How did chemists save astronomy? What’s so important about the spectrum, and how are they made? How can we tell what things in space are made out of? I discuss these questions and more in today’s Ask a Spaceman!
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EPISODE TRANSCRIPTION (AUTO-GENERATED)
You know, I kinda feel bad for astronomers in general, but especially astronomers before the mid eighteen hundreds. I mean, by this time, they had telescopes, big telescopes, big observatories. They were studying the planets in the solar system. They were discovering moons and asteroids and comets. They were looking at nebulae and classifying them and drawing sketches.
They were observing double stars and triple star systems. They were measuring stars as they moved across the sky. I mean, they were learning a lot, but they had no clue what anything was made of. They were just making a bunch of records of where things were and what they looked like, but you could say, like, say, hey hey, astronomer, what's the sun man of? I don't know.
Why is Jupiter kinda orange? I don't know. Why is that star blue? I don't know. They just didn't know what stuff was made of.
And so I get this sense, and this is just probably my own personal bias. Then the mid eighteen hundreds when it comes to astronomy, astronomy itself as a field was just kinda stuck. Like, they they were just measuring stuff with just going along, like, okay. Here's another night, another map of the sky. Okay.
Oh, look. Comet. Yay. What's a comment don't ask us what a comment's made out. We don't know.
And it's funny when you look at the past few hundred years of science, how scientists in a completely different field can be working on something in parallel just for their own interest, their own field without realizing how they're about to completely and totally revolutionize another field. And in this case, it was started with Newton, Newton himself, studying optics and the behavior of light. And one of his favorite tricks was to take a prism and split sunlight into a bunch of different colors. And, of course, people I mean, people have just prism since forever. People had looked at the colors of light coming out of prism forever, but Newton did a really, really good job of writing down what he was seeing and proving that it wasn't the prism that was creating the colors because that's the obvious thing.
Like, oh, you shine light into this weird glassy thing, and then the glassy thing creates a bunch of colors. He was able to prove that white light itself is a collection of many different colors. And what was happening inside of a prism was something we call dispersion. Dispersion's the nerdy jargon term for what happens when white light or light in general gets split up into its component colors. And what happens with the prism is that the light enters one side and different wavelengths of light, like red light and blue light, travel at different speeds in the prism, and so they end up coming out at all slightly different angles.
So it's like a color sorter or a color spreader. Puts the blues way over here and the reds way over here and then the middle colors in the middle, so it splits up light. And his work with prisms and later work throughout the seventeen hundreds and eighteen hundreds, you know, if you're studying light, you need a bright light source. And this is before lasers. This is before light bulbs.
Really, the only way to get a bright light source was either from flames like a fire or the sun. There really just wasn't much else to work with back in the day. And so you see experiments, especially in the eighteen hundreds, of throwing stuff in fires and seeing what happens because that's basically all they had. And and one common experiment that physicists and chemists, you know, scientists will perform is to get a nice hot flame and then just just toss something in it, like some sort of salt. Put salt in a flame, and you get this big yellow flash.
And, like, wow. Big yellow flash. What does that mean? I don't know. Or they put it like a metal in the flame, and it and it glows blue, and there's a big flash of blue.
And they're like, wow. That's neat. With light sources available to them at the time, the sun and flames, that's all they had. Using things like the sun, they were able to figure out that light has a spectrum, that light is actually a combination of many other different kinds of light. And with the flame experiments, they were beginning to put the pieces together that different kinds of elements might have different spectra that just might look different when heated up.
Like, the kind of light that a particular element gives off is different for each element. And the person to really start clenching this in was Josef von Fraunhofer. Physicist, chemist, kind of sort of astronomer, overall smart dude, Fraunhofer developed what he called a spectrometer or spectrometer. It's a thing that measures a spectrum. You put in light in one end.
There's a big prism. The prism spreads the light out. Instead of just putting it on a screen and, you know, saying, wow, isn't that cool, which is what Newton did, You projected onto, say, a ruler or something that you can very, very carefully measure. So instead of just spreading out the blues and the reds, you say, okay. The reds are over here.
There's this much red. And I can measure how wide and how bright the red is versus the blue versus the yellow. This is a spectrometer, a thing for measuring the specter, for for quantifying it and characterizing it. And Fraunhofer was the first person to be really curious about these two different sources of light that everyone had available. You could if you wanted to do spectroscopy, which is the study of spectra, you can either use flames or the sun.
