.

Pamela: Right.

Fraser: And you kind of give a hint — it’s a red supergiant star, hundreds…what, 600 times bigger than the Sun?

Pamela: Yes, 640 times bigger than the Sun, so…

Fraser: So, that would stretch out to like what, past Jupiter?

Pamela: Uh, way more than that.

Fraser: Like past Saturn’s orbit?

Pamela: [laughing] Yeah.

Fraser: Wow! Can you just imagine, like, having that in the Solar System?

Pamela: Yeah, it wouldn’t fit so well.

Fraser: So what does it look like if we got closer?

Pamela: Well, that’s the neat thing about these giant, red stars is they have very diffuse atmospheres, so it would be kind of like sneaking up on a big, red, burning fog bank, where the outer parts of the star’s atmosphere are a lot like playing in thick clouds, they just happen to be thick clouds of plasma. So this is a giant star; it has a puffed-out atmosphere. This is one of the stars in the sky that’s most likely to go supernova in our lifetime – that doesn’t mean it will, that doesn’t mean it will even do it in the next 10,000 years, but it’s still sitting there waiting to potentially do it, and if this giant, red star does go supernova, it will actually be visible for almost the entire planet during the daylight.

Fraser: And like, the next 100,000 years? The next million years?

Pamela: Yeah, we don’t know…so it could be tomorrow, it could be 100,000 years from now…yeah, somewhere in there.

Fraser: Yeah, but it’s a fairly young star. It just happened to be…

Pamela: Massive.

Fraser: …very massive, and is in this red supergiant phase of its life, so if we looked at it like what a million years ago, it would have looked quite different, right?

Pamela: Right, and well, so a million years ago… These stars — they move off the main sequence very quickly, so we don’t know exactly where it is in its phase right now, so a million years ago, it might not have looked too different, but in its past this would have been a giant, blue star. So, it’s a large star; it’s currently a red supergiant, and in the past it would have appeared a whole lot bluer.

Fraser: Wow. So, I mean, it is one of the most spectacular stars. It’s one of the stars that I know Hubble is able to resolve a disk of the star it’s so big.

Pamela: And they can actually see Sunspots on its surface, so this is something that it gets Sunspots that would take up large fractions of its surface that we can track and measure the variations in the light due to these Sunspots.

Fraser: Yeah, like Sunspots bigger than the orbit of the Earth…it’s just mind-bending. OK, so that’s that. Right away, one of the most important stars in the whole night sky is in Orion, but then one of the other ones…like, why don’t we skip down to the bottom, what is it, the bottom right-hand corner? With Rigel, right? That’s one of the other most important stars in the night sky.

Pamela: So this is also known as Beta Orionis; it’s another supergiant. Here, it’s not quite done doing the interesting stuff, so Betelgeuse — it’s moved over to the right-hand side, it’s still undergoing a lot of mass loss, but Rigel because it’s bluer, it’s much, much hotter, it’s burning much, much faster, it’s at a much higher temperature and when it’s done, it’s also likely to go supernova, but the fact that it’s still blue tells us that we probably have a little bit longer to wait compared to Betelgeuse, which, because it’s red, means that it’s further along in its evolutionary state. It’s kind of neat how looking at this system you can see two giant stars that are both going to end their lives in similar, but not identical ways, but because of the difference in color, we can tell that they’re actually undergoing different things down deep in their cores.

Fraser: Right, and I wonder, again, if we can see…there’s a really great animation which I’m sure a lot of people have seen, where it’s these comparisons of the different planets, and eventually you’re comparing the planets against the Sun, and then the Sun against other stars, and I know Rigel is on this animation (we’ll link to it in the shownotes), but it’s kind of like the Sun compared to Rigel is like the Earth compared to the Sun. It’s unbelievable to see how big Rigel is when you see it compared to the Sun — like it’s, what is it, 78 times bigger than the Sun? And just astonishingly different, and it’s super-hot, right?

Pamela: Yes, it’s super hot – thus the blue color, and one of the neat things about it in terms of just giving yourself perspective on things is, so Betelgeuse is fainter and closer, so when you look at it, you see it as fainter, and it’s closer. Now, when you look at Rigel it appears brighter, and you think, “well, maybe it’s even closer than Betelgeuse,” and these two things are similar in brightness, but…or similar in luminosity, but the reality is that Rigel is 770 light years away, so it’s a more distant object that appears brighter in the sky. It’s also a smaller object because it hasn’t bloated itself out yet, so the fact that it’s young and blue means that it’s still pretty compact – it’s 78 solar radii, which is still giant, but the smaller size, it’s bluer color means that even though it’s at a greater distance, it still appears brighter in our sky.

Fraser: And Rigel is a neat target for telescopes because it’s a visual binary.

Pamela: Right, so if you have a small telescope — you can’t do this one with binoculars, sorry, unless they’re giant binoculars…but unless you have truly insane binoculars, find yourself a small telescope, and when you take a look at it with sufficient magnification, you can tell that there’s actually two different stars there. Now, the separation between these stars is huge — they are 2200 a. u. apart. You’re not going to see them moving; you’re not going to see anything else. They’re simply two stars that are very loosely orbiting around each other.

Fraser: Wow! Any of the other stars in Orion very cool, or doing something interesting?

Pamela: Well, I mean, they’re all kind of cool. What’s amazing is this is all part of the giant star-forming complex as you look out in that direction, and individual stars don’t necessarily belong to the complex, but as you scroll through the system, there’s bits and pieces of nebulosity just about everywhere. And then as you look at the individual stars, you start being able to pick up things such as Mintaka, which is Delta Orionis, which is upper right-hand if you’re in the Northern Hemisphere belt star; lower left-hand Orion’s-hanging-upside-down belt star if you’re in the Southern Hemisphere. This is another one of these multi-star systems, and in this case, you have a blue star, you have a white star, and it’s an eclipsing binary system, so you can actually see slight variation in the brightness of the stars, which is just something really cool to be able to go out and watch with your own eyes.

Fraser: So, when you say eclipsing binary…so, you’ve got one star that’s orbiting in front of the main star?

Pamela: That’s exactly what’s happening. So, this is one of those times when if you very carefully watch the star over time, you can see every 5.73 days that the stars slightly change color and slightly change brightness.

Fraser: But they’re too close for you to actually be able to pick them apart with the telescope.

Pamela: Right.

Fraser: Right, OK, you just measure it in terms of brightness. Well, then I think then we should talk about the probably the prize of the whole constellation, which is the Orion Nebula. And this is something you can actually see with the unaided eye. I mean, if you go out with really nice, dark skies – like where I live – you can see this kind of furred, you know, this kind of fuzzy, blurry bit just underneath the belt stars, and that’s the Orion Nebula.

Pamela: Right, so this is a giant star-forming complex. There’s the Trapezium stars – these are bright, white, hot “O” stars, many of them varying in brightness. As you watch this entire region over time, you can…actually if you get extraordinarily lucky – and this has happened before – occasionally, you get to see a new star blowing off the cloud of gas around it. Now, admittedly, this has only been known to happen once, but it’s kind of cool to know that it’s happened. So, as you look at this system and explore it, or you get to see what stars look like when they’re just forming and haven’t fully pulled themselves together yet… And what’s interesting is you get all the different types of nebula in here in terms of gaseous nebula: you have dark nebula like the Horsehead Nebula, which is this cold cloud of gas in front of the more brightly luminous background, you get reflection nebula, where you have the light from stars passing sideways through clouds, and getting reflected towards us, and the reflected light we see is blue, you also see the red nebula, where light is trying to pass from the back through the clouds of gas and only the red makes it through to us, so there’s lots of physics to be understood by staring at some of the most beautiful things in the sky that you can access with a backyard telescope.

Fraser: And so if you can get your hands on even a good pair of binoculars, you can actually see (I’m trying to think of what you can see with good binoculars) sort of a fuzzy, blurry bit still there.

Pamela: Right. You can make out the Trapezium stars.

Fraser: Yeah, and it’s almost like they’re kind of enshrouded in fog a bit, and then as you get to better and better telescopes, you don’t really see the color until you’re actually taking some photographs.

Pamela: Right, so some of this where we really suffer is the human eye. It’s made up of two different cells, and only some of those cells are sensitive to color, and the ones that are sensitive to color don’t work in low-light conditions, and so it’s the cells that are simply sitting there going “light/no light,” and forming black and white images that typically trigger when you’re looking at the night sky.

Fraser: So, to really appreciate what it looks like, you want to hook up a CCD camera to your telescope.

Pamela: Right. So all of these things are part of the Greater Orion Molecular Cloud Complex. It’s 1500-1600 light years away, and M-42 — Orion’s nebula is just part of this, the Horsehead Nebula is just part of this, and it’s made up of all these different types of nebula. It includes Barnard’s Loop as well, which is a loop of gas, the Flame Nebula, which is famous for its red color… This is one of the areas of the sky that in order to see amazing things, really, all you have to do is take an everyday — admittedly film, but an everyday film camera, point it at the sky, and try taking a photo for a couple of minutes. Now, if you’re not zoomed in at all, not tracking at all will cause it to be slightly blurred, depending on what type of lens you have, but it’s enough to start picking up color in a very dark site, and the color is a reflection of the fact that this is where stars are being born. Now, if instead of using a 30 mm, or a “whatever mm,” small camera in your backyard, you resort to the Hubble Space Telescope (which is always fun, I highly recommend it), the Hubble is actually able to make out what are called “proplets” — these are caterpillar-like structures embedded throughout the Orion Molecular Cloud Complex that are stars getting ready to emerge, but they’re still surrounded by the nebula of gas that they’re forming from, so these are basically stellar cocoons waiting for solar systems to emerge.

Fraser: And, there’s actually…you said M-42, and there’s also M-43. They’re all kind of connected in that exact same region, so you can actually…they are almost a little hard to distinguish.

Pamela: Right, so M-43 is the one that I think is in many ways has been made famous from the Hubble camera. If you’ve ever seen the image that’s all blues and yellows fading to oranges with bright white stars embedded throughout it, and it looks kind of like a watercolor? I know that’s kind of vague…google M-43 and you’ll know what I’m talking about.

Fraser: Yeah, yeah that’s M-43.

Pamela: Yeah, it really looks like a watercolor painting.

Fraser: And then you just sort of mentioned briefly the Horsehead Nebula, but what’s that?

Pamela: And so this is what’s called a dark molecular cloud. So what happens is if gas gets thick enough, just like if clouds get thick enough, light can’t pass through them. So in these cases, you have a background that’s made luminous through stars that have emerged, have burned off the gas and dust around them, and that backlighting of the cloud causes the cloud to just stand out the same way a thunderhead might stand out against a bright summer’s day. And it’s these dark molecular clouds that collapse in on themselves to begin forming stars, so where you see things like the Horsehead, where you see things called Bok Globules – these are the future sites of star formation, but right now they’re just sitting there going, “I’m cold, and I’m dark, and I’m going to block all the light behind me.” And so, the Horsehead is just stars waiting to be born.

Fraser: So if you have a powerful enough telescope, can you make out the Horsehead, or does it still require CCD camera?

Pamela: I’ve been able to see it through a 30-inch telescope, and actually…

Fraser: [laughing] A 30-inch telescope? Yeah, OK!

Pamela: [laughing] There are amateurs out there… Now, I haven’t tried with smaller telescopes, but…

Fraser: Well, of course! Why bother if you’ve got access to a 30-inch telescope?!

Pamela: Well, it was random. It happens… but based on when it was discovered, I’d assume that you don’t need too giant of a telescope as long as you have really, really good skies. Now, it was discovered in 1888 on a photographic plate. This isn’t an easy target that your friendly 4-inch telescope is going to pick up, but with perfect skies, perfect telescope you should be able to pick it up with smaller telescopes.

Fraser: Very cool. Was there anything else in the constellation, or have we kind of covered all that?

