Image credit: Argonne National Laboratory

Image credit: Argonne National Laboratory

at are the many sources of light in the universe? Why is a spectrum so important? What’s the difference between synchrotron and Cherenkov radiation? What’s the best word ever in physics? I discuss these questions and more in today’s Ask a Spaceman!

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Music by Jason Grady and Nick Bain. Thanks to WCBE Radio for hosting the recording session, Greg Mobius for producing, and Cathy Rinella for editing.

Hosted by Paul M. Sutter, astrophysicist at The Ohio State University, Chief Scientist at COSI Science Center, and the one and only Agent to the Stars (http://www.pmsutter.com).

 

EPISODE TRANSCRIPTION (AUTO-GENERATED)

Let's talk about the spectrum. Let's say it out loud together because it's gonna come up a lot in this episode, and we better get used to it. So on the count of three, with a clear confident voice, all together now, we will say it together. One, two, three, spectrum. Thanks.

Thank you for participating. This is not the only word I'm gonna ask you to say out loud today so you better get used to it. More on that later. For now, the spectrum. The spectrum of light is by far the most important tool at the disposal of the astronomer.

If you wanna be an astronomer, you must get friendly with the spectrum. The spectrum is simple. It tells you how much of what kind of light is coming off an object. How much of this wavelength? How much of that wavelength?

How much of every wavelength? How much green, but more detailed than it? How much lime green? How much kelly green? Asparagus green, neon green, sea green, mantis, malachite, springbud, viridian, hunter?

Even more detailed than that, how much light at 500 nanometers, which is green light? How much at 520 nanometers? How much at 530.1 nanometers in beyond the visible? How much radio? How much microwave?

How much infrared? How much red, green, blue, ultraviolet, X-ray, gamma ray, the works. How an object interacts with light is determined by what it's made of. So by looking at the light in a really detailed way, we can tell what an object is made of. For example, look at some grass.

It's green. Right? Not necessarily the grass in in my yard, but but, you know, healthy grass. If you looked at the spectrum of grass, there would be a big spike around 500 nanometers, which is what we call green. The chlorophyll in the grass is absorbing all the colors of sunlight but reflecting some around the wavelength of 500 nanometers, and hence, we see that green light.

Look at some red paint. The paint might be red because it has iron oxide in it, and iron oxide has a specific spectrum that peaks in, you guessed it, red. Imagine you were looking at a barn that had some red paint, some good iron oxide paint on the side of that barn. But imagine you went so far away from that barn that the entire barn is just a tiny dot, a tiny dot of light. So far away, you couldn't even tell if it was a barn or a house or a sailboat or a tree.

It's just a tiny dot of light. There's a tiny dot, but no image. It's too small to make an image, but you can still take that light and collect a spectrum. You can collect a spectrum and divide that light up into all its little wavelengths and look at where the peaks and valleys are. The spectrum from that barn, even though you don't know it's a barn, the spectrum will reveal a specific spike around the color red, the signature of iron oxide, and you might conclude that there was a barn in the distance even though you can't see an image of the barn.

That is another reason why the spectrum is so important. One, it's because it helps us tell what things are may have. And two, in astronomy, almost everything is just a tiny little dot of white. And you can hardly ever build an image in astronomy, but with any amount of light, you can still build a spectrum and learn a lot. Many cases, you can learn more than by having an image.

And if you're in a glowing mood, you're giving off light. Giving off light, emitting light, creating a spectrum is all about changing energy at a molecular or subatomic level. The stuff that makes us us is constantly, well, doing things. It's wiggling and jiggling and dancing. Sometimes light can strike it, adding energy, and sometimes light can be emitted, removing energy.

I was going to say that this constant exchange between light and matter is a hidden world surrounding us, but it's it's literally visible. We are awash in radiation of all kinds all across the spectrum. In this radiation, this spectrum of radiation, it's constantly interacting with matter. Some of it we do see the visible light. Some of it we feel, the infrared heat.

Some of it we don't see, but it's always there. Matter and radiation are constantly interacting with each other and constantly changing the spectrum. You might emit light or something might emit light with a particular spectrum, but then it interacts with matter and that will change the spectrum depending on what that thing is made of. The process of changing the spectrum in this way is is due to, like, scattering and reflection of light. This is what we especially see here on the earth.

