Astronomers gather electromagnetic radiation with the telescopes: mostly visible light. But sometimes they’ve got to be clever about where they look for these elusive photons. Light can get emitted, absorbed, reflected, and each method tells astronomers a little more about what they’re looking at.
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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: Astronomy Cast Episode 398, seeing things. 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 Cane. I’m the publisher the universe today. With me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville, and the Director of Cosmo Quest. Hey Pamela. How you doing? Happy New Year!
Pamela: Happy new years to you. It’s 2016, and this means that we’ve been doing this show –
Pamela: for ten years, basically. So we started September of ’06. It’s now ’16. Kinda cool.
Fraser: There you go. We were podcasting before it was cool
Pamela: It’s true.
Fraser: We were podcasting before it was even acceptable.
Pamela: Hey, no.
Fraser: We were podcasting while it was still embarrassing.
Pamela: Yes, that’s true. That is entirely true.
Fraser: Cool. So we got lots of good shows planned for 2016.
Fraser: All the way through until our summer break. So hang on, it’s gonna be a fun ride.
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Fraser: Astronomers gather electromagnetic radiation with their telescopes. Mostly visible light. But sometimes they gotta be clever about where they look for these illusive photons. Light can get emitted, absorbed, reflected, and each method tells astronomers a little more about what they’re looking at. Okay, Pamela, so you put this on the docket. We’re gonna be talking about different ways that we see the light, and the different journeys that this light takes to make it to our telescopes and eyeballs.
Fraser: So where do you wanna start?
Pamela: Well, first of all, I wanted to describe why I came up with this crazy topic. Because this was one of those things that has never appeared on any list we’ve ever had, or been requested by any listener. But the other day I had a drive-by moment on Twitter, one of these zoom in to look something up on my computer and see this amazing photo on Twitter. And it was the Twitter equivalent of a driveway moment, where you have to pause to figure out what you’re looking at, like you do sometimes with an NPR story, where you sit in the driveway to figure out what the story’s about.
Pamela: And it was this photo that showed ionized – light emitted through the ionization process by a meteorite going through the atmosphere. It showed reflected light from a comet, and then it had all the stars and galaxies in the background. And it was one of these amazing moments of ionization, reflection, and emission, all –
Fraser: All in the same picture.
Pamela: And it was this moment of seeing these are the three ways that we’re able to see our universe.
Fraser: Okay. And so I guess that’s the part that people maybe didn’t realize before they were getting – listening to this specific show, is that there are these different ways that light – that photons get emitted.
Fraser: And that we can then absorb, them and each one tells a different story. So let’s start with the sort of the most common one, the source.
Fraser: Which is the emission.
Pamela: Exactly. So when we look at anything that is hot, it’s going to be emitting what’s called black body radiation. If it’s a stone, a rock, a star, a piece of asphalt, if it’s any of those things that Captain Kirk could light up with his phaser, all of this warm stuff – your hamburger does this if you eat a hamburger for lunch – all of this warm stuff is emitting light in a continuum of colors, an entire rainbow. And there’s going to be a specific color where it gives off the most of its light that corresponds exactly to the temperature of the object. As objects get warmer and warmer and warmer, they go from giving off the majority of their light in the infrared – which is what we humans do; we’re little black bodied thermal radiators – to giving off their light in colors that are more like green, which our son does, although our eye perceives the sun as white just due to our inability to perceive green really well.
Fraser: Well, but also just that it averages out, right? I mean this is the –
Pamela: It averages out, yeah.
Fraser: Right, that we are emitting, or that the sun is emitting photons, and we’re emitting photons, the cosmic background radiation is emitting photons. There’s photons coming from all these things.
Pamela: Any warm source, any hot source – and by warm I mean anything greater than, well, absolute zero – everything is emitting photons in this spectrum of colors to a point. Now the to a point is exceptions like if I look at a compressed sodium lightbulb, I’m going to see stark emission lines from the sodium, because that’s a pure substance that doesn’t have the ability to give off light in every single color. It doesn’t have thermal emission. It has emission lines, due to the chemistry involved.
Fraser: And so when we think about a thing that’s a color, right? When we think about, you mentioned the sun, but when we think about, I don’t know, a light that is green, or a light that is blue, then we’re seeing blue photons coming off of that object. We’re actually seeing, as you said, the black bodied radiation, we’re seeing a spectrum of photons coming from that object. And it’s just where the majority is that’s the color?
Pamela: It’s how they add up and are perceived by our human eyeballs. So there are stars out there that give off the majority of their light in the ultraviolet, because they’re extremely hot, bright stars. But our eyes don’t see those colors. But what we see is all the colors that are, well, visible in the colors our eyes can see. So we perceive these stars as bright, bright blue. At the same time, there are stars that have peak radiation out in the infrared, which again, our eyes can’t see. So we’ll perceive them as a deep, deep red, because they’re still giving off a lot of light in that tail of the block body spectrum that’s in the visible red.