Pamela: I think this is just the perfect place to start learning. It’s one of those places that you can say, “this is where star-forming starts; this is where my observing of the sky starts,” and then use it as a branching point to explore. One of the neat things about Orion, well, it’s so stupidly easy to find: you look up, and you look for the giant red star, and the giant blue star in the corners of the box. Once you’ve got that, you can use it to move over to Taurus and Aldeberan – the giant burnt-orange eye of the Bull. You can go across the diagonal from Rigel, up through Betelgeuse and find yourself over at the constellation Gemini, and see Castor and Pollux. You can follow the line of the belt stars into the left if you’re in the North, and you can get yourself down to the Dog Star, Sirius. So, this is a starting point to learn your way around the sky. When I was a kid learning the constellations, I could always find Cassiopeia, and I could always find Orion, and just like some people’s knowledge of cities grows around subway stations, my knowledge of the sky grew around these two constellations.

Fraser: Yeah, exactly the same for me…you know, I learned the Big Dipper and I learned Orion. It was almost like that was a gift that I got from my father was…you know, early on he taught me where Orion was, and he taught me where the Big Dipper was, and then later on, when I took up Astronomy as my own hobby, and I got myself the sky charts and I started to learn all those other constellations, again now, I can find Cassiopeia, and I can find Andromeda, and I can find, you know, Boötes, and all of those, right? But in the beginning, it’s those two, and especially Orion, when it’s like the night sky, and it’s winter and the sky is so clear and it’s crisp and it just blazes on the night sky. So, if you haven’t taken the time to teach your children, teach your friends, teach your parents, your students how to find Orion. It should last a lifetime.

Pamela: And if you really want to blow their minds, while they’re standing there looking at them, tell them that all the things they see as single stars…well, most of them are actually multiple stars, and most of them weren’t there to be seen when the Earth was just starting to form.

Fraser: Yeah. Cool! Well, thanks a lot, Pamela.

Pamela: It’s been my pleasure, and go to Astrogear.com! Buy t-shirts!

Fraser: [laughing] Cool!

This transcript is not an exact match to the audio file. It has been edited for clarity.

Ep. 160: Eclipses

admin — Wed, 11 Nov 2009 17:34:13 +0000

Every now and then, the Moon destroys the Sun. Okay, not destroys, covers. Well, not really covers, but from here on Earth, sitting inside the shadow of the Moon, that’s what it sure looks like. These events are called eclipses, or more precisely, transits and occultations. They occur whenever one object passes in front of another from a 3rd perspective. They’re beautiful and exciting, and deliver a tremendous amount of science as well.

Ep. 160: Eclipses

Shownotes

Eclipse: An astronomical event that occurs when one celestial object moves into the shadow of another.
Transit: Event that occurs when one celestial body appears to move across the face of another celestial body, as seen by an observer at some particular vantage point.
Occulation: Event that occurs when one object is hidden by another object that passes between it and the observer.
Solar Eclipse – Mr. Eclipse
What causes an eclipse? — Earthview.com
NASA’s Solar and Lunar Eclipse Calendar
Moon’s inclined orbit — Fourmilab
Sun-Eating Dragon — Exploratorium
Traditional tales of solar eclipeses – Color of India
Lunar Eclipses for Beginners – Mr. Eclipse
How Lunar Eclipses Work — How Stuff Works
Eclipses on other planets – Cornell U
Transits of Venus
Transit of Venus in 2012
James Cook and the Transit of Venus — Science@NASA
Using the transit of Venus to measure an AU — Exploratorium
Transits of Mercury
Safety during solar eclipses — NASA
Using a kitchen colander to see an eclipse — NASA
Thousand Oaks Filters
Oceanside Photo and Telescope
Herschel Telescope
Transit method to look for extrasolar planets
Terrestrial Planet Finder
Pluto’s atmosphere — Universe Today
Asteroid Occultations

Transcript: Eclipses

Fraser: Astronomy Cast Episode 160 for Monday October 19, 2009, Eclipses. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain, I’m the publisher of Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville. Hello, Pamela.

Pamela: Hey Fraser, how’s it going?

Fraser: It’s going very well. Alright… so, every now and then the moon destroys the sun. Ok, not destroys but covers… ok, not really covers but from here on Earth, sitting inside the shadow of the moon, that’s what it sure looks like. These events are called eclipses or more precisely transits and occultations. They occur whenever one object passes in front of another from a third perspective. They’re beautiful and exciting and deliver a tremendous amount of science as well. Now I think you, once again, let’s sort of delve into Pamela’s personal life history, you had a pretty exciting little adventure this summer.

Pamela: I did… I did. This summer, 2009, I traveled to the China Sea and off the coast of Japan and China was on a boat called the Costa Allegra with the Eclipse of the Century tour group attempting very hard to view what was supposed to be the longest eclipse of the century that we’re currently living in. It was supposed to last about 6 minutes 30 seconds, depending on exactly where you were along the line of the eclipse you’d get a little longer or a little shorter. It was a valiant attempt because that particular time of year is typhoon season and the weather is chronically awful, but we tried. And we did experience utter darkness for 6-ish minutes while getting rained on.

Fraser: Oh.. oh no… so you flew half-way across the world…

Pamela: Literally, yes…

Fraser: … got a great big cruise ship, went out into the south China Sea, right to the eclipse point and it was raining and…. Did you see anything? Did you see any parts of the sun being chopped away?

Pamela: There were three different moments during the lead-up to the eclipse where a crescent sun peaked through the clouds. In one case it was kind of funny because if you rotated the picture 90 degrees, it ended up looking like a smiley face because of two darker points in the clouds and then the sun beneath. So we saw these small fragments leading up to the eclipse, and then at the eclipse and after the eclipse it just rained… hard… a lot. There’s pictures on starstryder.com. You can go share in my rainy-day experience.

Fraser: See, I was seething with jealousy the whole time, and really that made me feel better.

Pamela: We got rained on and I got motion sickness. For the first time in my life I had to leave my own public presentation to go become very ill.

Fraser: So have you ever seen a total eclipse, then… work out?

Pamela: No. I saw a partial eclipse under gorgeous northern Michigan skies back in undergraduate, but it was a partial eclipse.

Fraser: We had a pretty big eclipse in Vancouver… I can’t even remember… it was back in the early ‘80s… like ’81… ’82… Things got a little dim, but that was it.

Pamela: Yeah. So it did get amazingly dark.

Fraser: Alright, so then what is an eclipse… just to sort of fill up the textbook.

Pamela: Well, an eclipse can take many different forms, and like you said, it’s always when one object passes in front of another and blocks out its light. So, for instance, I can drive a semi-truck in front of floodlights and cause a truck-eclipse… or , I guess, a floodlight eclipse. I can put the moon in front of the sun and cause a solar eclipse.

Fraser: So if you have a truck sitting on a pier and a boat goes past the truck behind….

Pamela: Then that would be a truck eclipse.

Fraser: Would that be an occultation, though? A boat occultation?

Pamela: This is where the language gets tricky because historically, we’ve used the word eclipse when it was the moon or the sun disappearing. We’ve used transit for little tiny things passing in front of larger things. If Venus or Mercury passes in front of the sun, if an extrasolar planet passes in front of an alien star, those are transits. We tend to use the word occultation for when an asteroid nearby passes between us and a far off distant star. So, in all cases we have this eclipsing behavior… we have one object blocking out the light from another. With eclipses, we’re typically referring to the complete destruction of light from an object in our own solar system by another object in our own solar system. Moon blocking Sun… Earth blocking light from Sun from hitting moon. When we talk about transit, it’s small thing not completely blocking light of a star and when we talk about occultations it’s typically objects in two completely different solar systems… one in our solar system blocking the light from an object in another solar system.

Fraser: Ok, so let’s focus on the traditional solar eclipse first. So, what is going on to make a solar eclipse happen?

Pamela: Well we live during a wonderful time when our Moon is periodically just the right distance from the planet Earth that its disk can pass exactly between us and the sun. This doesn’t happen every single month because the orbit of the moon is inclined just enough that most months, if you were to draw a line from your eyeballs to the sun through the new moon, the moon would actually not be on that line. It would be above the sun or below the sun, so you’d be making this crazy triangle as you went from eyes to moon to sun. It’s that crazy triangle… the fact that the moon appears in the sky above or below the sun that prevents eclipses from happening. That inclined orbit has two points… two nodes… where the orbit of the moon crosses the ecliptic… it crosses that line that the sun is on in the sky. When the moon does that, if it happens to be during a new moon, then the moon does block all the light from the sun. The ancient Chinese refer to this as the dragon eating the sun and would bang pots to scare away the dragon. Throughout all of history there have been stories of, in one case, a king dying of fright of a solar eclipse and battles ending due to a solar eclipse. It’s a very dramatic and creepy experience. It’s strictly because the moon is just the right distance. Now the thing is, the moon is getting further and further away over time. And its orbit… it’s not a perfect circle. So even now there’s some times when the moon just happens to be at the right point at its orbit, when it’s on that point where it’s crossing the ecliptic, it’s too far from the earth to be large enough in the sky to completely block the sun. In those cases we end up with what’s called an annular eclipse where you see this annulus of light around the moon… a donut of sun essentially hanging in the sky.

Fraser: Right, because the distance between the earth and the moon changes over the course of the moon’s orbit, and the distance between the earth and the sun changes over the course of the earth’s orbit, and so when you have both at their furthest point, then… or actually when you get the moon at its furthest point from the earth it’s actually smaller in the sky and doesn’t completely cover up the sun. And other times when it’s at its closest point in its orbit, then it nicely covers up the sun.

Pamela: And with the sun’s changing distance as well, all of this affects how long eclipses can potentially last. This year it just happened to be that the sun was about as far as it gets from the earth during the eclipse… not entirely as far as it gets but about as far as it gets from the earth… which made it small.

Fraser: So, that’s the smallest sun, right…

Pamela: And the moon was about as close as it gets… not as close, but about as close… which made a big moon.

Fraser: Biggest moon…

Pamela: And so that bigger moon was able to block out the smaller sun for a longer period of time. Now other eclipses might only last a minute or less because you don’t have just the right combinations. In some cases, you actually end up with hybrid eclipses where the changes in distance for the moon actually lead the eclipse in some parts of the world to be annular and in other parts of the world it’s a total eclipse. Those are the weirdest eclipses of all.

Fraser: So, then why do only certain parts of the earth get to see the eclipse? Why doesn’t everybody get to see the eclipse?

Pamela: The problem is that you have to be in the shadow to see the eclipse. Just traveling a few miles in some cases can take you out of the area where the shadow of the moon touches the earth. You can sort of imagine this if you go out on a sunny day and hold a golf ball up. The shadow of that golf ball only touches one area on the ground. Now the closer you bring the golf ball to the planet, basically because human beings are short, you don’t really within your height see that big a difference in the size of the shadow. But if you took that golf ball up to the top of the building and used the golf ball and a really long stick to cast a shadow onto the street below, you’d see that the shadow was much smaller. The further you move that golf ball from the surface of the planet, the smaller the shadow appears. You have to be exactly in the shadow to get the sun blocked out to your eyes. It’s like holding up your hand to block the sun except you’re holding up the moon, which is way cooler than holding up your hand to block the sun.

Fraser: Right, so you can kind of imagine as the moon passes… is orbiting around the earth… and things are working out perfectly, that the moon’s shadow kind of shows up on one side of the earth and then zips across the planet and then is off into space again. Because the moon is always casting a shadow from the sun, right? It’s just when that shadow happens to cross the earth that some lucky people on Earth will happen to see the eclipse.

Pamela: Exactly. One of the really weird things is the combination of the earth’s rotation and the moon orbiting around the planet and the earth and moon together orbiting around the sun cause all sorts of motions of that shadow. So if you pull up maps of eclipse paths, you’ll see that there are series of different identical-looking paths… they may cross slightly different parts of the planet, repeating as they go… but there are many, many different types of shadow paths across the planet. It’s one amazing way to get a real sense of all the motions that are going on. We’ve all moved flashlights around to cause shadows to dance on the wall, well all the different motions of the planet, the moon, and the sun and the earth and moon around it cause the moon’s shadow to dance around on the planet Earth.