Well, he compared the spectrum. Now they had this really, really precise way of measuring spectra. He compared the light from the sun to light you get from a flame, And he actually found something very, very surprising that in the spectrum of the sun compared to a flame that there were little dark lines. There were little bits of the spectrum taken out. Now these dark lines are hard to spot.
No one spot them before him because he had a really, really, really good prism that could really spread the light out really well, enough so that these dark lines became apparent. And then he compared it to the flame, there's no dark lines in the flame, but there are dark lines in the spectrum of the sun. That was funny and no clue what was going on. He was just able to notice like, hey, by the way, I don't think the sun is on fire. Because fire makes a spectrum that looks like this and the sun doesn't quite look like this.
There are some missing things. But there's only so much he could do with a prism. Prisms can only spread light so much. The bigger the prism, the more you can spread the light. But the bigger the chunk of glass, the better you have to be at making sure it's a really clear, perfect chunk of glass, and that's just harder and harder and harder.
So he needed something better. And to finance this operation of making something better, he invented Patreon. Go to patreon.com/pmsutter to learn how you can keep this show and all all of my education outreach activities going. I sincerely appreciate it. That's patreon.com/pm sorry.
There's also links on the website, askaspaceman.com, which was not invented by Fraunhofer, but we can pretend it was. Instead, Fraunhofer invented something else to go better than prisms. Other people played with this. He did a really, really good job of it, so his name stuck to this, which is a diffraction grating. And a diffraction grating is a really, really, really good light spreader.
And it works on completely different physics than a prism, but it produces the same results. You put white light in or any light in and it gets spread out into all of its individual colors. So white light, you get a whole rainbow. And a diffraction grating works by having many, many, many, many, many, many, many, many, many, many, like hundreds or thousands of tiny little gaps all right next to each other. I've talked about in previous episodes the single slit and double slit experiments where light enters a very, very narrow opening and from there spreads out on the other side.
And if you have two slits, these two spreading out waves of light can interfere with each other. Well, you can do that with two slits or three slits or four or a bajillion. And what's key here, what makes this work as a thing to study spectra is that how light spreads out, how light interferes with itself on the opposite side of a very narrow opening depends on the wavelength. It's like you you send light through some slits of one specific color. Let's say you send blue light through some slits, the waves on the opposite side interfere with each other.
Sometimes they cancel out, sometimes they add together, and you'll get a pattern. You'll get this alternating pattern of stripes on the screen of blue. And then you send red through it, and you'll also get these alternating stripes of red but at a slightly different spacing because how the waves interact with each other depends on the wavelength of the light. And yellow and green and all the same, so at the opposite end of this diffraction grating, you get the colors all spread out, and it does it a much better job than a prism can. It can spread light far wider, and so you can get a lot more detail.
You can get a lot more resolution, and it's much easier once you get the knack for making diffraction gratings. They're much, much easier, much more compact than a big old prism. So the diffraction gratings will work as a color sorter, a color spreader. White light comes in or any kind of light comes in and the fraction grating says, okay. Blues, you're over here.
Reds, you're over here. Yellows, you're over here. Greens, you're right there. And and you just they end up coming out in different directions. A while later after that, a couple chemists, by the name of Kirchhoff and Bunsen Bunsen, you may be familiar.
Bunsen burner fame and Kirchhoff as in no one remembers Kirchhoff except for the nerds, but that's okay. They made a very key discovery here. And remember, these people aren't necessarily astronomers. Fraunhofer did some astronomy stuff like looking at the sun because it was an interesting light source. But really, he was just interested in optics in general.
He was just interested in lenses and diffractions and gratings and colors and prisms. He was just interested in that for its own sake. And Kirchhoff and Bunsen weren't astronomers. They were chemists. But these people who are studying light for their own interests are in the middle of totally revolutionizing astronomy.
And the big key came from Kirchhoff and Bunsen where they had, you know, they had a burner. They just called it a burner. They didn't call it a Bunsen burner. Then that name came later. They just had a burner.
And they were dropping stuff into the burner, which had been done before. These salt flame experiments had been done for decades, but now they were using a spectrometer. Now, they're being much more careful about it. Now, they were studying it in much more detail. And what they found in these flame spectra, the spectra of light coming off of things when they get really, really hot, is that certain substances, certain elements, when they got really hot, they gave off very specific colors of light that they could pick out.
