Ep. 337: Photometry

There’s a lot you can learn by just staring at an object, watching how it changes in brightness. This is the technique of photometry, and it has helped astronomers discover variable stars, extra-solar planets, minor planets, supernovae, and much more.

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This episode is sponsored by: Casper, Swinburne Astronomy Online, 8th Light, Cleancoders.com

Show Notes

  • Donate to Cosmoquest! 
  • Variable stars in the Ursa Minor Dwarf Spheroidal Galaxy: a story of stellar formation as told by the horizontal branch — Gay, PL
  • Variable Star Photometry — NAAP
  • An Introduction to Astronomical Photometry using CCD’s (pdf) — University of Oklahoma
  • History of Photometry in Astronomical Observations — McCormick Museum
  • Transcript

    Transcription services provided by: GMR Transcription

    Female Speaker: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest running online astronomy degree program.  Visit astronomy.swin.edu.au for more information.
    Fraser Cain: Astronomy Cast episode 337; Photometry. Welcome to Astronomy Cast, your weekly facts-based journey through the cosmos. 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 Everettsville and the director of Cosmo Quest. Hey Pamela, how are you doing?
    Dr. Pamela Gay: I’m doing much better this week, and I have to apologize for last week. We recorded, and I was really stubborn and determining not to admit how really damn sick I was, and the next day got diagnosis of pneumonia and scolding and yeah. So if last week’s episode, the episode on units, was not the best ever episode we’ve ever produced –
    Fraser Cain: For once, we can blame you.
    Dr. Pamela Gay: It’s totally my fault. Totally my fault. Dayquil, Nyquil, were not enough to get the neurons firing at full capacity. I’m better now. Not 100 percent, but –
    Fraser Cain: Yeah, you look like you’re well into your recovery.
    Dr. Pamela Gay: Yes. I’m at the getting the energy back stage instead of getting the oxygen back stage.
    Fraser Cain: All right, let’s get on with the show.
    Female Speaker: This episode of Astronomy Cast is brought to you by 8th Light Inc.  8th Light is an agile software development company.  They craft beautiful applications that are durable and reliable.  8th Light provides disciplined software leadership on demand, and shares its expertise to make your project better.  For more information, visit them online at www.8thlight.com.  Just remember, that’s www.8thlight.com .  Drop them a note.  8th Light; software is their craft.
    Fraser Cain: So there’s a lot you can learn by just staring how an object and watching how it changes in brightness. This is the technique of photometry, that has helped astronomers discover variable stars, extra-solar planets, minor planets, supernovae, and much more. And this episode is going to come straight from your head, because you are a variable star astronomer. This is your bread and butter.
    Dr. Pamela Gay: Well, and it’s not just variable stars. You can also do photometry on galaxies, and most of the research that I’ve done, and all of the research that I’ve done that hasn’t made me hate my computer, has been based on photometry. So, when I was in graduate school, I studied variable stars in the Ursa Minor dwarf spheroidal galaxy, I studied galaxy clusters at what we would now call low red-shift, but back then we called moderate to high red-shift. Which basically means that they’re far enough away that they’re only a little bit fuzzed out compared to stars, and you just kind of treat them like stars when you study them with CCDs. So most of the work I’ve done has been, “Let’s take pretty pictures of things, except ignore the prettiness and get it just the science.”
    And even as an undergrad using much earlier technology, it was all about, “Let’s just see that light.”
    Fraser Cain: So I want to hear some stories. So what was your research and how did photometry play into it? So what is the title of one of your research papers?
    Dr. Pamela Gay: Oh man, The Blazhko effect in RR Lyrae, I’ve done work on AH Leo, I’ve done – Man, my, unfortunately, master’s thesis research got totally sniped by the Hubble Space Telescope, but there’s some posters out there on using the variable stars in the Ursa Minor dwarf spheroidal galaxy to understand the stellar population distribution. That may be one of those things where my poster led to one of my greatest moments of sadness in my research career. I was at the American Astronomical Society meeting, and one of the awesome things about photometry is if you have a reasonably good telescope and you are careful in what you do, you can make up for the telescope being small by just observing long enough. And I observed these faint 20th magnitude variable stars in Ursa Minor using photometry to measure how their light varied over time.