Fraser: Right. So then let’s reverse the scenario, and let’s talk about a lunar eclipse because that… everybody gets to see. So this, I guess, is obviously the opposite. The moon has traveled completely around the earth, and now it’s going through the earth’s shadow.

Pamela: And we consistently get two lunar eclipses about every six months. Sometimes we can get lucky and we’ll actually have two eclipses one lunar month apart, which is kinda cool, where you can get three… and if everything lines up absolutely perfectly… and I don’t remember the frequency of this… even four eclipses of some sort or another in one year.

Fraser: But they are bunched up, I know, in one month increments. So you get two eclipses, and then six months of waiting, and then two eclipses.

Pamela: And these are again occurring when the moon crosses that ecliptic line, but now it’s happening during full moon. The reason we get a full moon every month is because the moon’s orbit keeps it out of the earth’s shadow most of the time and it’s able to reflect back at us an entire face-worth of sunlight. Now, if the moon happens to get in the way of the earth’s shadow, we see it glow only from light that’s refracted around by our atmosphere. It glows this bloody-red color, in some cases, of refracted light.

Fraser: And the lunar eclipse is like in all of astronomy… it’s one of the things that everybody can do. About half the lunar eclipses will be visible from any place on earth. So you have a 50-50 shot of being able to see the lunar eclipse… mostly. And it’s gorgeous… it’s amazing. If you have a clear sky, you get chomps taken out of the moon and then at the moment of total eclipse, it turns this blood-red color. It can turn different colors of red or brown, depending on what’s in the atmosphere. It’s just amazing. So if you… once again… lifeless, right? See Saturn… see a meteor shower… see a lunar eclipse. If you don’t make a holiday out of getting outside and gathering your friends together and watching the eclipse… and in many cases you don’t have to do anything… you just look out your window and you can see a lunar eclipse.

Pamela: Some of them are more noticeable than others. The earth’s shadow has two different parts to it… the umbra and the penumbra. This is the effect of… does the edge of the sunlight from the sun pass on one side or the other of the planet Earth. Each point on the sun is giving off light in all different directions, so if you draw complex ray diagrams… we’ll put these on our website… you end up with the shadow that has the darkest part of the shadow and then it has a lighter part of the shadow. There’s the occasional unfortunate eclipse where you see absolutely nothing because the sun and the moon and the earth line up so that the moon only goes through the lesser part of these two shadows. But when you get lucky, it lingers for a long time in the darkest part of the shadow. You never know what color the moon’s going to be. What color you see depends on when was the last time a volcano went off… what is the current level of pollution in the atmosphere… which is a bit depressing but it does affect what you see with lunar eclipses. They are truly fabulous and I have to admit I’m still bitter because the very first baseball game I was ever taken to see was during a lunar eclipse. And where we were sitting in the stands…. the eclipse was directly behind us and I couldn’t see it. Then, the Red Sox had the audacity to lose the game. So I saw a losing game and missed my lunar eclipse. It was very sad.

Fraser: Ohhhh… yeah… so thanks to pollution for making pretty eclipses… that’s something that pollution’s good for. So, as we said, those are happening with the moon passing into the earth’s shadow… opposite of the solar eclipse. Now are there any other kinds of eclipses that we get just even here in our solar system? We can sometimes see shadows passing on the face of Jupiter, right?

Pamela: Right, so here what we’re seeing is transits of the moons of Jupiter cross the face. You can see the shadows touching the face of Jupiter, and what’s neat is often the alignments allow you to see the well-lit-up planet off a little bit lagging or above the shadow itself so you can see both worlds and then the little dark dot on the surface of Jupiter.

Fraser: So, if you were on the surface of Jupiter, what would you see?

Pamela: If you were on the surface of Jupiter you’d be seeing a solar eclipse at that moment if you were under that little dot floating in the gaseous atmosphere of Jupiter.

Fraser: But I guess it wouldn’t really be an eclipse because it would just be the tiny moon passing in front of the sun, but it wouldn’t be blocking the same way that our moon does.

Pamela: Well, as long as you’re within the shadow… it is. That’s the amazing thing. Even for worlds like Jupiter, what matters is how close is that little moon…

Fraser: And how small is the sun… right….

Pamela: If that little moon is close enough to you and the sun is so far away once you get to Jupiter that it’s much smaller in the sky, you can still get that full solar eclipse.

Fraser: So when we see that little shadow, that means that from that point on Jupiter’s surface… obviously it doesn’t have a surface, just cloud tops… the light of the sun is being blocked by that moon, and so they would appear similar in the sky. That’s amazing. But from here on Earth, watching that happen, we’re seeing a transit.

Pamela: Yes. Yes. And then we also get to see transits of Mercury and Venus in front of the sun, and this is actually one of the first ways that we were able to start mapping our solar system rather accurately.

Fraser: Here comes the science!

Pamela: Yeah. So people were able to predict… so, we suspect that Venus will pass in front of the sun on these given days… and Venus passing in front of the sun doesn’t happen very often because you have to get the earth, which is on a tilted orbit, and Venus, which is on a tilted orbit, to have their tilts… have their points in their orbits such that where they are exactly puts them on a line with the sun at the same time. And this tends to happen in groups of two, where you’ll get a pair roughly every 21 ½ or 105 ½ years. We’re in fact in the middle of a pair right now where there’s a transit of Venus in 2004 and will be another one in 2012.

Fraser: 2012! Isn’t that the end of the universe?

Pamela: We’ll still be around to report on it. Back in the 1700s, the pair in 1761 and 1769, they realized that we could use these transits to figure out how far away the sun is because if you have people on north-south lines on the planet Earth go measure where on the disk of the sun they see Venus crossing… depending on where you are on that north-south line you’ll have an angle that puts Venus a little higher up on the disk, a little lower down on the disk, and we know the size of the earth… so we can get one side of the triangle. We can then by looking at that change at where it appears on the sun, that’s an angle. We know the ratio of the distance from here to Venus and from Venus to the sun from Kepler’s equations. Putting all these pieces together, we can finally calculate what is one astronomical unit. So they were finally able to start to make that measurement in 1761 and 1769. And then they were able to get good solid measurements in the 1870s and 1880s with the next pair of Venus transits.

Fraser: Right, and there are some amazing stories… the quest for the earlier explorers to get to the places they had to be to view the transits… the trials and tribulations… attacks by pirates… a horrible nightmare just to get that really precise astronomical data. They’re some of the most important measurements that have ever been made in astronomy and in science. The stories are amazing, so I highly recommend…. look them up… there’s some great books on the history of trying to get those measurements.

Pamela: Then join us somewhere on the planet in 2012 to replicate the measurements. If you happen to be in the Boston area, Harvard University has in their Science Center, on display, at least last I looked, some of the telescopes that were used in those early measurements to try to track Venus crossing the sun.

Fraser: So, before we move away from eclipses in the solar system, because we’re going to talk about some other ones, let’s talk a bit about safety and observation and how to do that. So, let’s say that you want to see a solar eclipse. What are the ways to see one safely?

Pamela: So my favorite way is you can actually take a kitchen colander, a strainer like you use for spaghetti, hold it up and cast the sun’s light through the little holes. This acts like a pinhole projector and you can get hundreds of little images cast on the ground of the eclipsing sun.

Fraser: Yeah, don’t look up through it, just let the shadows fall on the ground.

Pamela: And watch the shadows. Now there’s of course all sorts of different filters you can get… Thousand Oaks is just one of the many companies that makes filters. They’re one of the ones here in the United States. Call up the people at Oceanside Photo and Telescope… tell them, “I’m going to go see a solar eclipse—what should I get?” That’s what I do. They’ll help you pick out the right equipment for your binoculars, for your eyes, for your telescope. Any old solar filter that you can use on any old normal day will allow you to watch up until the moment of totality safely. Then during totality, they’re a little bit too overzealous in blocking the sun’s light, and so you’ll need something slightly different.

Fraser: You can actually see it with your unaided eye during totality, can’t you?

Pamela: Yeah… it’s during the really long eclipses that you have to start worrying about the ultra-violet. But, kitchen colander is always the best way to go. For transits of Venus and Mercury, where Mercury transits are a lot more common, my favorite thing to do is to get one of the little Astroscans, they look like little cherry tomato telescopes, and use them to project the image up on a wall. Then you can get a nice magnified view.

Fraser: Yeah… what I did was I had a little spotting scope, and also I’ve done this with binoculars, and you just hold the binoculars and you point them up towards the sun… not looking through them, so you’re letting the light of the sun come out the other end of the binoculars and land on a piece of paper or on the ground, and you’ll see the sun. Now the problem is that if you do it too long, you’ll wreck your binoculars.

Pamela: Right.

Fraser: And that’s what I did. You’ll heat up the interior of the binoculars and you’ll ruin the optics. It’s good to just catch a glimpse of it but not to study it for any length of time.

Pamela: Right. And the Astroscan… the one that I used… did survive the ordeal, but I will admit that the light was going through a very dirty office window, into my Astroscan, and then getting projected onto my office wall because I was really lazy in graduate school during the last transit.

Fraser: But the great thing about using binoculars or a really… this is what you could use a department store telescope for… something you don’t mind destroying… is that because then you could kind of blow up the image. Then you can have a really big image, and you can really see the eclipse or you can see the sunspots… you can see all that kind of stuff on it. So that’s really great. And with a lunar eclipse… completely safe. If your eyes don’t hurt looking at the regular full moon, you’re fine to look at a lunar eclipse. Now I want to talk a bit about science because we’re now finding eclipses, transits, on other solar systems. It’s a way that astronomers are looking for planets.

Pamela: And it’s also a way that we’re better understanding the planetary bodies in our own system. So, with extrasolar planets, we go out… and this is actually one of the things the Herschel Space Telescope is dedicated to doing… is you watch a star, a nearby star, any old star, but with high, high precision. So, this is generally only done with the nearest and brightest stars that you can get the best… we call it signal to noise… the highest accuracy measurements. You can do this with a four-inch telescope if you have the right CCD, the right digital camera attached to your telescope. You look for slight, slight variations in the amount of light from the star, and you see if these slight variations repeat themselves. You should be able to make a plot where you see a nice large flat period of time where the star is very, very bright, and then depending on the size of the planet it will either drop straight down to a little bit fainter or gradually drop down to a little bit fainter… stay there for a little while while the planet goes in front of the star… and then reverse itself and come back out and go back up to being nice and bright again. That very slight variation allows you to start to understand what’s the period of the planet’s orbit? How long does it take for this cycle to repeat over and over? It allows us to understand how big is this star… how long did it take it to get from being completely not in front of the star to being completely in front of the star? A big planet will gradually cause the light to get fainter whereas a little tiny planet will cause the star’s light to very quickly get a little bit fainter. By looking at all these different things we can understand the size of the planet, the orbital inclination… does the planet cut across the top edge of the star or cut across the middle of the front of the star? It’s a great way to learn all the little details that we can’t measure directly about those stars and planets.

Fraser: Yeah, but it’s just like a fluke thing… it’s like there maybe tens of thousands of stars in our nearby neighborhood, but only a fraction of those are the ones that have the star and the planet in the perfect angle so that we can see it here on earth.

Pamela: And the other side to that is there’s only a fraction of the stars out in the universe that are lined up so that when they look at us, they see us and Jupiter and the other planets cutting in front of the sun. Now what’s amazing about this technique is as the planet passes in front of the star, that star’s light goes through the planet’s atmosphere. We can measure what are the chemicals in that planet’s atmosphere. We’ve already found water on planets around other stars. Now this means that aliens out along that line that allows the earth to pass directly in front of the sun, if they can see us and they can see our atmosphere, they can see all the pollution that we’ve put into the atmosphere, and they’ll know we’re here.

Fraser: And the oxygen, right?

Pamela: Well, and the oxygen… but oxygen just means plants.

Fraser: And so this is how we’re just right on the edge of being able to discover life on other planets because we now have the technology to detect, with Kepler, just about Earth-size worlds going around other stars. With the Terrestrial Planet Finder, after we nag NASA to bring the mission back, it will be able to sense the atmospheres of Earth-size planets going around other stars using this method. It will watch how the atmosphere of the planet changes the starlight coming from the star and be able to detect whether or not there’s pollution in the atmosphere and make a pretty good guess about whether there’s life on that planet. That’s about the most important question we could possibly ask…. which is why the Terrestrial Planet Finder should be resurrected.