Now you can pick it out with an eye. You can say, wow, that's a different color, which is what we had been doing before. But now Kirchhoff and Bunsen were measuring this and say, oh, no, no, it's more than this or it's more nuanced than this. They're giving off very specific spectral lines. When we see the spectrum that comes out of our spectrometer when we get things really hot, it's very, very narrow colors.
So think of a rainbow like this broad, you get all the hues of green and all the hues of blue and all the hues of red and so on and so on. But what was coming out of the spectrum was just very narrow like, you know, just just a little bit of red, very specific color of red, very specific color of green, very specific wavelength color of blue, etcetera, etcetera. They were very specific. They were learning that individual elements have their own spectral fingerprints, that when they heat up, you know, sodium, they get a certain set of spectral lines, not just a certain color, but very precise, very specific spectral lines. And then they heat up something else like helium or oxygen, and they get a completely and totally distinct set of spectral lines.
So they learned that these lines are absolutely critical, that you can use these lines to identify an element. An element gives a very specific spectral feature, a set of lines that you can measure with the spectrometer, which means you can go backwards. And if you can spot the spectrum, you can spot the element. But they didn't know why. And that is the most hilarious part.
They didn't know why certain elements had the spectra that they did. They didn't know why oxygen made these kinds of lines. Neon made these kinds of lines. Sodium made these kinds of they didn't know why. But for the nineteenth century astronomer, it didn't matter.
It didn't matter. Who cares how it works? What matters is that it does work. The nineteenth century astronomer is able to identify for the first time elements outside of the Earth. Like, oh, we can look at a nebula.
Oh, wow. There's lots of oxygen in that nebula. We don't know how spectral lines work. It's all voodoo. But we can verify in a laboratory that oxygen gives this very specific spectrum, and then we can point our telescope, attach a spectrometer to the business end of a telescope, and we can see the exact same spectral lines when we point it at a nebula.
And, wow, there's oxygen. You know? There's water in that nebula. There's iron in the corona of the sun. Like, there there's elements there.
Don't know how it works. Doesn't matter. But we can still do astronomy. So astronomy was rescued. Astronomy was saved using a technique that they didn't understand, and nobody understood spectra for, like, half a century.
Kirchhoff and Bunsen are operating in the mid eighteen hundreds. The answer to spectra would come with quantum mechanics, which wouldn't come until the early twentieth century. Imagine could we get away with that today? A technique, a a a thing that we don't understand how it works, but we're using it to gather really useful data about the universe. Could we do that today?
I don't think so. I don't think we'd be allowed to get away with it, but, you know, eighteenth century, nineteenth century, different time, different rules. You can get away with stuff you can't get away with now. I'm not talking about their beards. I'm talking about their astronomical techniques.
Eventually, quantum mechanics was able to explain spectra and, actually, the presence of spectra. We knew it was connected to elements. We knew it was connected to atoms. Somehow, it became one of the big pieces of the puzzle to drive towards quantum mechanics is how do we explain spectra. And the explanation for why different elements have different lines came with our realization that the energy levels inside of an atom inside of a molecule are quantized.
They are discrete. The electrons surrounding an atomic nucleus can't have any old energy they want. They can have only very, very specific levels of energy. And because they can have only very specific levels of energy, they can only absorb radiation at very specific frequencies or wavelengths, and they can only emit radiation or light at very specific wavelengths, very specific frequencies that corresponded to the differences in energies. It's like it's like a staircase.
It's like a staircase. Like, okay. If I wanna get up five stairs, I need a certain amount of energy to do it. So, I need to eat, say, five crackers. I can't go anywhere with just half a cracker.
If you give me half a cracker, that's not enough to make it over one step. I need at least one cracker to have enough energy to go up one step. If you give me three and a half crackers, that's not gonna happen. I can't hit half a step. I'm gonna stub my toe.
And do you want me to have a stub toe? No. So, you can't give me three and a half crackers. You can give me three crackers, I'll go up three steps. You can give me four crackers.
I'll go up four steps. But you can't give me half a cracker and expect anything to get done. And it works in reverse too. I can release energy by going down the steps. And I'll leave it to your imagination how that energy is released, the opposite of eating, I suppose.
If I go down one step, I release one cracker's worth of energy. If I go down two steps, crackers worth of energy. But I can't go down half a step. There's no such thing as half a step. There's only the steps.