    And with RR Lyraes, the period of the stars related to metallicity, to type of star, to a whole variety of different things, and then the brightness of the star is related to distance and all of them have basically the same luminosity. So you can pull apart all these different, “How many stars have this type of period? How many stars have this kind of period?” And look at all of this and realize, “Huh, all of these, even though they’re so faint I can’t get a good spectrum of them, are probably made of the same stuff. All of these are basically the same age,” and that’s just the RR Lyraes. As you keep going looking at stellar populations, when you start to find other kinds of variable stars, each kind is a test particle saying, “Hey, there’s stuff this age and this metallicity here,” because if there’s more or less metals it wouldn’t pulsate. If it was older and younger, it wouldn’t pulsate.
    And when I looked at Ursa Minor and put together all the information I could learn from the variable stars, I was able to pull out the details that this little tiny faint system that I observed with a 30-inch telescope, all the stars had come out of a single epic of star formation, which, for a galaxy, is pretty cool. But it’s such a small galaxy that it was basically one of these great situations where this little tiny galaxy used up all of its material for star formation in a single go, and I was stupidly proud of my results, hanging my poster up, getting ready to write up the research paper for publication, and right next to me, Ken Miguel is hanging up his poster on the Ursa Minor dwarf galaxy showing the exact same results using an HR diagram with the Hubble Space Telescope and much better quality of everything.
    Fraser Cain: Did you just high-five each other though, and that’s like confirmation for both of you? Like, “Done?”
    Dr. Pamela Gay: There was a certain element of that followed by the, “I don’t need to waste my time publishing this.”
    Fraser Cain: No. Did he not even let you participate in the paper? Could you not be a co-author?
    Dr. Pamela Gay: Well I had nothing to do with his results.
    Fraser Cain: Well I know, but you had your results, and then the same as his results that verify – That would just make him look better.
    Dr. Pamela Gay: Yeah but my poster was already going up in the proceedings.
    Fraser Cain: You should have just shuffled your papers together and then just handed it in.
    Dr. Pamela Gay: It was just one of those, I spent two years of my life and 100 nights at the telescope getting my results, and it can also be accomplished with Hubble very quickly and much better than it was.
    Fraser Cain: All right, so photometry. It is detecting the changes in brightness from an object, be it a star, be it a galaxy –
    Dr. Pamela Gay: A creation of disk around a black hole –
    Fraser Cain: Exactly. So what is the gear? What is a photometry thing? If I could hold a photometer in my hand –
    Dr. Pamela Gay: Photomultiplier tube, photometer, yeah.
    Fraser Cain: What would this thing look like?
    Dr. Pamela Gay: They come in a variety of different forms. The old school ones, that still get used if you’re trying to do high speed work, is just a simple photomultiplier tube. It literally looks like a tube kind of laying that way. Light goes in one end –
    Fraser Cain: A tube you say. Okay.
    Dr. Pamela Gay: Light goes one end, and it goes light, no light. And it very very precisely counts the number of photons coming in. You can stick a filter in front of it so that you know what colors the photons coming in are. And what is glorious about these is there’s really not a whole lot of data reduction involved. You point your photomultiplier tube at the star, galaxy, whatever it is that you’re measuring. You then point it at the sky, subtract the value of the sky from the value of the object you looked at, and you have more data.
    Fraser Cain: And you say you can count the number of photons. How precise can you get with this amount of light year [inaudible]?
    Dr. Pamela Gay: They claim 95 percent quantum efficiency.
    Fraser Cain: Wow. So in other words, your error bars are going to be, like if it’s within the five percent then you’re probably going to say it’s error, but if it’s 10 percent, 15 percent, then you feel pretty confident.