Pamela: And my personal bet… and I’m willing to take this up with Seth Shostak at an “I’ll pay for his dinner if I’m right” type of level… or actually he should pay for my dinner if I’m right and I’ll pay for his if he finds aliens his way first… I think we’re going to find intelligent life on other worlds first by finding how they’ve destroyed their atmosphere, and only later via radio communications. I think this is a much more straightforward way to say ah, there’s industrial something going on over there.

Fraser: Yeah, I totally agree. If we get more and more powerful telescopes, we’ll be able to study more and more stars and their planets, and be able to turn up larger and larger area… or volume of space… more and more of these planets and be able to detect their atmospheres and eventually we’ll be turning up civilizations one after the other…. I hope.

Pamela: I hope, too. But for now, if you want to get involved, look up when a solar eclipse is… look up when a lunar eclipse is… find a transit… watch a moon pass in front of Jupiter. And if you want to do science, the one science that’s still open for anyone is watching an asteroid passing in front of a star.

Fraser: Yeah, this is one thing… maybe we should just take a second because we really didn’t talk about occultations too much… which is the method that we just talked about for finding life… this is how astronomers found the atmosphere on Pluto, right?

Pamela: Right. So, any time that a planet passes in front of a star, any time an asteroid passes in front of a distant star, that star’s light gets blocked out and before it gets blocked out if there’s an atmosphere, it passes through the atmosphere. And one neat way to map the shape of all those potato-shaped asteroids out there, is to get a bunch of people on the planet earth lined up perpendicular to the passage of that object across the sky and say ok, I see the star blocked out for this long… I see the star blocked out for this long…. and because each of them will have a different angle between the asteroid and the star, they’ll see the star cut across a different part of the asteroid, and whoever sees the star’s light blocked out the longest, just looked across the fat part of the asteroid. The person who sees it not blocked out for very long at all has looked at the skinny part of the asteroid. So we can build three-dimensional maps of asteroids over time as they rotate and pass in front of multiple stars, and that’s just really cool.

Fraser: There’s always a need for amateur astronomers to get involved in doing those timings.

Pamela: And if you’re into Ham radio you can get involved with people all over the world doing the same thing, watching the star’s light blink out and blink back on.

Fraser: Perfect. Alright, well thanks a lot, Pamela. We’ll talk to you on another show.

Pamela: Sounds great, Fraser. I’ll talk to you later.

Fraser: Bye.

This transcript is not an exact match to the audio file. It has been edited for clarity.

Episode 131: Submillimeter Astronomy

admin — Wed, 01 Apr 2009 17:41:22 +0000

Artist's impression of the ALMA array

Last week we examined the largest wavelength in the electromagnetic spectrum: radio. This week we get a little smaller… but not too small! And look at the next step in the spectrum, the submillimeter. Astronomers have only recently began exploiting this tiny slice of the spectrum, but the payoff has been huge.

Ep. 131: Submillimeter Astronomy

Jump to Transcript or Download (coming soon!)

Shownotes

Submillimeter Astronomy — U of Arizona

The spectrum of light — UIUC

Submillimeter Telescopes

Naturally occuring microwave lasers, or masers
Mira Variable Stars– Internet Encyclopedia of Science
Observing comets in submillimeter
Kuiper Belt Objects in submillimeter
Quaoar
Paper: Problems of Star Formation Theory and Prospects of Submillimeter Observations
Star Formation History of Submillimeter Galaxies -- BLAST
GRB afterglow observations in submillimeter

Transcript: Submillimeter Astronomy

Download the transcript

Fraser Cain: We’re back in the swing of things. Last week we examined the largest wavelength in the electromagnetic spectrum – radio. This week we get a little smaller, but not too small and look at the next step in the spectrum – submillimeter.

Astronomers have only recently begun exploring this tiny slice of the spectrum. The path has already been huge. Where can we find the submillimeter wavelength?

Dr. Pamela Gay: When we talk about submillimeter astronomy we’re typically talking wavelengths that have a peak to peak distance of about point 3 millimeters out to about a few millimeters which aren’t necessarily submillimeter.

We’re confined in what we can look at by our atmosphere. If you ask me to find the geography of where we can observe submillimeter I’d say go up. Go up really high.

Fraser: Radio goes from meters down to a few centimeters to I guess a few millimeters. It’s such a huge difference. While some millimeter, a millimeter or two and you’re out again, right?

Pamela: The thing to remember is there is a ton of differences in what lines you can see at each of these little fine gradiations. Just going from a few hundred nanometers say 300 nanometers out to 800 nanometers, you’ve now grabbed most of the light looked at by a typical visual telescope. That’s a very small region of the electromagnetic spectrum.

Fraser: That’s true, who am I to complain about a few millimeters when the visible spectrum fits within nanometers?

Pamela: Right so it is all about how much you break up the light. With submillimeter we still are able to break it up a lot within those small regions that we can measure on a meter stick basically.

Fraser: The other side of the submillimeter is infrared? So we’re in-between infrared and radio?

Pamela: Yes.

Fraser: Has it always been sort of considered its own separate wavelength? I guess we’ve got microwave too, right?

Pamela: Microwave actually gets lumped in with the submillimeter a lot. What makes submillimeter what it is we have this tendency to break things up according to how hard they are to look at in some ways.

So, we have visible light [Laughter] which we can see with our eyeballs. Infrared light which we can still detect with detectors very similar to the ones that we use for visual astronomy and snakes see in the infrared so that’s not all that different.

Fraser: I get it though it’s like we can’t see it with our eyes therefore it is a different wavelength. It’s another region and the same with ultraviolet I guess?

Pamela: Right and then when we get to radio. Radio is nice big blocky wavelengths that you can use nice big chicken grate detectors if you want to detect.

Submillimeter, in the last show we mentioned that you have to have a surface perfect to within one part in 20 of the wavelength you’re looking at. With submillimeter dishes we need to be able to build in this case using technology similar to a radio dish.

We need to be able to build it so that the surface of this radio dish has no imperfections that are even the size of someone leaving behind a dollop of metal the size of a strand of hair.

Fraser: Then if I looked at a submillimeter telescope what would it look like?

Pamela: It looks a lot like an overly massive radio dish. Because the surface has to be so perfect, they’re a lot more fortified than your typical radio dish. They are solid surfaces unlike radio dishes that often have big holes in them.

If you look at Arecibo it has big holes in it. They are perfectly solid surfaces and they are really well fortified so that as you tilt them there is absolutely no movement in the surface of the dish. They’re rugged, they’re robust…

Fraser: So it’s really like a cross between a mirror and a radio dish. It has the perfection of a visible light mirror but it sort of functions in the same way as a radio dish?

Pamela: Exactly. Radio dishes you can pretty much stick anywhere and use any time of day. It is convenient that way. Radio light quite happily passes straight through the atmosphere, straight down to the surface of the earth, goes through clouds and doesn’t care if there is daylight. Radio is easy to detect.

Anyone listening if they wanted to could go to Radio Shack and get all of the components to make a radio dish and go outside and detect Jupiter or check the center of the galaxy.

With submillimeter though, we have to start worrying about the amount of water in the atmosphere because water molecules in the atmosphere are capable of blocking submillimeter light.

You have to get yourself someplace exceedingly dry and you also want to be at a high altitude so you can get as much of the atmosphere as possible below you.

Fraser: Once again it is very much the same techniques for putting together a visible light telescope top of a mountain, in the middle of Antarctica, or out in space.

Pamela: It is even worse for submillimeter. We don’t actually launch them into space because there are things that we can’t observe from Earth at all but we do stick them on tops of mountains.

With visible light, the atmosphere just smears the light out which is inconvenient but we can at least look through the atmosphere. With submillimeter light the atmosphere actually flat out blocks the light.

We measure the amount of water in the atmosphere by how many millimeters of ocean it would make if you decided to take the entire atmosphere all at once and turn it into an ocean.

If you have enough water in the atmosphere above the telescope to just make a sea 4 millimeters deep that can block out more than 50 percent of your light, more than 80 percent of your light at some wavelengths.

We just can’t get the light through the atmosphere unlike visible where it is just blurred out and you’re sad.

Fraser: So just to use sort of an analogy when you talk about the radio wave. You point your detector at some spot in the sky and you either get radio or you don’t have radio. It is very binary.

Pamela: Right.

Fraser: Does submillimeter work the same way?

Pamela: Submillimeter works the exact same way. If you have a single dish you look at a single point.

Fraser: Right, if it is working the same way you’re either getting your signal blocked by or decreased by water or not?

Pamela: Yeah and that gets kind of frustrating. So, the best submillimeter telescopes in the world are in neat places at the tops of cool mountains. There is one in Spain called IRAM that’s actually the middle of a ski resort.

Astronomers will be sitting there observing away and watching people ski past their telescope. That’s just amusing.

Fraser: That would be a hard temptation to resist I think. [Laughter]

Pamela: Yes it would. There is also a really good one out in Hawaii that is owned by Caltech and a consortium of universities.

We’re also in the process of building as a community a brand new one in the Atacoma desert which is where the Very Large Telescope is located.

Fraser: Let’s compare and contrast then the functions of a submillimeter telescope. We talked a bit about how the radio works. It scans the sky in a certain region and measures the strength of a radio signal but it doesn’t create an image.

It is only by successive sweeps across the sky you actually build up an image. How does the submillimeter detectors work compared to it?

Pamela: The submillimeter generally works the exact same way. When we combine multiple dishes then we can start to build an image all at the same time. But a single dish working with a single receiver at the same time, you get basically one pixel on the sky.

There are detectors that are working to figure out to essentially multiple pixels. In general it is basically one dish one pixel. It’s when you combine multiple dishes and use interferometry that you’re able to build up simultaneously much more complicated pictures.

Fraser: That’s kind of strange because infrared telescopes work the same way that visible light telescopes work. You point it at the sky and it can make an image.

Pamela: It is a matter of we’re still trying to figure out how do you build the multi-pixel detector for a radio dish. We’re just not quite there yet.

Fraser: I guess the most important question is why on Earth are we developing telescopes to analyze this teeny tiny part of the spectrum?

Pamela: So we can look at lasers in space!

Fraser: Space lasers.

Pamela: There are more reasons than that. You can use submillimeter to look at a bunch of molecules but one of the coolest things is there are naturally occurring microwave basically lasers. Lasers is light processes, microwave is a type of light.

There are places in the universe where for a variety of different thermodynamic complicated reasons the electrons and the atoms end up in an energetically inverted state. You end up with a bunch of electrons that for a variety of reasons ended up at a higher energy level than they should be at if the system is in equilibrium.

As these electrons cascade down to lower energy levels they met a laser beam and that’s just kind of cool. We can use this to study a variety of different environments.

There is a classic pulsating variable star. They are semi regular we’re not entirely sure how they work called Mira stars. There was a lot of press about Mira itself a few years ago because as it is shooting through space it is leaving behind bits of its atmosphere and it is basically a stellar comet in some ways.

In the atmospheres of these stars we actually get these maser processes taking place. We also find this in clouds of interstellar material. We can also use submillimeter to look at a variety of different molecules that we don’t see any other way. We’re just studying a different part of the cold molecular universe.

Fraser: You said that it shoots off like a laser so wouldn’t we see that invisible light?

Pamela: When I say laser it refers to the thermodynamics of the system. It refers to any system in which you end up with more excited electrons than non-excited electrons and a certain type of resonance where you end up with all of these electrons jumping to lower levels and emitting light.

Fraser: Right but you’re getting a coherent beam of photons.

Pamela: It doesn’t have to necessarily be a coherent beam but you have a coherent thermodynamic process that is releasing all of these electrons. When we make these things in the laboratory we end up with a nice coherent beam.

The universe doesn’t generally shoot lasers like you see in Star Wars but it does have this coherent emission of light just like think of it at a spherical laser beam.

