Ep. 128: Dust

Dark nebula in Orion.

Dark nebula in Orion.

You can’t make a Solar System without a whole lot of dust. And that’s the problem. This dust has blocked astronomers views into some of the most fascinating parts of the cosmos. It shields the galactic core, enshrouds newly forming stars and their planets, and blocks our view to churning supermassive black holes, actively feeding in distant galaxies. But new telescopes and techniques are allowing astronomers to peer through this dust, and see these events like never before.

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    Dr. Pamela Gay: Hey Fraser. It’s so good to have you back finally.

    Fraser Cane: I know…I know. It’s been about…I’m ashamed to say. I have never been so sick in my entire life and it was some crazy head cold and I lost my voice…like completely. Like we’re two weeks after I’ve lost my voice completely and I still haven’t totally got my voice back. But it’s good enough to record a radio show, so let’s do it.

    Pamela: Yay. I have my co-host back.

    Fraser: So thank you very much to Chris and to Bob and to Scott and to you, of course, for filling in while I was gone. Hopefully we’ll be able to move on without any more breaks. Lots of question shows…lots more cool episodes. Speaking of…let’s get on with it.

    You can’t make a solar system without a whole lot of dust and that’s the problem. Dust has blocked astronomers’ views into some of the most fascinating parts of the Cosmos. It shields the galactic core and shrouds newly forming stars and their planets and blocks our view to churning super massive black holes actively feeding in distant galaxies. But new telescopes and techniques are allowing astronomers to peer through this dust and see these events like never before.

    Alright, well…let’s talk about dust.

    Pamela: It is the smallest, most annoying thing in the universe.

    Fraser: Yeah. So, what would be dust? If I was to go out and grab some dust with my spacesuit on and bring it into a spaceship and take a look at it, what would I be looking at?

    Pamela: Well, most cosmic dust is made out of some sort of silicate materials; things that are based around silicon atoms and is about the same size as the particles in the smoke from a cigarette smoker. But it’s just a slightly different composition.

    This is material that comes off of in a lot of cases, very old senior citizen stars. The red giants they breathe out this material as they’re going through their final days of life. But we also find that it comes in huge amounts from supernova remnants. In some cases you can get 6,000 times the mass of the sun in dust coming out of supernova remnants.

    Fraser: So this silicate material is just kind of being breathed out by stars. It’s being exploded out by supernova and then it’s just kind of collecting? How is it obscuring our view?

    Pamela: Well, and it’s about 6,000 times the mass of the Earth when dust comes off of a supernova. Sorry about that minor unit issue there. So basically if you try and look through the smoke from a cigarette smoker and I’m just going to keep returning to this because it’s the right size grains, you can’t see through the smoke necessarily with your eyes.

    It’s because light trying to pass through the smoke it hits the individual grains and then it reflects off in a different direction. This is called scattering. It can also get absorbed by the individual grains of dust. This causes the molecules to vibrate; it can increase the velocities of the particles.

    So you have both absorption and scattering processes going on that prevent light from passing straight through the dust. In fact, if you have a large cloud of dust in space, a reflection nebula, and you have a star off to the side, light might be coming from off to the side trying to go horizontally through your field of view. But it encounters a cloud of gas and dust and it scatters off of the dust grains towards you.

    What we find is blue light is scattered a lot more than red light. Shorter wavelengths are easier to scatter. So when we look at reflection nebulas, they are largely blue objects because that’s what is getting scattered. In fact, that is why our own sky is blue.

    Fraser: And those are the ones that are given off by supernova.

    Pamela: The reflection nebula can be collections of gas and dust from supernova, or it could be just general gas and dust that is hanging out within our galaxy that has gravitationally gotten pulled together.

    This is one of the amazing things is dust is everywhere. But it has variable densities as we look through the universe. Just like the stars, just like dark matter it is affected by gravity and it sinks down into the gravity walls of galaxies. One of the really neat results that comes out of the Sloan Digital Sky Survey recently, is they looked at the light from distant quasars, distant galaxies that have super massive black holes in their centers that are actively feeding on material and thus giving off huge amounts of light.

    That light as it passes through the outskirts of nearer-by galaxies, the light gets shifted to the red because the blue light is getting scattered off in other directions. We are able to see that depending on where the light cuts through the different galaxies, we get different amounts of what we call reddening. This is preferential removal of the blue light through distant objects.