So I can only emit or absorb very specific amounts of energy. In the staircase, the size of the steps in the staircase is different for every single atom, every single element, every single molecule. You have a different element, a different atom, a different molecule, then the staircase is gonna look different. The gaps between the steps will be larger. They'll be differently placed.
You know, there might be one big step and then a couple little steps and then another big step depending on the atom. It's very strange staircase. Okay? So different elements have different staircases. Different staircases means they can only absorb or emit very specific amounts of energy, which means you will only see that element associated with certain kinds of light, the kinds of light, the wavelengths of light that have the exact right frequency to match the exact right energy so that these jumps can happen.
Now it doesn't have to be just electrons around a nucleus or in a molecule, especially if you have molecules, a bunch of atoms linked together. There can be vibration. They can wiggle back and forth. They can twist back and forth. They can rotate, and those are going to be quantized too.
If you have a few atoms in a molecule and they're vibrating, they can't vibrate with any old energy they want. It has to be quantized. This is quantum mechanics. So that molecule can absorb a very specific amount of energy and jump to a higher rotational or vibrational energy state, and it can drop back down and release a certain amount of energy, a very specific wavelength of light, all stays the same. Like, it's all quantum mechanics.
It's all discrete. That's why you get the lines with these elements. Not broad continuous spectra. You get very specific lines. And there are different ways that you can identify these lines.
One is through emission. If you have a hot thing where the energy needed is coming from something else, from it just being hot, from slamming into each other, then that's where the energy is coming into the system, and then it's coming out through the emission of light. This is what Kirchhoff and Bunsen saw. They heated something up. They lit it on fire, made it nice and hot, gave it a bunch of energy.
The electrons in the atoms jumped to very, very high energy levels. They they got a lot of energy. And then they came back down this staircase. And as they came back down the staircase, they were releasing energy here and there in very discrete packets, very specific wavelengths generating the lines. Another thing you can get is absorption.
If you have if you're just a cold blob of gas and there's a bunch of light coming through you, then you might absorb some of that radiation. But you will only absorb the radiation that has the exact right energy to allow you to go up a few steps on the staircase. Like, if you get three crackers from some background light, then, man, you're going up the staircase. But if you just get, you know, half a cracker, you're not going anywhere. Usually, what we see in a typical astronomical scenario is both, where there's a little bit of emitted radiation.
There's some radiation absorbed. Sometimes you can absorb radiation, and so you'll see some missing gaps in a spectrum because that's the energy that those are the wavelengths that are getting sucked up. Those are the crackers getting eaten. But then the electrons come back down, and they generate some lines somewhere else. In the what gets absorbed, what gets emitted, it depends on very complex quantum mechanical rules.
Happy to jump in and do an episode about that. Just ask me about, you know, rules of quantum mechanics, rules of energy levels, forbidden transitions, It depends on the nitty gritty physics of each atom in each molecule. It determines what kind of jumps are allowed, what does the staircase look like, and so you can get both. Say there's a background source of light. Some of that light gets absorbed, so you get some missing gaps, but then the electrons come back down the staircase, reemit that energy, but in a different frequency.
And so you get emission somewhere else on the spectrum. It's not just limited to visible. There's also infrared and x-ray spectroscopy. Basically, if there's a wavelength of light, you can do spectroscopy. There was a great physicist in the twentieth century, I have to mention this, Peter Zeeman, who actually found that strong magnetic fields remember those magnetic fields, can affect the spectrum?
And this is because the little electrons that are in these atoms, they have spins. They respond to magnetic fields. And so if you have a strong magnetic field somewhere, you can actually get some extra lines. It's like taking a staircase. Like, okay, you take that step.
It's just one step worth one cracker. But if you add a strong magnetic field to it, you can actually split this step in half. And so you only need half a cracker to get up because now it's only half a step. That's what magnetic fields can do. And this is how we know that places like the sun have strong magnetic fields because we see this kind of splitting of energy levels, right there.
And so this amazing tool of spectroscopy that was invented and developed by people who are interested in optics, who were interested in chemistry, and explained by people who were interested in physics, you know, the whole quantum mechanics thing, but ended up creating perhaps the most important tool in astronomy nowadays. Yeah, we still take pictures. We still do very precise measurements in astronomy, but, man, a spectrum is worth a thousand pictures. Spectrums or spectra tell us how we know what stuff is made of. You know, you look at some nebula, and it turns out most nebulae are ridiculously hot.