Fraser: Then why is the submillimeter the great tool for looking at this?

Pamela: Because the electrons and a bunch of different molecules just happen to have these transitions for the electrons occurring at energies that emit light in the microwave. The microwave is part of the submillimeter.

It just happens that for instance silicon hydrogen molecules, a molecule with one silicon atom and one hydrogen atom happen to have a transition that creates a maser situation.

It’s just a matter of some things emit things in colors we see with our eyes and some times they emit things that we see in the submillimeter. We have red lasers, green lasers and microwave lasers. That’s just one of the neat things about thermo is it allows all of these different colors to exist.

Fraser: Just to reference last week’s show so you’re saying that if I kind of did the math as the electrons stepped down in their energy levels, they’re emitting these photons, right?

Pamela: Right.

Fraser: And the amount of energy in the photon relates to the submillimeter wavelength. Is that right?

Pamela: Yes.

Fraser: If I look out into the universe in this very tight focused sort of part of the electromagnetic spectrum and I see a source that is bright in that then I can assume that this very specific process is going on thanks to the genius theorists who [Laughter] worked up the math to figure it out.

If you see light at this very specific wavelength then that means that this interesting thing is happening. So what were the interesting things that were happening? We talked about Mira which is a pretty amazing star but what exactly is going on do you think?

Pamela: It is basically a situation where you end up with the material is getting bathed in light of just the right temperature or color – we use them interchangeably – from some other source that it is bumping electrons up to a higher energy level.

Lasers and masers have this weird you can’t just bump things up one energy level you actually need to bump them up a couple. Then there is a multi-decay process. It just happens to be that the light flooding this region is of the right energy to set up the population in the wrong set of energies.

If you look at the temperature of the system, the electrons want to be at one energy level. Then you start hitting them with light and the light excites the electrons to a higher energy level. As they cascade down you can end up with this laser maser situation being given off.

Fraser: And maser that’s…

Pamela: It’s just a microwave laser basically.

Fraser: It’s like mega-laser [Laughter] microwave laser.

Pamela: There are people who are trying to change the terminology to mean molecular laser because any of these that are taking place in the microwave wavelengths are actually molecules doing the excitation.

Fraser: Right, okay so that’s one thing that the submillimeter is good for. What are some other things that you might want to look at?

Pamela: There are also different molecules in clouds of cool gas that are giving off their light in submillimeter. This is where we can start looking for formaldehyde in space just in case you want to go pickle something.

We could also look for various carbon organic molecules in space that give off their light in cool clouds of molecular gas in the submillimeter.

Fraser: The same deal here, we’re seeing electrons changing in energy levels and giving off photons in a very precise wavelength?

Pamela: In general with molecules it’s not even the electrons that are jumping up and down levels. But because you have this system of multiple atoms working or bonded together I guess is the best way to put it, they can be vibrating.

As the atoms vibrate, and they can also be rotating, so as the system rotates or vibrates in different ways it has different energies. If it changes from vibrating one way to vibrating another way like a string getting plucked a couple of different ways you can end up with different energies tied in and an electron being given off as it changes the vibrational or rotational energies.

Fraser: It is amazing that we can see that and astronomers can know what they are looking at which of these situations they’re analyzing.

Pamela: It gets to be really ugly math. When you take quantum mechanics they start off friendly and they say here is the hydrogen atom and you learn all about the transitions in the hydrogen atom.

Then they make you sad by giving you the helium atom which is more complex. As you continue to take quantum they eventually hit molecules where you cry because you’re dealing with what’s the moment of inertia of different molecules. What are the vibrational levels of different molecules?

It becomes very complicated very quickly. But we have the abilities if you chew through enough math and use enough computers we have the ability to figure out what are all of these different energy transitions.

Fraser: Wow and so is there some stuff that is maybe closer to home? It sounds like a lot of that stuff is way out in deep space. You’re looking for clouds of molecules or anything useful here in our solar system. Is there any way to observe moons, planets, anything there?

Pamela: Submillimeter is just the study of cold stuff basically. Comets are conveniently nice and cold. In looking at them in the submillimeter we’re able to trace out the nucleus of comets.

We’re also able to look out at the small cold bodies in the outer solar system where for instance and I can never pronounce this poor small body’s name correctly or however you say the small thing that begins with a ‘q’ that is near Pluto. It is something that has been imaged in the submillimeter and we can use this to measure the diameter of these different cold bodies.

Fraser: In this case we’ve got objects that are at that perfect temperature. They’re so cold that the radiation that they’re giving off is in the submillimeter. So it is the perfect gadget for finding those bodies out there. That’s pretty cool.

Pamela: So while we see them in visible, we can see them and learn a little bit more about them in the submillimeter as well. We also use the submillimeter for other things.

We can use it too to look at the envelopes of evolved stars. That’s what we’re doing when we look at Miras. We can also use it to study star formation.

Fraser: Isn’t that a hot process?

Pamela: Stars start off somewhere and they don’t start off particularly warm. Quite literally what’s cool is when you take a giant molecular cloud and let it begin to condense. You start to see the nuclei of stars forming or in this case the protostars forming as small points of heat that appear in the submillimeter.

Fraser: Oh, so because they start out as completely cold clouds and then they start to form and start to heat up and then you’ll see them in the submillimeter. Then they’ll pass that and become hot enough where you’ll see it in infrared and eventually visible light when the star itself appears.

Pamela: Right and we’re also using submillimeter to look at [Laughter] this is going to sound strange but at gamma ray burst afterglows. We’re able to provide some constraints on what is the physics involved in the system around the gamma ray burst as we look at it in a whole variety of wavelengths.

We look at them in the gamma rays, in the x-rays, in the optical, in the infrared and in the submillimeter as well. It’s only by observing objects across all of the different colors of light that we can get a full physical understanding of what is going on in this system.

Fraser: Just a last thing, I want to talk a bit about the hardware, the technology out there that does this. What are some of the observatories?

If I’m reading the news or Universe Today for example, and I [Laughter] see an instrument or observatory mentioned what would sort of let me know that the trigger that okay that’s a submillimeter observatory?

Pamela: It is kind of hard to know when you’re reading through a newspaper if you’re dealing with a telescope that can do infrared in submillimeter or submillimeter in radio and what wavelength it’s working at unless you read the details and happen to know which molecules give off light at which wavelength.

Fraser: And if they mention the wavelengths in the article?

Pamela: Right.

Fraser: We were studying at the point zero five millimeter wavelength…. [Laughter] like “aha! That’s submillimeter!”

Pamela: Some of the big facilities for instance IRAM in Spain which I already mentioned, the Caltech submillimeter observatory. They are working on building the brand new Atacoma submillimeter telescope down in the Atacoma desert. Herschel when it launches is going to be able to work in the submillimeter. We have the Antarctic submillimeter telescope.

There are a variety of them scattered around the world. There are a variety of radio dishes that when push comes to shove can push themselves just barely into the submillimeter if they need to. The real neat experiment that is coming up is the Atacoma submillimeter telescope that is getting built slowly but surely and its first dish was actually delivered in December of 2008. It’s actually starting to show up out there.

Fraser: It is interesting to see sort of as now astronomers are using more and more of the spectrum slices of it are getting a lot of attention. They’re getting dedicated instruments, dedicated observatories. They’re getting some serious attention which is great. There are some amazing new observatories.

I don’t know if this has somehow turned into a tour of the electromagnetic spectrum. [Laughter] There is some really interesting stuff for x-rays and gamma rays. I think we could kind of try and mention that in the past there was sort of like telescopes that looked at light.

Then they figured out there are radio telescopes. But now they’re really slicing it up and saying let’s just focus on this tiny little wavelength and let’s be able to draw the best science we can. Like you said make our force our big radio telescope to drive down into the submillimeter.

Let’s make a really good submillimeter telescope that is finely tuned like a mirror, like a visible light telescope but can receive in that spectrum. It is just great the one in the Atacoma is going to be amazing and Herschel as well.

Pamela: Herschel is going to be amazing.

Fraser: On many fronts.

Pamela: But it is dedicated to one fairly narrow science. What’s amazing about the Atacoma Large Millimeter Array which is generally called ALMA is this is a major international project with the UK, the European Southern Observatory, the Japanese National Astronomical Observatory and the U.S. National Radio Astronomical Observatories all working together building in fact three different types of receivers and transporting them out to a desert plateau.

You really don’t get more difficult than getting many ton instruments to the middle of nowhere. This truly defines the middle of nowhere.

Fraser: Right and if it was a big radio telescope you could tweak it a little. These have to be kept very, very perfect, moved carefully.

Pamela: It is a huge number of dishes and it is going to be years putting it all together.

Fraser: When it is done…

Pamela: Yeah, once they get these 50 to 60 different antennas all installed out on the plateau the universe is ours to observe.

Fraser: No kidding, it’s going to be a monster. That’s great. Well, thanks a lot Pamela we’ll talk to you on the next show.

Ep. 129: Interferometry

admin — Tue, 17 Mar 2009 17:44:41 +0000

ESO Very Large Array. Image credit: ESO

When it comes to telescopes, bigger is better. But bigger is more expensive. Way more expensive. To keep the costs reasonable while improving the sensitivity of their instruments, astronomers use an amazing technique called interferometry. Instead of building a single huge telescope, you can merge the light from several telescopes to act like a much larger telescope. It’s a technique that has already revolutionized Earth-based observing – but just wait until it gets into space…

Ep. 129: Interferometry

Jump to Transcript or Download (coming soon!)

Shownotes

Interferometry– JPL
Episode #85: Detectors
Observable Universe — Wise Geek
Kuiper Belt Objects — Nine Planets
Episode #41: Rise of the Super Telescopes
Segmented mirrors – Wiki
The Giant Magellan Telescope
Waves and Particles -- Colorado University
Constructive Interference – Windows to the Universe
Collimated light
The Very Large Telescope Interferometer
Video: First Fringes of the VLTI
PowerPoint presentation on Radio Interferometry by Jeffrey Kenney at Yale
Terrestrial Planet Finder
Atomic clocks – How Stuff Works

Transcript: Interferometry

Download the transcript

Fraser Cane: When it comes to telescopes bigger is better. But bigger is a lot more expensive, a lot more expensive. To keep the costs reasonable while improving the sensitivity of their instruments, astronomers use an amazing technique called interferometry. Instead of building a single huge telescope you can merge the light from several telescopes to act like a much larger telescope. It is a technique that has already revolutionized Earth-based observing. Just wait until it gets into space.

Okay Pamela, when I wrote that intro I kind of wanted to like not scare people. [Laughter] Interferometry, don’t let the title scare you. It is one of the coolest technologies that has been developed in kind of modern astronomy. I think it has led to part of this golden age that we’ve been talking about in astronomy.

As I said wait until we get to the space missions. Let’s talk about first the dilemma of building a gigantic telescope. What are kind of the limits of telescope technology right now?

Dr. Pamela Gay: You build giant telescopes for two basic reasons. One reason is you can get just so much more light. The more light you collect with your mirror, with your dish, with whatever light collecting surface you’re using, the fainter an object you can look at because you are getting more photons from that faint distant object or even that faint nearby object.

Cameras require a certain number of photons before they can go “yes I believe there is light here”. At the same time you also want to have high resolution images. You want to be able to make out the separation between close stars. You want to make out the details in galaxies. You want to be able to see all the bumps and wiggles in patterns of star formation regions’ blobs of gas and dust.

Both resolution and light collecting area depend in different ways on the radius of the telescope. For the light collecting area it is simply how much light are you gathering; it is what is the area of your detector. With resolution, all it cares about is how far apart the two edges of your detector are. How many wave lengths across is your detector? What’s cool is when you’re figuring up the resolution of the detector the detector doesn’t actually care if the middle part is there or not.

You can get the exact same resolution with a giant donut shaped mirror that is only maybe 4 inches wide in the donut part and 10 meters across from outer edge to outer edge as you’d get from having that entire solid mirror. That donut, amulus of mirror is going to weigh a lot less, is going to cost a lot less to produce and it doesn’t even care if it is a contiguous donut.