    And so galaxies have a halo of dust around them. But then within that halo, they have knots that are coming from star-forming regions, knots that are coming from the remnants of supernova. In the case of supernova, that is where the dust is getting generated. In the case of other types of nebula, it is just where the dust is gravitationally collecting together in a pile like a dust bunny in the corner of a stairwell.

    Fraser: And because I mentioned in the intro, one of the most difficult things…we can’t even see the center of our own Milky Way.

    Pamela: Yeah this is a serious problem. If you go out during a good summer night, the plain of the Milky Way disc for people in the Northern Hemisphere is bright through the sky during the summer.

    In a lot of cases you’ll see that it is called the Milky Way because it looks like someone spilled milk on the sky if you are in a very dark location. This is the combined light of thousands, and thousands, and thousands of stars packed together in the disc of the galaxy.

    But woven through this bright spilled milk are dark snakes of dust. So what you have is in other places the dust is collected within the spiral arms of the galaxies while dragged in by the over density of mass in the spiral arms. As it collects you get areas where the more dust there is the less light can get through.

    Just like on a foggy day, as the fog gets thicker and thicker and thicker, your ability to see through it goes down and down and down. What is interesting is because of the difference in how dust scatters light as a functional color, if you try and look through the disc of the galaxy, you’d see a really blue light.

    If you try and look through the disc of the galaxy in even ultraviolet, you’re not really going to see much. And the bluer the light is the less you’re going to see because the dust is scattering so much light. But as you start looking through in the red, you start seeing more. Then as you start looking through in the infrared, these really long wavelengths are able to pass right through the dust.

    Fraser: Right. And this is the solution.

    Pamela: And this is, in fact, how Andrea Getz has been able to do her wonderful work measuring the orbits of stars going around the black hole in the center our own Milky Way. She looks through in infrared and is able to peer through all of this dust.

    Fraser: The infrared revolution has only come about the last couple of decades, right? And now we’ve got space telescopes and the upcoming James Webb telescope, which is going to be the most powerful space telescope ever built and is going to be largely an infrared telescope.

    Pamela: It has required some serious technological breakthroughs to get to the point that we can look through this annoying dust and start to understand the centers of our galaxy, the centers of other galaxies. The problem is that while you and I are both infrared flashlights, anything that is warm is giving off infrared light, so the atoms in your body, my body, anything that is warm, the electrons periodically jump energy levels.

    As they jump to lower energy levels, they give off light that is in the infrared. The color of the light corresponds to the temperature. This means that if you have a warm telescope, the atoms in that warm telescope are going to have electrons jumping around at different energy levels. Those jumps are going to correspond to infrared light. So if you’re trying to look through a telescope that is itself flooding the inside of the telescope to you with the color of light you are trying to detect, you’re going to blind yourself.

    We had to figure out how to cool telescopes down to the point that all of this thermal noise, all of this infrared radiation that just comes from transitions and atoms could get blocked out. What we end up doing is cooling telescopes with liquid helium in some cases getting as close down to absolute zero we can.

    With the James Webb space telescope, they have had to go to amazing lengths to figure out how to shield the telescope from the light from the sun. That’s really the ultimate way to cool down a space telescope. You just prevent it from getting hit with sunlight and there is nothing to warm it up.

    Fraser: Right I can sort of imagine as an analogy it would be like installing a bunch of spotlights on the inside of the Hubble Space telescope. You know, turning them all on while you’re trying to look at Hubble.

    Pamela: And that’s just a really bad thing.

    Fraser: Yeah, that would be no good; it would be of no use. Now I know that with the Spitzer space telescope they are using liquid helium on it to keep it cooled down. But there is only so much liquid helium that they can launch on the spacecraft.

    Pamela: Right. And so this limits the lifetime of the telescope. It’s not quite like the Hubble Space telescope which works mostly right now on optical wavelengths and ultraviolet, which allows it not to have to worry about the cooling.

    But any time we want to go into the infrared, we have to worry, well how much coolant can you have on board and that limits the useful time for getting infrared data. The Hubble Space telescope did have infrared observing capabilities, but it runs out of its coolant as well over time.

    One of the problems we had early on with one of the infrared spectroscopes was that it ran out of fuel far faster than anyone had imagined and so it ended up with a much more limited life. Every time we service Hubble, we end up having to reinstall more coolant.