    Dr. Pamela Gay: The bigger issue comes from the atmosphere. So you do have to correct for sky glow, you have to worry about changes in transmission of the sky. As your object wheels from straight overhead towards the horizon, the atmosphere is going to start knocking out more blue light. But at the end of the day, the detectors are really good, and this is why, with the even not as good CCD cameras, we can spot planets going in front of other stars. That’s a photometric process. Kepler used photometry to discover planets.
    Fraser Cain: And so what will you get as a result? You’re going to have the photomultiplier, you’re going to have two in front of the telescope. It’s only going to be able to see the photons from a very small portion of the sky, right? Like, I’ve seen ones that are more elaborate where they’re gathering multiple stars all at the same time, or multiple galaxies.
    Dr. Pamela Gay: Right, so that’s a CCD.
    Fraser Cain: Right, but mostly you’re aiming for one little chunk of your sky, right? The resolution, the – You know what I mean? Like, the size of the sky that you can see is pretty small.
    Dr. Pamela Gay: Right. So when you’re doing photometry, what you’re interested in is the light from a single object or a single region of light from an object. And if I take a big picture of the sky and I look at the distribution of the light pixel to pixel to pixel, it will be essentially central pixel that gets a lot of light, and then the light tapers out towards the edges. The distribution of how the light tapers off depends on the sky, depends on the detector, but it tapers off in a nice, neat curve. And there’s what’s called a full-width half-maximum to this curve. And so that’s the width of the fall-off curve where the light at that point is half of the maximum light coming in. And in general, when we’re doing photometry, we try and collect everything that’s within three to six times what that full-width path max is, and that’s what we consider the useful portion of the light coming in from the star.
    Now if you keep going, our atmosphere blurs that light out for a long time, but that’s the core light that allows us to get the bulk of the science done without getting a lot of noise from the sky in.
    Fraser Cain: Now you said that you could put filters in front. So, for example, if I’m only looking for the blue end of the spectrum or the red end of the spectrum, if there’s enough light, I guess, coming from the star, I can start to segment that stuff out and see. So could you have a situation where the overall light of the star seems to be the same, but there’s changes in brightness from some of the photons, from some of the types of photons, that are changing, like the heat? I guess if it’s heating up you would get –
    Dr. Pamela Gay: That changes the entire curve because of black body radiation. So this is one of the awesome things about doing photometry is when you’re doing spectroscopy, when you’re dividing the light of the star, galaxy, a source, out into that full rainbow to measure how many atoms of this, how many atoms of that? What molecules are there? What red-shift is that? You have to spread that light out a whole lot. So you’re starting to get less than one angstrom of light per pixel, and that’s like 10 to the 10th of a meter, 10 to the -10th of a meter. Tiny fraction of the light into one small bit. Well, with photometry, you’re looking at sometimes a couple hundred angstroms, a couple thousand angstroms, depending on what you’re trying to accomplish.
    And so you’re getting a whole lot of colors of light in. But because the shape of a black body curve is very distinctive, that curve of light that describes the temperature of an object, the peak of the light shifts towards bluer and bluer colors the hotter an object is. That shape is very distinctive, and if you sample an object’s light at three different places, you can fairly precisely get at the object’s temperature because only one curve is going to fit to those three lines. And if you have extra information, you can sometimes even get away with just two colors of light if you have to.
    Fraser Cain: Right, and I guess with spectroscopy, as you mentioned, the difficulty is getting that light spread apart. I guess with photometry, the real challenge is to measure those slight changes in brightness. That’s how you find the planets. That’s tough.
    Dr. Pamela Gay: Well, and you’re not always looking for the changes in brightness. Sometimes you’re just looking for stuff. And that sounds lame –
    Fraser Cain: Super lame.