I could actually instead of having this ring of mirror, this ring of collecting surface; I could instead have maybe 4 different dishes at the north, south, east and west equivalence of that giant donut. As I break down to smaller and smaller areas, it gets cheaper and cheaper to build.

Fraser: Give me an example then of something where you want a lot of photons.

Pamela: A lot of photons are I’m trying to observe a galaxy at the very edge of the observable universe. This is a very faint object, very far away. We’re just not getting a lot of light. Here we want to collect more and more light or I’m trying to observe faint quiper belt object.

One of these blobs of frozen ice that is out around the orbit of Pluto, a very small very not necessarily reflective object I need to collect as much light as possible to try and see if it is there at all.

Fraser: Okay so it is like every photon is precious and if you can’t collect the photons you don’t even know that the thing exists.

Pamela: Exactly.

Fraser: What is the situation where high resolution is key?

Pamela: I’m looking at the star Betelgeuse. It is near enough by that if I use a high enough resolution detector I can actually make out the disc of the star. I can look at it and go, oh star spot.

I can look at it and I can measure how big it is. I know the distance to Betelgeuse and this allows me to actually calculate the physical size of the star.

Fraser: Then in this example Betelgeuse is giving off plenty of photons. No more photons are needed but the key is that you need to be able to have your resolution to be able to see the disc of the star, to be able to see sunspots.

As in the case as you mentioned before some kind of binary object where you’re trying to sense the separation between two stars. So then the traditional way is to build a big telescope. We talked this a bit in our rise of the super telescopes episode. Prices rise exponentially as the size of the telescope increases, right?

Pamela: Not only that but just the mechanical skill needed to get bigger than we are currently able to build, we’re just not there yet. Some of the largest telescopes in the world right now have 8 meter to 10 meter mirrors. These giant mirrors are right at our limit to spin cast them, to transport them to mount them so that gravity doesn’t deform them.

As we get to bigger and bigger mirrors we’re going to have to develop new technologies in using segmented mirrors; building new mount systems and being able to handle all of this weight without gravity deforming the systems. We’re reaching a point where engineering problems just as much as cost problems make it prohibitive to build these giant telescopes.

Fraser: There are plans in the works for 30 meter telescopes which will have all of these segments kind of lashed to make one great big telescope. The cost on that telescope like the Magellan is going to be enormous. You’re really kind of reaching the feasible limits but that is like a big light pocket right?

A big telescope like the Magellan is going to give you a 30 meter telescope to collect a whole lot of photons. That’s going to be seeing these faint quiper belt objects in these galaxies at the edge of the observable universe. Then here comes the solution, interferometry. So, what is interferometry?

Pamela: Interferometry basically goes light is particles that also act like waves. If you combine waves in a meaningful way, making sure that the peaks of one wave line up with the peaks of another wave they interfere in a way that we call constructive interference.

In this way you can collect a bunch of waves, line them up and it’s just as though you’ve collected all the waves at the exact same time. This sounds like a relatively simple idea. I go out, I collect my light, I somehow maybe using fiber optics, maybe using mirrors recombine this light and everything works.

Fraser: That’s kind of funny because it kind of sounds like gibberish to me. Let me just kind of parse this because I barely am wrapping my head around it.

You’re getting the light from one location and you’re getting the light from another location. You’re putting that light together? Is that right?

Pamela: Yeah.

Fraser: Then you are sort of seeing how the waves, as you said they construct or destruct one of those. I remember in physics we had the situation where you have waterways and you have two waves running into each other and if the two peaks come together then you get a double wave.

If the peak and the trough come together then you get flat water. I know light works the same way so I’m having trouble understanding how you can take light from two different positions and merge it together.

Pamela: This is where it gets very tricky. We’re really good as astronomers at doing this with radio waves. We know how to tune our receivers. We know how to detect the peaks and the troughs and the wave patterns very well.

With detectors like the Very Large Array, they know okay so the object is over in that part of the sky so I tilt all my dishes towards that object. I know exactly where on the surface of the Earth all of my telescopes are located very precisely.

I can use geometry to figure out this dish at this angle is this much closer to the object being observed; this other one is this much farther. They use different travel paths, different computers to take the signal from each of these telescopes and combine it with delays that allow the radio light that is received by each of these dishes in a slightly different location to be mixed. So it is as though the light was hitting each of the dishes that have left the object at the same time, hitting the telescope at the same time using artificial delays.

These artificial delays allow the peaks to stay lined up with the peaks and the valleys to stay lined up with the valleys and to get constructive interference and to artificially create a giant telescope aperture that gives you these extremely high resolution images.

Fraser: Are the two telescopes recording the same photon? You know, light could be a wave and you’re saying you’re trying to line up them together. Is that what’s going on that the photon is spreading out over a large area and so it is hitting the two telescopes and that is how you can kind of get at your better resolution? Is that what’s going on?

Pamela: We talk about light being what is called columate. This means the light that is coming off of an extremely distant star in an extremely distant galaxy, the light that is coming off of the source has the peaks and valleys within it lined up so that you get a coherent light beam.

So, they’re not detecting the exact same photon in two different dishes, but we’re detecting photons that are acting together in a columated fashion such that if I don’t have the moments at which I’m recording my data artificially lined up then I’m looking at a packet of photons that were released at one time, a packet that was released at another time. The peaks and valleys from these two different times might not be lined up.

I can get like you pointed out a trough lining up with a peak which gives me no light at all. It’s because it is coherent light coming off of the source that acts in what we call a columated fashion that we have all of these peaks and valleys, peaks and valleys lined up as the light travels through space.

Fraser: The timing is the key.

Pamela: Yeah, we have to maintain that lined up and we do it by shifting the signal from the telescopes until it is as though all the light was hitting the telescope at the same time.

Fraser: I guess I find that kind of, I’m sorry, I find that a little confusing as to why you say it is a columated light. I guess I find that part just a little confusing. A lot confusing which is why that is important?

I guess I understand if you don’t have the timing then it’s kind of like you have a telescope over here and a telescope over there. This one is taking images, that one is taking images that you could merge the images together – and this is what a lot of amateur astronomers do, right – they run video of some object that they’re trying to collect.

They take frame after frame and then they use image stacking software to kind of stack the images together to get a longer exposure but also to be able to remove the bad frames. That’s not what this is about.

Pamela: No.

Fraser: This is not taking two separate telescopes and merging the light together until you’ve got a thousand photons on the right telescope and a thousand photons on the left telescope. You put them together and you get a little bit better image. This is different. I think that is important.

Pamela: This is getting at increasing the resolution of the image.

Fraser: Right and as you said it is key to the fact that the photons were emitted at the same time and are connected is the way to put it?

Pamela: Connected is not quite the way to put it. First of all you do have the object varying. In theory I could take light from two different telescope dishes and combine it so that I have the peak of one wave combined with the peak of the next wave. In theory I’ll still get everything interfering in a way that allows me to get nice good sharp image.

But I’m observing the object in two different periods of time so I want to get a snapshot of the object in the now such that all of this light that I’m receiving traces the same behavior in the object.

There is also a matter of the resolution is directly related to how wide is my aperture. How wide is my reflecting surface? In this case that width is a reflection of how many wavelengths fits across the telescope.

If I have a ten meter optical telescope, it is going to have absolutely amazing resolution because optical light is extremely small. It is hundreds of nanometers. These are smaller than anything that you can believe or imagine because it is smaller than what you can see with your eye.

We have something that is several hundred nanometers peak to peak whereas with radio wavelengths I can have a ten meter dish and very, very poor resolution. An entire galaxy is nothing more than one pixel of well, this is light, this is dark. No you can’t see any details, all you know is there happens to be an object that emits radio over there somewhere.

This is because the radio wave ones are meters and meters in length. I’m trying to compare something that is hundreds of nanometers to something that is tens of meters in size. This is such a huge difference and it means a huge difference in the resolution.

Fraser: I think that is key though, that this is very tricky. With optical telescopes to line up the wavelengths between two optical telescopes you have to make sure that your wavelengths that are nanometers across are perfectly lined up.

That requires timing at an insanely complicated level. While you can imagine these radio wavelengths which can be meters across where you can kind of miss a little bit and it’s no big deal. To use this technique radio is the larger wavelength is where it really shines, right?

Pamela: Conveniently it is the radio wavelengths where we need this technology the most. A single dish has such terrible, terrible imaging resolution. It is only by starting to combine dishes that are actually spread across half the globe that we’re able to start getting really good resolutions.

One of the amazing things about this is with radio telescopes we’re able to very precisely record incoming radio light from distant objects in New Mexico, Massachusetts, and in Spain. Using dishes spread across the entire part of the planet that are capable of looking at some distant object at the same time we record the signal onto hard drives onto magnetic tapes. We record the timing of the observations.

Then artificially in a computer combine all of this light in a process that involves this neat thing called fringe finding where you carefully adjust the timing offsets between the data. We artificially combine everything to create this artificially large telescope.

With optical light we don’t have the ability to record the incoming light in the same way where we’re able to keep track of every peak and valley, every change in incoming photons. It is because of this difference where we end up resorting to things like using fiber optics where we physically delay the light travel time to the detector and physically combine the light so that it has the correct delays from one telescope to another.

Fraser: Let’s talk a bit about sort of what the set up of one of these inferometers looks like. There are a couple operating now and so sort of the visible light telescopes lay it out for us. What does this kind of look like?

Pamela: Visible light is still very, very experimental. There aren’t many systems in the world that are getting actively used for things that you and I would be able to see easily with our own light. The most famous of the systems is the Very Large Telescope Interferometer down in Chile.

We have the large 8.2 meter telescopes in the Atacama Desert that have the ability of using fiber optics, combine the light and get extremely high resolutions along what we call the baselines. These are the lines connecting one mirror’s center to another mirror’s center.

When you combine two telescopes you get extremely high resolution only along the direction in the image that has from one edge of one telescope to the other edge of the other telescope. Then you’ll get single dish resolution 90 degrees to that because we don’t have that added size to the telescope mirror in that direction of the telescopes aren’t spread out from one another.

Fraser: It’s like imagine a big long skinny telescope that is 8 meters high and how far apart are they?

Pamela: These are actually about 200 meters apart.

Fraser: Okay so it is 8 meters high and 200 meters wide. It is a very long skinny mirror. As you said it can go the other dimension too, right?

Pamela: Right so here they have multiple telescopes and they also have the additional little one meter telescopes that they can move around the facility and add in additional rays to end up getting as many as six different dishes on at the same time I believe.

By combining different mirrors in different ways all of this working in the optical – well here I have to say it is kind of a cheat to say the optical – it is the optical if you’re a snake.

This is near observing. There are systems that are working in what we do see as visible light but these are mostly systems that are still experimental. We’re still working on developing this technology.

Fraser: Right, the future has yet to be written.

Pamela: There are experiments in Sydney, in Hawaii, there are experiments all over the world trying to make this work invisible light but if you’re a snake, it is visible because we do have the strength you need for that.

Fraser: So, infrared and down sub millimeter into the radio microwaves, you’ve got the ability to merge these telescopes. As you mentioned the most amazing example of this is the fact that one whole half of the Earth can be called upon as one great big radio telescope. If what you need is a telescope the size of the planet there’s one available to you.

Pamela: Right and with the radio, we’re already doing this. We already have plans or at least different people have put together different plans, no one is working on building them at least right now that I know of to extend the baseline by putting dishes in orbit.

The plan to extend the baseline where instead of being confined to the diameter of the planet Earth, we’re instead confined to where in the solar system that we put our telescopes.

Fraser: We could put up one radio telescope on one side of the Earth’s orbit and another on the other side…

Pamela: Or stick one out on the moon. There are lots of different ways that we can start combining things.

Fraser: Right and the resolution goes up. Then there are plans in the works to develop space telescopes that would use interferometry. I guess our favorite cancelled space program that was where they were being planned, right with the terrestrial planet finder?

Pamela: Yes. This was the system that was looking to combine the light from three different telescopes to a central observing hub. It is a complicated system but in space it is using its lasers to very precisely locate the spacecraft.