Most of the colors, most of the spectra of what we see is due to the scattering or the reflecting or the absorbing of other sources of light, light like the sun. The light gets bent, transformed, absorbed, reemitted, the works. How that light gets scattered or reflected? Well, it depends. In the case of our atmosphere, we have some nitrogen and oxygen molecules, actually a lot of them floating around.

Those molecules are much smaller than the wavelength of visible light, and this will scatter the light in a very peculiar way. The blue wavelengths of light, the the shorter higher energy wavelengths of light end up bouncing off of these objects. You can think of it as bouncing if you want. Bouncing off these objects and getting scattered all over the place while the red light, the longer wavelength stuff just passes on through. This is called Rayleigh scattering, and this is why we have a blue sky.

We have white light coming from the sun. Blue light gets scattered. The red light passes through. White minus blue equals yellow. That's why our sun looks yellow.

But if you look at, say, a cloud, a cloud is made of water droplets, and those water droplets are about the same size of the wavelengths that are getting scattered. And so all wavelengths end up bouncing around equally, and we get just white light. This is called my scattering. My scattering is why clouds look white even though the sky looks blue because they're made of bigger stuff. They scatter that sunlight in a different way.

And as the clouds get thicker, of course, there are too many layers for the light to get through, and the cloud ends up looking totally black. There's lots of pretty colors in the world, like I said, and those colors are determined by how much of that white sunlight and what kinds of that white sunlight, whether preferentially red or green or blue or anywhere on the rainbow, their individual atoms and molecules reflect or absorb. It's the interaction between sunlight and objects that give us the color and variety of the world. Scattering and reflection also, of course, happens out in space, but it's far less important. Most stuff in space is just various shades of gray because there's a lot of dust particles floating around, sticking to surfaces, being boring.

Sometimes you get some very pretty colors like the iron oxide in Mars atmosphere or the ammonia in Jupiter. These will scatter and absorb and reflect the white sunlight from the sun and produce some very pretty colors. There's also something called zodiacal light. There's a bunch of dust in our solar system itself and these will very, very gently, a little bit at a time, reflect scatter sunlight. And you can see that a band of light following our solar system, this is called zodiacal light.

Outside our solar system, the biggest examples of scattering or reflecting light are the reflection nebulas. These are blue. These are nebulas, clouds of gas and dust. They're nearby very bright stars. The bright stars illuminate them.

The redder like the redder colors just pass on through. The blue like gets scattered and bounced around, and so we see it. These exact same reason why we see a blue sky, we see blue nebula. But all this is not making your own light. It's taking somebody else's light and changing the spectrum.

There are a lot of processes out there that do indeed make their own light. And the number one source by a huge margin of emitted light in the universe is through a process known as black body radiation. Say it out loud with me. Go on. Black body radiation.

I've used the word before, but here's the quick definition again. It was developed in the late eighteen hundreds, this term, because a bunch of scientists were trying to figure out the thermodynamics, the the properties of heat, the relationship between heat and radiation, and they were using special devices that were coated in all black. They were bodies that were black, hence, black body, and they would glow in a very particular way, hence, black body radiation. Black body radiation is the light given off by hot things. If you're hot, you're glowing in with black body radiation.

The spectrum of black body radiation is very broad. Almost all wavelengths are represent at least a little, but definitely not equally. All the atoms and molecules inside of us are, like I said, wiggling and jiggling and bumping and rubbing and flipping and rotating and swinging and waltzing and cha chaing, all sorts of energy changes going on as they interact with themselves. This causes all sorts, all sorts of wavelengths of light to be released. The hotter the thing is, the more it can cha cha or rotate or flip, the higher energy light it can release.

And for light, higher energy means shorter wavelengths, which means bluer colors. So while any blackbody will give off light of all sorts of different wavelengths represented across the spectrum, there will be a peak. And this peak is connected to the temperature of the object. For example, the sun surface temperature is 6,000 Kelvin. It emits as a blackbody that peaks in the visible, but also a lot of infrared, a lot of UV, also radio, microwaves, X rays.