    Dr. Pamela Gay: So, this is one of those rare shows where we hit nail on with my research. And when you’re looking for galaxy clusters, you’re taking images of sections of the sky and then going through and basically counting the density of objects across your image and what blurriness are they? Blurriness isn’t quite the right word, but what spreads is the full width half max half. Because if you have a beautifully focused, no optical imperfections, idealistic, doesn’t actually exist, but we’re going to pretend for the points of this episode, telescope, when you look at stars all across your entire field of view, all of the stars will have exact same point spread function.
    When you look at how their light forms that hopefully circular distribution that fades from the center towards the edge, all of them will have the same full width half max, they’ll all have the exact same shape, and you can even, at a certain point, go, “Huh, they’re all slightly teardrop-shaped,” but all stars look the same. Now a galaxy, even if it’s far away, it’s not a single point of light the way a star is. Its point spread function is going to be larger, it’s going to be not necessarily the same shape, and by looking for clusters of things that have not-star point spread functions, and that’s where you have to star it as not-star, by looking at things with a not-star point spread function, you can say, “This cluster of things that I tried to do photometry on, this is actually a galaxy set,” and then you can start looking at them and going, “Does the profile of colors match the profile of a bunch of galaxies that are located at the same distance?”
    You can start to put all of these pieces together to map out the structure of our galaxy by looking at the density of stars. By mapping out the density of galaxies you can start to get at where are the clusters? Where is the large scale structure? And in this case yeah, some of that stuff is varying in light. You have black holes that get accretion disks that vary around them, but if I’m looking for a galaxy cluster off the bat, I don’t care about those variations. I just care that that point spread function is not a star.
    Fraser Cain: And so can you do things like, I can imagine like when you take, like, just look at a picture of the galaxy of Andromeda, and you just see the structure and you see the star clusters and the central core and all of it is pretty, but I guess if you’re doing photometry and you’re just carefully going across the galaxy bit by bit by bit, you’re going to get these changes in brightness that’s going to tell you things about what’s going on in there.
    Dr. Pamela Gay: And you start to use a different technique when you’re doing that. So in general when you talk about photometry, you talk about something that we say you can put an aperture around. You can draw a circle around the object you’re interested in, and you count all the photons inside of that. You then draw a donut around that aperture, measure the sky, the nebula, the whatever that it’s embedded in, subtract the average value of that donut, and that tells you about what’s inside of your aperture. When you start looking at measuring the brightness of distributed objects, of things that actually take up space across the sky –
    Fraser Cain: Like the sun or like Jupiter, would that –
    Dr. Pamela Gay: Well there you’re even getting into completely more different science. So we start talking about with galaxies what’s called surface brightness, where you start, instead of talking about what is the total brightness of the object, you start talking about what is the brightness per arc second on the sky. You start talking about what is the – Then you turn this around with math. And what is the luminosity per unit of volume or unit of area? Because you can’t actually get at volume, unit of area on the sky of what you’re looking at. And in this case, you’re looking at a changing boxcar average. Draw a box, move the box around, see how much light fits inside the box.
    It’s a different type of technique, but the detectors that you use for doing photometry on a whole bunch of little tiny things, a CCD, you can also use to do surface photometry instead of aperture photometry and start getting at that surface brightness profiling, start using for dust lanes, start forming your regions. How does nebulosity vary? It all comes down to how much light and what color. That’s astronomy.
    Fraser Cain: So I want to talk about some of the kinds of the research that we’ve been wanting to do. You’ve talked a bit about the highlights, but what are some of the different kinds of observations, the kinds of answers we’re looking for, and then what would our setup kind of be like? Like, for example, finding extrasolar planets, right?
    Dr. Pamela Gay: Well what’s kind of awesome is the setup is the same, and the technique is the same for all of the science cases. So, your setup is you need a device, charged couple device, photomultiplier tube, depending on if you are only looking at one objects or a whole bunch of objects, and then you need a telescope that has a very good drive system so that it very precisely moves across the sky, and hopefully an atmosphere that doesn’t fluctuate a lot, because the atmosphere can actually be what kills you more than anything else with photometry. And once you have that nice, stable setup that can precisely start counting photons, what you do pretty much runs the gambit, and I’ll start at the sun and move my way out. People who do solar photometry, depending on exactly how they’re set up, they can actually start to measure asteroseismology, helioseismology. They can look for tremors, pulsations, modes set up in the surface of our sun.