We can get them spread out to much larger distances. Again this was a system that was looking to physically combine the light along these different path lights using physical delays.

We know how to do this. It’s not easy to do. It’s not cheap to do and this is why NASA hasn’t quite gotten around to actually doing it yet.

Fraser: Alright and I think the terrestrial planet finder this is one of those situations where the resolution is what you want. You want to be able to resolve a planet orbiting a star where it is a separation issue.

Obviously you want some photons but you really want to be able to just acknowledge its existence there. That’s not simple.

Pamela: Here we have to be able to resolve the planet separate from the star. We have to be able to block out the light from the star. It starts to become increasingly a more and more difficult system.

Fraser: When I first heard of the concept of interferometry I got really excited. I wondered if you could take, kind of like SETI at home. Amateur astronomers around the world could set up their telescopes, point it at some object that they’ve all been instructed to point it at and then gather light for whatever period of time.

They would then submit their images to some central clearinghouse that could then merge it all together to do interferometry in the visible spectrum. I emailed this to several scientists and I got some like polite laughter. [Laughter] Where was my thinking incorrect?

Pamela: Like I was saying earlier with the optical light the problem is we can’t just artificially recombine it later because you’ve been able to record with your CCD detector the over time variations and the life of peaks and troughs quite so conveniently.

With light you’re collecting photon after photon after photon and building an image with long integration times. You don’t do that in radio because the technology is just completely different. On top of that there are also all of the timing issues all of the positional issues.

This means that you can’t even do what you are suggesting if we replace all of the 12 inch amateur telescopes in people’s back yards with instead satellite dishes taken from the local cable television networks.

The issue with Inferometry is you have to know exactly the surface of the planet. You have to have atomic clock precision in the timing of your observations. Even with that, combining the data can be difficult to say the least.

The process of fringe binding the process of artificially lining up the radio data to get it to coherently combine, to get the peaks to line up with the peaks to enhance your signal is a complicated process. As you bring in the light from each different dish if you’re using for instance baseline interferometry you have to worry about things like the Russian’s telescope is off by three seconds. You have to worry about oh Spain was behind by half a second.

All of these small differences in timing have to be accounted for and they end up affecting your data as your telescopes move across the sky in several different ways and it is just hard to combine all of this data.

As you bring in more and more telescopes all of these small time issues become more and more things you have to worry about. It just becomes an untraceable puzzle fitting nightmare that no one wants to solve.

Fraser: It would work if we gave everybody an atomic clock and connected their telescopes by fiber optics and measured their position on the planet to an insane accuracy. Plus we would need to develop entirely new technologies and new kinds of computers.

Pamela: And we were able to wire the fiber optics in just the right way that we were able to exactly compensate for differences from multiple objects with multiple differences and travel time delays. It is just far too complicated of a problem.

Fraser: With radio dishes, it would be feasible.

Pamela: It’s all about the computer power there. Even then you would have to give everyone on the planet an atomic clock and military grade GPS.

Fraser: Oh well, all in the name of science. I’m up for it. [Laughter] Cool, well I think that this is going to be one of those technologies that over the next couple of years they will keep on developing the techniques.

They are going to crack it and I think that you’re going to really see some amazing research from ground-based telescopes and especially the space-based observatories that are hooked up in these baseline arrays. That’s going to really be amazing.

Interferometry – it is a complicated word but it is a very exciting technology. Read the stories when you hear something about some interferometer that comes online. It could be the next great technological advance. Thanks Pamela and we’ll talk to you at the next questions show.

Pamela: Sounds great Fraser, talk to you then.

Ep. 118: Sky Surveys

admin — Fri, 12 Dec 2008 18:56:06 +0000

SDSS Telescope

In the old days, astronomers had to beg for telescope time. They’d put together a proposal, convince observatories to gather data for them, crunch that data and release the results. No telescope, no results. But everything’s different now. Fleets of robotic telescopes constantly scan the skies, building up a vast database of raw data about the Universe. Anyone who wants can access the information through the Internet, download what they need to do real science. No telescope necessary. Let’s look at the development of sky surveys, and how they’re changing how astronomy gets done.

Ep. 118: Sky Surveys

Jump to Transcript or Download (coming soon!)

Shownotes

TEXOX Survey of Radio-Selected Galaxy Clusters (Pamela’s doctoral dissertation project)
McDonald Observatory
Palomar Observatory Sky Survey – a comparison of POSS to the Sloan Digital Sky Survey
Digitized (and searchable) version of POSS from the Space Telescope Science Institute
The Second Palomar Sky Survey
Sloan Digital Sky Survey
Apache Point Observatory (where the SDSS is done)
Sky Server (to search for SDSS images, as well as educational materials)
The drift-scan technique
Tidal tails – Internet Encyclopedia of Science
co-moving: indicates a state of expansion or contraction matched to that of the universe as a whole.
SDSS: 3-D Map of the Universe Bolsters Case for Dark Energy and Dark Matter
SDSS: Quasars Found at the Edge of the Universe
SDSS: Scientists Discover New Celestial Dwarfs
Galaxy Zoo
Galaxy Zoo: The Science
GZ and Overlapping Galaxies
Chris Lintott
Kevin Schawinski
VLA FIRST Survey
Near Earth Asteroid Telescope (NEAT)
MACHO Project overview
OGLE (Optical Gravitational Lensing Experiment) Project
Sky View Virtual Observatory

Transcript: Sky Surveys

Download the transcript

Fraser Cane: In the old days astronomers had to beg for telescope time. They put together a proposal, convinced observatories to gather data, crunch the data and release the results. No telescope, no science.

But everything is different now. Fleets of robotic telescopes constantly scan the skies building up a vast database of raw data about the Universe. And anyone who wants can access the information through the internet; download what they need to do real science – no telescope necessary.

Let’s look at the development of sky surveys and how they’re changing how astronomy gets done. Is that a little over the top you think?

Dr. Pamela Gay: No, I think sky surveys really are actually changing how everything is done and that’s kind of cool.

Fraser: Why don’t you then regale us with a story of you attempting to get science done the old way? [Laughter]

Pamela: I think the best example is my doctoral dissertation. I was working on a project called the Texox Survey where we were following up on a radio survey actually, looking for places on the sky where we thought maybe there was a higher probability of finding galaxy clusters.

We put in for observing time at McDonald Observatory, first on the 30 inch telescope, then on the 107 inch telescope. Hundreds of nights were spent out at the observatory.

When the weather went bad we didn’t get our data, I attracted clouds, and there was much crying and sadness. We got somewhere but we didn’t get where we wanted due to clouds and forest fires and all that sort of stuff affecting the amount of time we had at the end of the day. It was sad.

Fraser: It still kind of happens now. I know you’re telling me about some projects that you’re working on where you have to scrape together telescope time. You have to convince amateurs to let them do some observations for you.

You’re distracting telescope operators so you can sneak in and you know, quickly move the telescope. [Laughter]

Pamela: There are two different ways of needing to get data. There is the “I have a question that requires significant coverage on the sky. I want to look at a whole bunch of different objects and try and prove something in a statistically significant way.”

To do that you need a ton of telescope time scattered all over the sky. The data that I take might also be good for somebody else. Not the type of stuff that leads naturally to the type of surveys that we’re going to talk about in the rest of the show.

The other type of thing you need is: “Ooh, I found a really cool object! I need a bazillion hours on this one really cool object.” That’s where you start begging people. That’s where you need the dedicated time to look at just your object.

This is why the stable of astronomical equipment needs to include both telescopes that are dedicated to doing surveys, looking at the whole sky night after night and also telescopes that are set aside for astronomers to follow up on pet projects.

Fraser: Alright, let’s then segue into the survey. Cane you give us an example of sort of what was one of the first sky surveys put together? What’s involved?

Pamela: The first really big survey that people paid attention to even today, is the Palomar Observatory sky survey. It used glass plates, two different Kodak emulsions, one sensitive to red; one sensitive to blue to look at pretty much the entire northern hemisphere of the sky all the way down to minus 30 south. It had significant coverage of the sky.

The idea was to catalogue what’s out there. The survey has been used to look for galaxy clusters. It’s been used to get statistics on how many of what different types of objects are out there.

It’s used even today where some cool something or other happens and you pull out the digital sky survey and look to see what was there before. It’s a historic record of what was where on the sky when. It’s a map of the sky and it’s a way to do big science admittedly on the 1940s technological scale.

Fraser: You’ve got a really nice telescope every night, taking a picture, moving a little bit, taking a picture and moving a little bit. It’s just slowly cataloguing every single little piece of the night sky.

I guess as we talked about before to really get good science about an object, you want to point Hubble [Laughter] at it for about a hundred hours and get every stray Photon that’s coming from it. This is the opposite, right?

This is quick and dirty. This is click, move; click move and so as you said you’re able to count up galaxy clusters. You’re able to count stars. You’re able to get a general sense of what’s out there. But you’re not able to really dig deep and see the same thing pointing at one object for a hundred hours.

Pamela: No and in fact with these old glass plate surveys, it took them years to get coverage of the entire sky. Even today we’ve moved on and today’s new version – perhaps new version is too strong a word, but our new optical survey of the sky is the Sloan digital sky survey.

It’s probing the southern galactic pole. It’s looking at a very focused region of the sky and it’s studying extremely deeply. It’s doing it in 5 colors whereas the Palomar sky survey looked at the sky originally in only 2 colors and it’s getting huge swaths of the sky every night.

But to get as deep as it does, it takes significant amounts of time and it takes years to get a really detailed survey of the sky complete.

Fraser: Let’s take a look then at Sloan. When did Sloan get operating?

Pamela: They started building the telescope back in the 1990s and it first started taking images in 2000. It’s still here in 2008, reinventing itself, coming out with new ways of doing things.

Fraser: Then what does it capture? Obviously it takes a picture of a chunk of space but I know there’s more information that it’s helping gather, right?

Pamela: The Sloan digital sky survey is using a technique called drift scanning. You might start off with an object on the right-hand of the CCD and over time it slowly drifts from the right side to the left side.

As it is drifting the digital camera is taking that right-hand most column of data and shifting it one pixel to the left and shifting it one pixel to the left. It’s shifting the recording of the data one column at a time at the same rate that the object is moving across the CCD.

By the time it has read all the way across the chip you might have many, many minutes of observations of that particular object allowing you to get extremely deep images all across the sky.

Fraser: Okay, I understand, it’s like the big CCD isn’t moving so it is able then to just pick it up, see the same object again and again.

It’s almost like it’s taking multiple photographs of the same object because it’s grabbing such a big swath of the sky at the same time.

It’s using the rotation of the Earth to move the objects in it through its field of view.

Pamela: Yeah, that’s exactly what’s happening. What’s really cool is in addition to doing this detailed imaging of the sky, the Sloan digital sky survey is also going back and doing follow-up spectroscopy.

In this case they’re actually taking plates and drilling holes in them and then aligning fiber optics onto the holes on the plate. For each hole in the plate and each fiber, they get individual spectra that allow them to get a sense of what elements are in the objects that they’re looking at. What is the red shift of some of the galaxies that they’re looking at?

These are pretty big fibers so while they’re able to capture a lot of light all at once they also aren’t so good for dealing with really crowded fields like galaxy clusters.

But for isolated objects this system is allowing us to sample a huge number of galaxies scattered all across the sky to find out where they are in red shift space. That gives us a sense of their distance and to also give us a sense of what are other things we can learn by looking at the elements.

This starts to play more of a roll when we’re saying does this thing have absorption lines? You can start to differentiate different types of active galaxies by looking at what lines exist in emission.

This is where you have an angry super massive black hole in the center of the galaxy that’s chomping on things and heating them up and if the alignment is just right, the heated up elements end up radiating emission lines that we can detect. That tells us something about what’s going on down in the core of the galaxy.

Fraser: Then how much of the sky is Sloan going to be mapping?

Pamela: Sloan digital sky survey is looking to eventually map out about 25% of the sky. That doesn’t sound like a lot, but at the level of detail they’re getting this is actually a really powerful survey.