So that’s hot things that are giving off thermal emission where it’s everything bouncing around and emitting every possible color of photon. Then, like I said, there’s also that emission lines that come from specific atoms going, “And now I shall let my electrons jump from this level to this level.” And when that happens, a photon is emitted for emission lines, or there’s also cases where they’re absorbed out, which we don’t see with our eyeballs, which is an absorption spectrum.
Fraser: So can you talk about that emission, those specific emissions? So if we take a, I don’t know, if we take a rock, and we heat it up.
Pamela: That’s thermal emission.
Fraser: Thermal emission. Right.
Fraser: But in this case, it’s giving off photons that are in the infrared. They are getting emitted and they are sort of across the range.
Fraser: They are literally just different wavelengths. Some are a shorter wavelength, some are longer wavelength, and we perceive the warmth from the rock with the eyeballs that are our hands that we feel the temperature coming off of the rock. And it’s an average temperature, and that is sort of, again, this is kind of like the average of all of those photons getting emitted. But you said that you can have situations where photons will get emitted in a very specific line, emission line. So what’s going on there?
Pamela: So here we have a different kind of situation. With the thermal emission, it’s everything bouncing around, and that kinetic energy, lots of complex physics, you end up with a continuum of photons coming off. If you have a single element or a cloud of specific elements, for instance inside of a fluorescent light tube, inside of that compact sodium bulb. If you then look at that specific chemical mixture using a pair of those rainbow glasses you can get in novelty stores, you’ll see the light get spread out into specific lines of color. And different lines will be different brightnesses.
So if you go and look at a bright red open sign, a neon sign that’s actually probably filled with hydrogen, you’ll see a really bright red line, and some other fainter lines. And what’s happening is you have the electrons. Well, that red line comes from an electron falling from being excited up to the third energy level in hydrogen and dropping down to the second energy level in hydrogen. And when the electron changes energy levels, it gives off a discrete photon that has a very specific energy that is a shade of red. When an electron goes between two other levels, two to one, that’s going to be a different color. And in that case, it’s a color we can’t see with our eyes.
Fraser: And this is very useful for astronomers.
Pamela: Yes. Yes. So for instance, if I’m looking at hydrogen at any place in the universe, I know that those hydrogen spectral lines, that three to two energy jump that gives off a photon, that two to one energy jump that gives off a photon, wherever I look in the universe, the photons are emitted at the same color. But if that object’s moving relative to me, perhaps getting carried away by the expansion of our universe, that color is going to get shifted toward the red if it’s moving away, toward the blue if it’s coming towards us via its own volition – that’s not going to happen with most galaxies. And I can measure that velocity, that Doppler shift, by looking to see how these known colors get shifted redward and blueward.
Fraser: And so in this case, right, astronomers have a very specific filter that they can put on to their telescope to only filter that very specific wavelength into their CCD, into their cameras, right?
Pamela: So we do that when we’re looking at things within our own galaxy, where we know that the relative velocities aren’t that great. We’ll throw what’s called an H alpha, a hydrogen alpha filter, for that three to two energy jump, and this allows us to focus in on the hydrogen in local nebulas. The hydrogen in local star-forming regions, and see them detailed. We also might do this if we’re looking at the sun, in which case we also use a lot of neutral density filters to bring down the amount of light we’re looking at.
But when we’re looking beyond our own galaxy where things can get shifted to radically different wavelengths, there are times that we use custom built filters to look for things at very specific wavelengths. But most often what we’ll do is look at an object through a spectrograph, through a device that’s designed to spread the light out into a rainbow by either putting it through a slit or reflecting it off of what’s called a grism.
Through all of these different techniques, sometimes using prisms and slits in combination, using all of these different techniques to create this rainbow, we can zoom in and say, “I don’t know how fast this object is moving. I don’t know what atoms are in it. But I’m going to look, and I’m going to identify these fingerprints of atomic spectral lines based on their spacing and say now, by looking at how this fingerprint is shifted redward or blueward, I can get the velocity of this unknown object.
Fraser: Okay, so that was the first part of your, I guess, of that comet meteor star image that you were talking about, and that’s the emissions. And that’s sort of this great fingerprint for seeing what speeds that galaxies are moving away from us, or towards us, even some of the chemical constituents of some of those galaxies. Which ones are having more star forming regions, things like that. So let’s go for the second one, then. Let’s talk about the ionization that you saw in that image.
Pamela: So this is where an object decides to make its reality known by heating up the stuff around it until the stuff around it gives off light. So, for instance, if we have just a grain of sand colliding with the earth’s atmosphere, and plunging down through that atmosphere, which is what happened during meteor storms, which some of you might have seen with the geminids in December.