People are black bodies too. But our temperature at our body temperature, we don't peak in the visible light. We don't visibly glow. We glow in the infrared with basically no visible light coming off of us. But we do give off microwave radiation.

It's true. It's true. You can take a satellite dish. You know, the satellite that may you might use to get TV, that is a microwave detector. It uses microwaves to communicate with the satellites in orbit.

Humans glow just a little bit in the microwaves frequencies that can be detected by those satellite dishes. So you can actually rig these things up, One of these satellite dishes to a sound machine, and you can have people walk by it or wave their hands and they'll start going. You know, however you decide to connect the detection of microwave emission to a particular sound, but they can detect it. We're we're glowing in microwave. The hottest stars in the universe are so hot.

They give off black body radiation that peaks in blue and ultraviolet. In the dimmest stars, the weakest stars, they give off light down in red and infrared. And, of course, let's not forget the biggest black body of all out there, the cosmic microwave background emitted when the universe was just 380,000 years old. That light is very, very cold, just three Kelvin, three degrees above absolute zero. So its black body peak is firmly deep, deep in the microwave, much further than the microwave emitted by humans.

As a side note in understanding black body radiation, a dude named Max Planck accidentally invented quantum mechanics and immediately regretted it, although he still accepted the Nobel Prize for his work. But that's another story feel free to ask. So there's a lot of glowing things in the universe. There's a lot of black body radiation being emitted in our universe. There's also a lot of non glowing things, but those are different episodes.

Most of that light is due to the wiggling and jiggling of molecules like this black body radiation. But there are also individual atoms involved, and atoms can there do their own kind of internal dance. Inside of atoms, electrons would love to fall into the nucleus because of mutual attraction, but they're prevented by, well, some rules that say they can't electron degeneracy pressure. If you'd like to listen to that episode again, electrons can only pack themselves so tightly together. This is what supports things like brown dwarfs and white dwarfs.

And inside of atoms, it forces the electrons to separate into shells, arranging themselves like a teensy tiny onion around the nucleus. And the electrons can hop up and down between the levels. These are discrete levels like steps on a staircase or onion layers or, you know, shells, whatever metaphor you wanna use. The electrons can hop up and down between the levels if they give or receive just the right amount of energy. So if an electron gets just the right amount of energy, it can pop up.

It can be promoted to a higher shell. And if it gives off just amount the right amount of energy, it can be demoted. It can go into a lower shell. And you can only do entire steps at a time. You can't do a half step or a twentieth of a step.

That's like a fundamental basic quantum mechanics. You can do one of those big awkward jumps to do two or three steps at a time, but your minimum is one step, one shell at a time. So if light of just the right energy hits the atom, it can bump up an electron up to a higher level. And if electrons jump back down, they will emit that exact same frequency. And the energy levels, the differences in energies between these shells, the the height between the steps is different for every atom, for every element.

Each element has a unique spectrum, a fingerprint of emitted energies. Energies are wavelength. So if we heat up a gas, if we take a gas, a pure hydrogen, a pure neon, a pure oxygen, and heat it up, give it lots of energy, those electrons will start jumping up and down between their levels. They will start emitting light. They will emit light at very specific wavelengths, very specific colors.

Or you can have a hot glowy thing behind a cloud of gas, and that light might just have some of that light will have just the right frequency, just the right wavelength, just the right energy to bounce up the electrons, in which case the electrons in that that cloud of gas and dust will suck up very specific wavelengths of light, not letting it pass, in which case we would see the spectrum of the thing behind it, might be a star, might be a bunch of stars, might be a galaxy, whatever. Very nice spectrum, but there'll be dips in it. There'll be holes. There'll be missing gaps where the intervening gas cloud is sucking up light of just the right wavelength. We call this line emission because the spectrum looks like lines.

We have scattering. We have blackbody. We have line emission. There is so much more, child, when it comes to glowing things in the universe, especially once you get to very high energies. Because at very high energies, charged particles do some really funky stuff.

Why does that matter for radiation? Because radiation, all radiation including light, are themselves wiggles of electricity and magnetism. Light is a traveling wiggle of electricity and magnetism. If you start moving around charged particles, accelerating them, you start changing an electric field, which makes a changing magnetic field, which makes a changing electric field, which produces light. If you start moving around charged particles, you set up just the right conditions to generate light, like a radio antenna.