They're very, very thin plasmas, but they're very, very hot. Man, they have emission lines. You can see the nitrogen. You can see the water. You can see the free hydrogen.
You can see the free oxygen. You can see gold. Like, this is how we know it's made of because it's hot and it has emission lines. Or there's a bright light source behind, like a star behind a cloud, and we can see the missing gaps, and we can figure out that way what the nebula is made of. This is how we discovered helium.
Helium comes from Helios, a Greek word from the sun. We saw some missing lines in that upper most layers of the sun, lines that hadn't been assigned to any other element yet. And so a new element was proposed right there, helium. This is how we're able to study atmospheres. This is how we know that there's, like, ammonia in Jupiter because we can see the sunlight reflecting off of Jupiter.
And as the sunlight reflects off Jupiter, the elements and molecules in Jupiter's atmosphere will absorb some wavelengths of that sunlight. And so we know what sunlight looks like when it's not bouncing off Jupiter, and then we look at it bouncing off Jupiter, and we can see the elements. We can do this when we send robotic spacecraft to other planets. They have little lasers on them. They shoot lasers at rocks, and they look at the spectra of what comes off.
And they can figure out what those rocks are made of. Like, isn't this it's almost like a superpower where you don't have to taste something. You don't even have to touch something. Something could be on the opposite side of the universe, and you know what it's made of. That's astronomy's superpower, and it was invented by accident.
And using spectra, it unlocked the vast majority of our conception of motion in the universe. We had seen we can literally watch stars move over the course of years and decades and centuries. We can see stars shifting their position. We can see planets move. That one's easy.
No points awarded for seeing a planet move. But our knowledge of movement in the heavens was severely limited before spectra. But what the spectrum gives us is it allows us to measure the redshift or blueshift because we can identify lines. We can identify that fingerprint, but we're like, like, hey. Wait a minute.
I see the fingerprint of, say, oxygen. It's clear as day, but it's all shifted over by a certain amount. Like, the pattern is there, but it's in the wrong spot. And that's because of redshifting or blueshifting. Motion towards us or away from us will shift all the frequencies of light uniformly the same, and so any emission, any absorption, any spectral lines will be there, but they'll be just be shifted towards the blues or towards the reds because of that motion.
And so this allows us to measure motion towards us or away from us, which turns out in astronomy is way easier than trying to measure motion perpendicular to our line of sight. So we can see stars moving in and out towards and away from us. We can look at a single star, and we can measure rotation rate because we can see blue shifting on one side and red shifting on the other. We do this all the time. This is what allowed us to very carefully measure the positions of or speeds of globular clusters, which gave us our first hint of the size of the Milky Way and where the center of the Milky Way is.
Fritz Zwicky, in the thirties, was able to do this to entire galaxies and realized that they're moving too fast inside of galaxy clusters, which gave us our first clue on dark matter. Vera Rubin did it again in the seventies with stars inside of a galaxy, and then Hubble himself, like this is the expansion of the universe, was discovered through the redshift of spectra. Lines, man. It's all about them lines in astronomy. Lines, lines, lines.
If you ever wanna know what astronomers care about, they care about the lines. Thank you so much for listening. Quick promo, AstroTours cruise. That's astro dot tours. We got cruise to The Caribbean, checking out mine ruins, having a great time.
Lots of stuff to do onboard too. Basically, you get to go on vacation with me. Is that sound like fun? I like to think so. Go to astro.tours.
And thanks to Shanna Yu on Facebook and Steven on email for the questions that led to today's episode. And please, please go to patreon.com/pmsudder to learn how you can keep this show going. I'd like to thank my top Patreon contributors this month, Matthew k, Justin z, Justin g, Kevin o, Duncan m, Corey d, Barbara k, Nudredoo, Chris c, Robert m, Nate h, Andrew f, Chris l, John, Elizabeth w, Cameron Ellen, Nalia. Their contributions and contributions from everyone else that keep this show going. Can't thank you enough.
Go to askaspaceman.com for show notes, episode archive. Hit me up on social media at hashtag ask a spaceman. You can also email askaspaceman@gmail.com. Lines, man. It's all about the lines.
See you next time for more complete knowledge of time and space.