When these particles plunge down through our atmosphere, they have a lot of kinetic energy that they lose to the atmosphere as they frictionally interact, they lose their velocity. The energy ends up getting transmitted to all of the particles around them. And one way of looking at energy is temperature. And when that transferred energy hits the equivalent of about 10,000 degrees, that will cause the hydrogen to ionize. Different chemicals ionize at different temperatures. At the end of the day, when we see that blue green streak through the sky, what we’re seeing is the oxygen, the nitrogen, all of the different atoms that make up, and molecules that make up our atmosphere are getting hit with so much ionizing energy that they give off light.
Fraser: So the meteor, as it’s passing through the atmosphere, is heating up the atmosphere. This heat is then going in to these atoms in the atmosphere, and when they hit a certain point, they’re then giving off more photons.
Pamela: They’re just flat out giving out photons. A normal, everyday nitrogen atom is just hanging out going, “Hi! I’m nitrogen.” That molecular oxygen in our atmosphere, it’s just sitting there being quiet, blocking light.
Fraser: Very stable. Yeah. Very stable. Happy to hold onto its photons
Pamela: Well, happy to hold on to its electrons.
Pamela: And as that meteor streak by going, “Hi! Here, have my kinetic energy.”
Pamela: Exactly. Those electrons are no longer so happy to stay in their energy levels, and they cascade to higher energy levels, and then drop back down. And it’s in that drop back down that they give off all of this light that we see.
Fraser: And so, again, what does this tell astronomers?
Pamela: Well, it first of all tells us, hey, there’s something high energy right there. There’s a meteor passing through the atmosphere. And, based on what colors we see, we can start to see what is at different levels in our atmosphere that’s getting excited.
One of my favorite examples of this interaction with our atmosphere is the Aurora Borealis, which occurs when high energy particles cascade along magnetic field lines in the earth’s atmosphere, give up their energy to the surrounding particles, and we end up with these amazing light shows. And depending on what color we’re seeing, that’s different altitudes in our atmosphere that are getting excited. That’s different molecules that are getting excited. So we can see, based on the colors of the aurora, how deep into the atmosphere the particles are plunging.
Fraser: So just to sort of discuss that situation, right, so you’ve got these high energy particles coming from the sun. They’re moving with a tremendous amount of velocity. The earth’s magnetic field sort of pulls them, directs them –
Fraser: as they interact through, directs them down into the atmosphere, and same thing as the meteors, they are giving off energy into the surrounding atoms, and those atoms are then having the electrons rise and drop in energy levels, and that is generating photons that we then see as this ionized radiation in the sky.
Fraser: The colors, right, we get these blues and these greens. Is there a correlation between the – what’s causing those colors to be the colors that we see?
Pamela: Different colors are actually coming from different atoms and molecules in our atmosphere. So when you see different colors, you’re seeing different parts of our atmosphere, quite likely at different altitudes, getting excited. And we did an entire show on this a while back, so go ahead and look up that show, and it talks about specifically what color corresponds to what atom.
Fraser: Okay. So we’ve got emission. We’ve got ionization. And the last one, I think, is reflection.
Pamela: Exactly. And reflection is how we understood pretty much everything we knew about our own solar system other than the sun up until about 50 years ago. Everything we knew about other worlds came from either sunlight reflecting off of them, or from our own attempts to bounce radar beams off of things, which is another way of reflecting light. So when we look at Jupiter in optical colors, or in infrared colors, the light that we’re seeing is largely reflected sunlight. When we look at Mars, it’s largely reflected sunlight.
Now the reason I say largely is these are large, warm bodies, and Jupiter, in particular, gives off excess energy, because it is generating thermal energy just by being a large collapsed ball of gas that’s getting crunched by gravity. Mars has still not fully cooled off, and is a warm object. But the majority of the light we see, especially in the optical and the infrared, isn’t that, “Hi! I’m warm” light. It’s instead reflected sunlight. And it’s kind of weird to think about if our sun shut off, suddenly we’d be blind to all of these objects.
And this actually led people to start thinking about, well, is it possible that dark matter is nothing more than planets that are so small and so far from their stars that they are, as they run loose through the universe, creating the appearance of there being dark matter? Don Winget, one of my faculty at the University of Texas many, many moons ago, made the comment that you could explain dark matter fairly well by roughly one acme brick-sized chunk per solar system-sized volume of space in the universe.
Now that assumes even distribution, you have to adjust the distribution of Acme bricks to get at our actual distribution of dark matter. Programs like the Macho project looked really hard for these small blobs of undetected planet masses and didn’t find them. So between the Macho project and our own detection of things via gravitational microlensing over the years, it’s becoming more and more clear that Acme bricks don’t explain dark matter. But there still are, certainly, planets out there that are, via various matters ripped from their stars that we will never see because they’re failing to reflect light in a meaningful way.