Think of a big antenna. There's a bunch of electrons bouncing up and down in that antenna. They're moving back and forth. As they do it, they are generating electromagnetic radiation. They are generating radio waves.

And this radio waves get emitted in all directions surrounding that antenna. Now imagine a super antenna, something that you could use to to shove those electrons close to the speed of light. Like, let's not mess around anymore. Let's just get serious. This is typically hard to do back and forth.

There's a lot of started and stopping involved. So instead, you bend your antenna into the form of a circle and use some super magnetic fields to keep taking these electrons, shoving them around and around and around and around and around and around. You get the point. At these tremendous speeds, there's a lot of energy. There's a lot of emitted radiation, but because of the high energies, this radiation gets pushed up to really, really high frequencies, really, really short wavelengths all the way up to x rays.

And because of this relativistic effect called length contraction, which I haven't talked about very much, feel free to ask. Very cool topic. The twin of time dilation is length contraction and relativity. If you wanna know more, just ask. The radiation that did get emitted all in a shell around a radio tower, now at these high energies get shoved down into a narrow beam like a searchlight as the electron whips around in a circle.

This kind of radiation does have a broad spectrum of very peaked in those X rays. It was first discovered in space because nature is awesome, but we can make this kind of radiation ourselves using devices called synchrotrons. So say it with me. This kind of radiation, due to electrons charged particles moving around in the circle really fast is called synchrotron radiation. We see it in super massive black holes.

We see it near white dwarfs. All you need is a source of energy, some magnetic field, some electrons to make a move in a circle. Boom. You got synchrotron radiation. Synchrotron radiation is what happens when you shove electrons around really, really fast in a circle.

But what if instead we took those electrons and blasted them through something else? That sounds like fun. Right? Right. Say you blasted through water.

You took some electrons traveling at close to the speed of light and blasted through water. That sounds awesome. Well, the speed of light itself can change in a medium. Right? We have the speed of light in vacuum, but it slows down.

The speed of light in air is slower. The speed of light in water is even slower than that, which means it's possible for stuff like an electron to go faster than light in faster than light in that medium. You can ever beat light in a vacuum, but going through a medium like air or water gives you like a handicap advantage. This is a very curious case because so far we've been looking at examples of electrons, changing energy levels inside of an atom, moving and especially being accelerated either back and forth in an antenna or in a circle in a synchrotron. In this case, we can have an electron moving in a straight line with no acceleration at all and still emit light.

Why? Because it's traveling faster than the speed of light in that medium. It is gives off a kind of radiation that is like a sonic boom for light. Just like you generate sonic booms by going faster than the sound speed, you build up a wake, all the wakes that you leave behind as you plow through this medium like air or water, end up building in on themselves and pushing in front of you. That is a sonic boom.

This is a a light boom. It's kinda hard to make. You need just the right materials, and you need to generate high energy electrons. First detected by a guy named Pavel Cherenkov. So it's called, say it out loud with me, the kind of light given off by particles when they go faster than light in a medium is called Patreon.

Go to patreon.com/pmsutter to learn how you can support all of my education and outreach efforts. Huge thanks to all of you. It's Cherenkov radiation. Its spectrum is very smooth, very smooth. It's not like liney like liney mission, but it's also very narrow.

It's very, very, very peaked, much more peaked than synchrotron emission. And really the only kind of light that it emits is very high energy light blue ultraviolet because, you know, it's a high energy system in order to make this happen. Okay. So we've covered radiation by things moving a little, radiation by things jumping levels, radiation by things moving really fast, radiation by things moving really, really, really fast. What if we go all the way?

What if about radiation by things moving ludicrously fast? You know that energy equals mass. Right? Good old fashioned e equals m c squared. Get rid of the c because that's just a conversion factor.

What are you left with? E equals m. Energy is mass. Mass is energy. Two sides of the same coin.

Just like flipping a coin, matter and energy can switch sides. You can convert radiation into particles. You can turn radiation into particles and vice versa. But to turn radiation into particles, you need to give it enough energy to cover the cost of the mass. So if you have some really intense radiation, like say gamma rays, the hardest of the hard stuff, a single photon, a single bit of light can convert a gamma ray of sufficient energy can convert itself spontaneously into two particles, an electron and a positron.