    Moving outwards as we start to look at asteroids, near earth objects, small chunks of stone moving through the solar system, when they pass in front of stars, if we observe them from multiple points on the surface of the earth, the timing of how they block out the light from that background star will start to tell us the shape and more information on the position of that rock moving through the solar system. If we simply look at the reflected light coming off of an asteroid we can start to get at changes in its shape as more of it is facing, less of it is facing, as we move out, you can take pretty pictures.
    You start doing surface photometry of planets. That’s different. That’s pretty pictures, use a box, measure what’s in the box. But then as we move out and continue to move further, when you start looking at stars, you can look for flares. You can look for pulsation modes that means, “This is this type of star that has this sort of, “They’re actually what are called harmonic oscillations. These are just like when you blow into a bottle and it vibrates at a set noise, you blow harder and it blows at a different set noise, and there’s no noise in between those two you can get. Well stars pulsate the same way. They have allowed frequencies. And what pulsation modes you get depends on the composition, the age, the size of the star.
    Fraser Cain: So you can measure the pulsation and then that tells you some of those other variables that you’re interested in.
    Dr. Pamela Gay: And over very long periods of time, and this was the first research I actually started doing, you can look at some of these stars and look for changes in their pulsation modes. And as you measure these changes in periods, and this was one of my first papers that I did, as you look for these changes in periods, you can actually start to get at the density changes that are going on inside these stars. And we’ve been studying variable stars for so long that there’s now measureable changes.
    Fraser Cain: Wow, that’s really cool. And so that tells you other things about even more refinements on what’s going on –
    Dr. Pamela Gay: Stellar evolution models –
    Fraser Cain: And what’s going on with the star, yeah. Yeah, I mean you think about all of these events, I mean, I know like, what would be the absolute holy grail would be to see, for example, a supernova precursor and to be able to watch changes of brightness weeks before it actually detonates. That would just be the greatest thing ever, to see a supernova precursor right before it goes.
    Dr. Pamela Gay: You know, and that’s all cool, but what I think is just as cool is, what about those moments right before a star goes from main sequence to spending a couple thousand years becoming a red giant, that that death to a main sequence star is the next thing our planet is going to have the joy and depth of experiencing, and so there’s so many other stages out there. As we watch all these different stars over time with more and more surveys, we’re going to start catching stars going from main sequence to red giant branch, moving on and off the horizontal branch, and we see variable stars change modes periodically showing us their steps through the evolutionary trails.
    Fraser Cain: I wonder if Gaia, like is Gaia going to be able to help out with this at all? It’s going to be really precise measurements of the positions of the stars, but I wonder –
    Dr. Pamela Gay: And very precise photometry. Part of it is getting –
    Fraser Cain: Yeah, I wonder if it’s going to start to pick up some of these – Because if you look at one percent of all the stars in the entire galaxy and watch for changes in brightness and you’ve got some of these events that you’re looking for, chances are you’ll catch some of these.
    Dr. Pamela Gay: Well, so you have three things that you need to worry about. One is getting precise enough data. And with Kepler, we had the most precise data that has ever been accumulated, and people who do variable star work basically have a treasure trove that it’s going to take us a long time to sort out. Now, unfortunately, stars that Kepler realized were variable stars didn’t get observed for the whole duration of the mission because those weren’t prime targets for finding planets, but there’s still some really amazing stuff in there. So getting the amazing data is part of it. Part of it is getting data long enough. Variable stars are changing periods over 100 plus years, and we have 100 plus years of data from the ground, but not from the sky.