Already they’ve allowed us to learn new things about the structure of our own galaxy that we’ve never even guessed at. Galaxies are the cool part for me. In addition to looking at all the galaxies, it’s also mapping out stars in the halo of the Milky Way.

By looking at the colors of the stars we’re able to get a sense of how far away they are. So you look at the color and brightness and basically by taking a color magnitude diagram of what would a population of stars look like at what distance.

You can sample through the sky to find out where are the populations of stars that are 60 kilo parsecs away. Where are they that are 70 kilo parsecs away? We can start to see streaks, tidal tails of shredded dwarf galaxies out in the halo of the Milky Way by probing through using the Sloan digital sky survey data to find what we call co-moving populations of stars.

Fraser: I guess this is where the real power is because in the olden days you would take your telescope, look at a region of sky, and make measurements of whatever you wanted to do.

But in this situation Sloan is gathering images of everything that’s in that 25% of the sky and furthermore they’re writing down numbers. They’re saying: “there’s a star here at this location, there’s a galaxy at that location. There’s a quasar here at this location.” And then they’re figuring out what the elements are that are in those objects. You can then do searches.

This is where it becomes a database issue. You can say: “find me every star that has this level of metalicity, or find me all of the stars that are within this range in this region of the sky.” The data mining starts and perhaps suddenly you look at that line where you mapped out all the stars in the survey and they happen to be lined up in a very interesting line. That must be a tidal tail.

So there are whole new kinds of discoveries being made that could never be made before because you just didn’t have the raw data about the entire sky that you could just then mine and ask questions.

I know there’s some amazing work done for dark matter, quasars, as you said cataloguing active galaxies. Isn’t Galaxy Zoo using the Sloan digital sky survey?

Pamela: Right but before we jump on to galaxies I just want to make it really clear that the spectra that they’re getting with the Sloan digital sky survey really isn’t good enough to start getting at detailed metalicities of stars. It is rough spectra but it’s enough to get us what are their velocities and to get us broad information on them.

That’s what’s cool about the Sloan digital sky survey. It gives us a broad understanding of where things are, of how they’re moving, of what different types of things are out there that we can follow up on later.

It is allowing us to find more white dwarfs than we ever thought we could find some other way. It is allowing us to find quasars. It is in fact allowing us to determine the distribution of galaxies of different shapes and sizes and orientations on the sky. This is where Galaxy Zoo comes in.

Fraser: People are finding asteroids and kuiper objects that are sort of in the pictures.

Pamela: They’re starting to find really rare objects. One of the really cool things that came out of the Galaxy Zoo 1 project, a project that was originated by folks over at Oxford, my collaborator Chris Lintott, Kevin who is now at Yale, a whole bunch of different folks, were sitting around.

Kevin was given the task to go look at 50,000 galaxies and tell us where their distribution is in terms of are they spirals, are the elliptical? How are they oriented? After doing 50,000 objects he really didn’t want to do any more.

So in a pub, the idea was originated to get the public who likes looking at galaxies to look at almost a million objects from the Sloan digital sky survey and catalogue how they’re oriented and what their shape is. Basically are they clockwise, counterclockwise or edge-on spirals or are they ellipticals or are they mergers? They did this and the results of the public looking at all of these objects were just as accurate as getting a small number of professionals to look at a much smaller sample of objects because you can’t make professionals look at too many of them before we go crazy.

You ask 170,000 people to look at things and of course 170,000 people can look at a lot more objects than 5 or 6 professionals.

Fraser: Right and you’re saying that they turned up some amazing things.

Pamela: One of the really cool things that they were able to do is build up a catalogue of galaxies that are overlapping on the sky. These aren’t galaxies that are merging but rather two galaxies that are superimposed on one another’s line of sight, we say. One is nearby, one is further away.

Gravitationally they really don’t care about each other very much, but the background galaxy can act like a spotlight going through the dust lanes of the foreground galaxy allowing us to make out the details in structure that we might not otherwise be able to see.

Fraser: I guess it’s almost impossible for a computer to seek out those kinds of objects because it can barely tell that there’s a galaxy there at all. This is something that a human being is great at.

Pamela: Computers are generally pretty good about determining what is a star and what is not a star. Beyond going not a star, computers tend to get kind of confused.

At a certain level you can start to program them to look for things that are S-shaped or Z-shaped so we have clockwise and counterclockwise shaped galaxies.

We can program them to look for things that have a radial profile, things that are basically fuzzy blobs like elliptical galaxies. You can’t program a computer to determine if this thing looks like nothing anyone has ever seen before. They’d be showing that up every time an asteroid happens to pass in front the galaxy.

You can train human beings to determine what an asteroid looks like when it is passing through an image; this is what a satellite looks like when it’s passing through an image. This is a really weird nebula; this is reflected light inside the telescope.

We can take human beings and train them to do things that we can’t train a computer to do. Human beings will naturally note “this is cool and unusual” and bring it to the attention of other people. That’s one of the wonderful things that came out of the original Galaxy Zoo project. What’s even cooler is there is now a second generation, Galaxy Zoo 2 that’s been launched.

If you go to galaxyzoo.org there is now a link off of it to Galaxy Zoo 2 but you’re going to have to take a survey if you participated in Z1 so that some of the folks working with Galaxy Zoo, which in this case includes me, so please take the survey, can find out a little bit about why is it that you love using Galaxy Zoo. I know why I love using it; I want to know why you love using it.

Fraser: What are some other surveys that are happening?

Pamela: Surveys aren’t restricted to just being optical telescopes. There are lots of other surveys out there. The Very Large Array out in New Mexico is working on a survey called ‘First’ and it’s looking to do both the northern and southern galactic poles.

It’s a survey that’s looking at 21 centimeter continuing radiation. This is the type of light you get from disks of galaxies from blobs of gas. It allows you to see star-forming regions. It also allows you to see jets off of radio galaxies.

The first survey has about 5 arc second resolution which is what you get on a really bad day with an optical telescope. It’s about 5 times what you’d get from a reasonable sight with an amateur telescope. It’s going pretty faint and it’s looking to collect data that will allow us to figure out where are all the galaxies that are actively forming stars.

Where are the most distant radio galaxies – galaxies that are actively feeding black holes in their center and are thus giving off radio emission? This is an ongoing survey that’s constantly working to increase its area and it’s working in radio. It’s just another way of looking at the Universe.

Fraser: So, same deal, right? Gather as much raw data as you can, catalogue it as best you can and then make that information available to the scientific community and I mean the general public – it’s all on the internet – to look for whatever they want in the data as it stands which is just amazing.

Pamela: And we’re working to try and cover as much of the sky as we can and as many different colors as we can. There’s 2MASS out there working in micron radiation that used two 1.3 meter telescopes in Arizona and Chile to look at the sky.

There have been infrared satellites and x-ray satellites that have also worked to cover the sky. We also look at data that was perhaps taken for other purposes as another source of perhaps serendipitous observations.

There’s a telescope called the near-Earth asteroid telescope, neat. It is primarily out there trying to make sure nothing hits the planet Earth. It’s a good goal, but as it is out there surveying for this very specific purpose, it’s also picking up supernova. It’s also picking up variable stars. It’s picking up lots of other stuff that just happens to be in the background of the same fields it is looking for asteroids in. We can use that data as well.

There are very specific surveys looking at very small regions of the sky but because they’re building up data over years, we’re able to learn interesting things. There were two projects, the MACHO project and the OGLE project that looked at the magellanic clouds specifically looking for gravitational lensing events. This is where a nearby object, in this case nearby being on the outskirts of the Milky Way Galaxy, passes in front of the background object, something in one of the magellanic clouds and causes it to brighten through gravitational micro-lensing.

In the process of looking for these events, they’ve also done things like find light echoes from supernova moving through the interstellar medium. So there is a supernova hundreds of years ago, thousands of years ago and the flash of light from that supernova is forming an expanding shell of light. As that light passes through the gas and dust between the stars, it temporarily illuminates whatever section that it happens to be in.

By looking in the same direction for year after year after year, we can watch these shells of light move and then track them backwards and figure out where they originated.

This is one of the neat ways that we’re starting to figure out what was Kepler’s supernova actually like? What was the supernova in Cassiopeia actually like? We’re tracing back the light echoes to learn more about events that we weren’t around to see.

Fraser: This I think is a realm of science that has no limit. You can just imagine bigger telescopes, more telescopes gathering more of the data that are looking deeper. Are there sort of dream projects in the works to do really enormous surveys?

Pamela: What’s really cool is they’re not even dream projects, they’re actual projects. There are two really cool ones coming up, Pan-STAARS and LSST (Large Synoptic Survey Telescope).

These two different many meter telescopes are focused on trying to find things that are going to hit the planet Earth. Protecting the planet Earth is a good way to get money to build telescopes. Along the way while they’re out there taking snapshot after snapshot after snapshot of the sky they’re also going to be turning up supernovas. They’re going to be turning up variable stars. They’re going to be taking image after image of the same place on the sky.

Those images can be added together to get some of the deepest images we’ll have ever achieved of distant galaxies. It’s all a matter of adding up the data over time. With these two projects we’re going to have these huge telescopes taking in pretty much everything that’s visible every single night.

That data can get added up to allow deep imaging or it can get used together to get time sequence imaging to see what are all the transient events that we’ve been missing all these years.

Fraser: I think just to be clear all this data is available on the internet, right? If you know where to look, you can pull it down and crunch it.

Pamela: Different surveys have different proprietary periods. This is a certain amount of time where the people who invested the intellectual resources and the monetary resources to build a given instrument (telescope) or take a certain survey, get to have all the data to themselves. At the end of these proprietary periods all of the data becomes publicly accessible.

One of the greatest ways to go out there and access a lot of this data is through a portal called Sky View. Just open up Google and do Sky View Virtual Observatory. It will give you a form to fill out that will allow you to get radio data, x-ray data, and optical data, all of the same field on the sky.

It will allow you to map different objects onto these images to see where they happen to line up. It’s a great resource for both the professional astronomer and for people who are just trying to get a different wavelengths perspective of the Universe.

Everything is out there. Sometimes you just have to wait 6 months to a year to get your hands on it.

Fraser: I think the big need is for people in the computer industry, people who understand how to make a database sing and to be able to pull in that data and help crunch and help answer some of those basic questions.

But I think the reality is that anybody who wants, assuming they have the skills, can go onto those surveys, download the information and discover brand new objects, discover asteroids that have never been seen before. It’s all there; there are mysteries inside that data. All you have to do is go looking.

Pamela: And I can’t restate what you’re saying with enough emphasis. It is everyday people who are going out and discovering new things in some of these surveys. I’ve been at American Association of Variable Star Observers meetings where amateur astronomers have stood up and given talks on how they’ve gone through this database or that database pulling up variables that no one had known about before just because they knew how to do all the nice equal queries effectively.

With the Galaxy Zoo project there have been just everyday members who are participating and noticing things that are new that are making cool discoveries where go check out the forums. Look up the peas in the Galaxy Zoo forums. These are a new class of galaxies that were discovered by regular people who asked: “What are all these little green compact galaxies?”

There’s new stuff out there just waiting to be discovered. You might be the person to make the next cool discovery.

Fraser: And if you do let us know. We’d love to hear it. Well thanks a lot Pamela and we’ll talk to you next week.

Pamela: One more thing before we quit. Hopefully next summer I will both have my voice back and I will have the opportunity, perhaps with even you, to go out and see a really cool eclipse out in the Pacific Ocean. I’m going to be on the eclipseofthecentury.com tour. There are still seats available.

Fraser: That would be fun. I’d love to see an eclipse. I’ve never seen a total solar eclipse.

Ep. 108: The Life of the Sun

admin — Tue, 30 Sep 2008 20:02:18 +0000

We’ve talked about the Sun before, but this time we’re going to look at the entire life cycle of the Sun, and all the stages it’s going to go through: solar nebula, protostar, main sequence, red giant, white dwarf, and more. Want to know what the future holds for the Sun, get ready for the grim details.

Ep. 108: The Life of the Sun