If it has enough energy greater than the mass of the positron and electron. Why the two? That's a different episode, just ask. And in the particle world, what goes around comes around. If you get a bit of matter and some antimatter, which is like matter but opposite charge, you put them together, boom.

You convert mass into pure sweet unfiltered energy, aka radiation. So a single bit of light of sufficiently high energy, the really ridiculously high, ludicrously high energies can spontaneously become matter and matter and antimatter can collide and turn back into radiation. You can convert from energy to mass if you have the enough energy. This radiation is produced by the annihilation of two particles, and it doesn't really have a special name. It does have a very specific spectrum.

The annihilating particles, the ones that are slamming together have a definite mass, and hence there's a definite energy involved, so the light will be of a very specific wavelength. So there's one more, one more that I wanna tell you about today. And I saved the best for last, not because it's the weirdest or the most exotic or the most dangerous or the most cheeky, simply because it has the most awesome name I've ever encountered in all of science. The name is are you ready for it? Are you ready for it?

It's Bremsstrahlung. I'll say it again. Bremsstrahlung. It's German for breaking radiation, and we definitely, definitely need to say it all loud together. Yes.

Even you in the back. Ready? One, two, three. Bremsstrahlung. Savor that moment, folks.

Savor it. Hold on to it. It's not often that you get to learn such a fantastic word. How often do you get to learn a word as awesome as bremsstrahlung? I may have created this entire episode just so I had an excuse to say the word bremsstrahlung.

Bremsstrahlung is what happens when you have an electron buzzing around minding its own business closely encountering an atomic nucleus. The nucleus is positively charged. The electron is negatively charged. They're gonna attract. If the electron is slow enough, the nucleus can just capture it, making it an atom, and that's the end of the story.

But if it's too fast or too far away, the electron will simply get its path deflected, almost like a comet grazing past the sun, but, you know, really tiny and with charged particles and with electromagnetism, not gravity. The electron's path gets bent. It slows down a little bit. That is a change in energy. That means a little bit of light escapes.

Since this only happens in really hot or energetic systems, we only ever see Bremsstrahlung in the form of x-ray radiation. We see Bremsstrahlung most commonly in clusters of galaxies. These are the most massive gravitationally bound objects in the universe. These are homes to thousands of galaxies. The The galaxies themselves are buzzing around inside of a cluster like bees in a beehive, and they're swimming through a very hot but very thin soup.

A soup that we call the intracluster medium. The the stuff inside a cluster is a hot thin soup and it's made of electrons, it's made of nuclei, they're constantly interacting and countering each other, changing, getting, deflecting, moving around, generating light in the form of Bremsstrahlung. So, yeah, moderately interesting process, infinitely interesting name. I'd like to thank Rick Biss on email and either l also on email for asking today's questions. And, hey, have you bought my book yet?

Why not? Go to pmsutter.com/book. You should be ashamed of yourself. Also, go check out the movie UFO on streaming platforms. That was the science consultant.

Lots of fun stories about that movie. Let me know. You know what? Let me know if you'd like me to do a special episode about the science of UFO and the science that went into the movie and how I consulted for the film and the the role I played, I'd love to. I'd love to.

Feel free to ask. Also, go to astrotours.co for trips to Costa Rica, Ireland of, Colorado for CMFOS. All sorts. It's just so cool, man. So cool.

Just go astrotours.co. Of course, spaceradioshow.com if you'd like to chat with me live. Big thanks to my top Patreon contributors this month, Justin g, Kevin o, Christy, and Helghan b is your contribution. And all the contributions of your compatriots that keep this show going and all my education outreach activities going. If you don't wanna contribute, that's cool.

Please go to iTunes and leave a review. That really helps. You can go to askaspaceman.com for the show notes, discuss the show, put questions, follow-up questions on the episodes. Happy to answer them. You also follow me directly on social media.

My name is at Paul Matt Sutter on all channels. You can hit me up there with questions. You can email askaspaceman@Gmail.com, and I'll see you next time for more complete knowledge of time and space.

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