Fraser: And so the objects in the solar system are just one example of the kinds of things that you can see with reflection. You can also see nebulae, right?
Pamela: You can see nebulae through reflected light. Anytime you’re seeing a blue nebulae, that’s actually what’s called a reflection nebulae because the light is moving perpendicular through your line of sight, through the cloud, and then getting reflected off of particles and scattered back towards us here on earth. When you see a red nebulae, that is where the light, just like with the sunset, the blue has been scattered away, and the red makes it through straight to your eyes. So, with the red, that’s transmission. But when you see the blue, that’s reflected light.
Fraser: One of my sorts of favorite ideas in this sort of world is light echoes. And I think we did a whole show on light echoes. But just this idea that you can actually see this expanding sphere of reflection around objects, stars and things inside a nebulae, just blows my brain.
Pamela: And one of my favorite discoveries from the Macho project had nothing to do with what they were planning to do with the Macho project. But as they observed the areas around the large, I believe it was large Magellanic cloud, year after year after year, they were able to see these, what they thought initially we optical defects, these bands of light through their images, appear to move over time. And as the bands moved, it became clear these were not internal reflections. These were not defects in the images. These were echoes of long past supernova. So what happened at the dawn of civilization is still echoing through our universe, and what happened at the dawn of our planet is also still echoing, but a lot harder to detect, because those echoes are much more spread out.
Fraser: Yeah. And so if you miss the supernova on the first time, as it goes off, and you miss the light directly, the light is gonna escape through the surrounding nebulae, or the surrounding material that maybe the star had sluffed off years before, and that you can get other shots, other chances at seeing this light bounce back as it passes through these layers of material. It’s a fascinating concept. So, I mean, that’s another example. Are there any other examples, maybe, that astronomers will use reflection to study something?
Pamela: Well, one of my favorite examples of this builds on the concept of light echoes, and it’s V838 Mon. This is a very strange, strange star that had a couple of flares back in, I believe, the detection was in late 2002. It maybe have been early 2003. And these flares reflected off all of the gas and dust that surrounds this star, which is in a fairly dense pocket. And as the light continued to radiate and bounce off of more and more complex parts of the dust and gas around the star, we saw this evolving picture of clouds being illuminated one after another. And we learned about the structure around the star. We were able to detail how the color evolution of the flares took place by looking to see, okay, so this part is this color, this part is this color.
It gave us so much information about both the star’s flare event and the intervening matter. And then it was just plain weird. And anytime you can have something weird and unexpected and beautiful with the thing that you’re looking at forcing you to define news ways to define our universe, that’s kind of awesome. And this was one of those awesome things. And it happened to be a variable star. So I’m a fan.
Fraser: And there’s lots of other examples, really kinda cool tricks that astronomers have done with light. Like ways to study the reflectivity of earth by looking at how the light from the sun is reflecting off the earth and then onto the moon.
Fraser: And you can measure the earth’s shine on the moon. And then you can use that method in theory to search for extrasolar moons, to study extrasolar planets, all of these tricks that we’re practicing here in our solar system, will then have versions of that that are gonna be useful for studying whole other solar systems.
Pamela: And at the end of the day, the only way we can see the faint, tiny Kuiper belt objects that haven’t been visited by a certain little tiny mission called New Horizons is to look at them in reflected light. And we can guess at their compositions based on how much light they do and don’t reflect. Their albedo, is the fancy word for this. And highly reflective objects, we guess at being icier. Less reflective objects, we guess at being, well, a little less icy, a little more sooty or covered in carbon stuff. We have to guess at this information based on well how things reflect light.
Fraser: Very cool. Anything else on – or should we wrap this episode up? We’ve talked about the three ways that we see the light.
Pamela: I think we’re good. And I’d encourage everyone to go out and force yourself out of bed when the weather says it’s going to be clear, and take a look at the Catalina comet that’s up right now. And maybe you, too, can catch that moment of a shooting star, a comet reflecting light, and all of the beautiful stars in the background.
Fraser: Yeah. It’s very close to Arctururs right now, I think.
Pamela: So easy to find.
Fraser: Easy to find.
Pamela: And lots of information. Just give it a Google.
Fraser: All right. Thanks Pamela.
Pamela: My pleasure.
Fraser: Thanks for listening to Astronomy Cast, a non-profit resource provided by Astro Sphere New Media Association, Fraser Cain, and Dr. Pamela Gay. You can find show notes and transcripts for every episode at Astonomycast.com. You can email us at email@example.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 pm Pacific, 3:00 pm Eastern, or 2000 Greenwich Mean Time. If you miss the live event, you can always catch up over at cosmoquest.org.
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