    Fraser Cain: So is Kepler just, in addition to all of the work it’s done in helping to find all these planets, has it just dumped out mountains and mountains of garbage data for Kepler? Which is like, “Oh yeah,” the variable star astronomers are just digging through the trash with all of this, “Oh, you know what? It’s a variable star that no one’s ever heard of, but it’s not interesting to us, so you guys can have it if you want.”
    Dr. Pamela Gay: Exactly.
    Fraser Cain: I can just imagine how much of a treasure trove that is for the variable stars astronomers, because it’s been looking at so many objects.
    Dr. Pamela Gay: Well and what gets me is a lot of people are like, “Eh, we don’t care about variable stars,” but variable stars are what start to put the fine constraints on our stellar evolution models by saying, “Eh eh, you didn’t explain me. I’m doing this thing over here.” And so while a lot of people who admittedly need money and grad students and stuff will just ignore the variable star data because they’re deemed, “A solved problem,” because we know the basics, there is a whole lot of data – that it’s not as desperately sought after.
    Fraser Cain: Okay, so you were moving on. You got to variable stars –
    Dr. Pamela Gay: Yes, I got to stars.
    Fraser Cain: What about finding planets? I keep saying planets. I don’t know why you keep avoiding planets. You’ve done it again.
    Dr. Pamela Gay: No, I just haven’t got there yet.
    Fraser Cain: You’re not interested in planets at all, are you?
    Dr. Pamela Gay: I hadn’t got there yet.
    Fraser Cain: Okay, pulsars?
    Dr. Pamela Gay: So pulsars you can notice them in radio photometry, basically. It’s not all about the optical light. It’s all about the stable, precise setup. Yes, you can find planets, you can find binary stars, and then you can start looking at really neat physics. So there are systems with small compact stars, white dwarfs, neutron stars, sitting next to a companion star that they’re cannibalistically sucking material off of, and those accretion disks of material that build up around them will vary in brightness. So as we watch those vary in brightness and periodically undergo radical explosions and things like that, that’s photometry. Same physics applies to the accretion disks around black holes and galaxies, so we end up with active galaxies that flicker, and you can actually map out the course of the galaxies using photometry.
    Fraser Cain: Light curves of supernovae?
    Dr. Pamela Gay: Light curves of supernovae, yeah. It’s a whole lot of physics going on.
    Fraser Cain: Is there any sort of value even beyond that? Is anything done with the cosmic microwave background radiation? Because we know that it’s used for everything.
    Dr. Pamela Gay: There I don’t think you call it photometry anymore.
    Fraser Cain: Okay. All right. But there’s no point gazing at one spot in the cosmic microwave background radiation and just watching as it changes.
    Dr. Pamela Gay: Yeah, no. But with James Webb Space Telescope, with James Webb Space Telescope, we’ll be able to start doing photometry on the first stars and galaxies forming in the early universe.
    Fraser Cain: That is going to be mind-bending. Yeah that telescope, when it comes online, it’s going to change everything. I mean we just went through the big NASA 2015 budget, and I know everything you love was destroyed, but –
    Dr. Pamela Gay: There’s going to be no salaries left. We’re going to have the most amazing telescopes out there and no one working in astronomy.
    Fraser Cain: Now I don’t know if you’ve heard. I have volunteered the resources of Canada to help take over some of the maintenance of some of those projects like, I don’t know, Cassini, Sophia. Anyway, well thank you very much, Pamela.
    Dr. Pamela Gay: My pleasure.
    Fraser Cain: Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at astronomycast.com. You can email us at info@astronomycast.com. Tweet us @astronomycast. Like us on Facebook, or circle us on Google Plus. We record our show live on Google Plus every Monday at 12:00 p.m. Pacific, 3:00 p.m. Eastern, or 2000 GMT. If you missed the live event, you can always catch up over at cosmoquest.org. If you enjoy Astronomy Cast, why not give us a donation? It helps us pay for bandwidth, transcripts, and show notes. Just click the donate link on the website.
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