    Fraser: Top off the tank.

    Pamela: Yeah. Basically, just like you have to replace the batteries now and then. These are all of the different things you have to worry about when you’re trying to work the infrared.

    Fraser: So, I mentioned a bunch of places where dust comes into play. You talked about a sort of halo that surrounds the galaxies. We’ve talked a bit about how the dust obscures the center of the Milky Way. Now, what about newly forming solar systems?

    Pamela: Right, so and humans see bits of this as remnants in our own galaxy. As stars form, they have an accretion disc of material around them. You can imagine this giant blob of gas and dust that as it begins to collapse, it begins to spin. It is pretty much impossible to hit a blob of gas and dust so that you exert all of your force exactly on the center mass of the system.

    Any off center mass forces will start the whole system spinning. It’s a problem with torques and conservation of angular momentum and things. As the system spins, it collapses down flatter and flatter into a pancake just like making a pizza dough by flinging it into the air and spinning it.

    In the system you end up with the star in the center, but there is also a disc of material that we can see around young stars. This disc of material includes a lot of dust. This dust can glop together over time, gravitationally attracting, chemically attracting, and form planets and the planets will suck up more and more materials clearing the way.

    But there is leftover dust at the end of the solar system process. So when the star turns on it also will send out huge amounts of radiation, huge amounts of light pressure as a result. That clears a lot of the material in the solar system.

    The stuff that is leftover, this can cause what we see in our own system. This is a zodiacal light which is light that we see is sunlight scattering off of dust that is along the elliptical on the zodiac within our own solar system.

    Fraser: I’ve never seen it. I’ve had a couple of articles on Universe Today about the zodiacal light. Have you ever seen it?

    Pamela: Only at McDonald Observatory. You have to be someplace where it’s extremely, extremely dark.

    Fraser: Right. And can you sort of explain what you see?

    Pamela: It’s basically this after twilight, on a moonless night, you can go out and you can see this faint oval of glow directly opposite from where the sun has gone down on the horizon. If you don’t know north, south, east and west on the sky, you might think “oh, that’s the direction the sun just set in and that’s just the sky glow from the sun right below the horizon, that’s setting in the twilight.”

    Or it might look like light pollution from a city off in the distance. You have to know there is no city off in the distance. This strange glow that is exactly opposite from where the sun set, and looks strangely like light pollution is actually the scattering of sunlight off of dust in the solar system.

    Fraser: That’s pretty cool.

    Pamela: We have this inner-planetary dust in our own solar system. We are even regenerating this planetary dust in some ways. Where when you get comets passing through the solar system their tails leave basically a chain of dust behind as they go through the inner solar system.

    We pick up on the densities of this dust as our planet passes through the tails or the place where the tails formally happen to have been located. We end up with meteor showers. So meteor showers are really dust showers in the atmosphere of our planet.

    Fraser: Right. So you look at some of the damage done to the space shuttle and to the international space station. They are getting hit by little pock marks of dust from collisions.

    They are still going very fast, still lots of them out there. They’re not really going to damage the shuttle, but still it is like it is getting sand blasted, right?

    Pamela: Right. And it’s all this little dust that is basically the size of cigarette ash. That is what the majority of it is. Sometimes you do get stuff that is millimeter in size, more like sand I guess than dust. It is these larger particles that can cause serious harm to astronauts, to space crafts. Luckily the majority of the stuff is much more along the lines of cigarette ash. It does make it a pain though to look out and understand the entire universe.

    It gives us a misdirection of what color things are on the sky. We have to make maps of the dusts so we know where the dust is in the Milky Way. Now we’re starting to figure out where the dust is in other galaxies as we look out. It’s everywhere and we have to take it into account as we consider everything that we do.

    Fraser: Now, I’m going to anticipate a question from the listeners. Which is that, can this dust be an explanation for dark matter?

    Pamela: People have spent a long time trying to figure that out and the answer is no. There is just not enough of it. If dark matter were nothing more than dust, the consistency of cigarette smoke, we wouldn’t be able to see out of our galaxy.

    One of the amazing things about the universe is that we can see all the way across, or at least we can see all the way back to the shell of where the cosmic microwave background radiation was released at the right time in space.

    That we can see that far across the universe means that all of the mass that is necessary to understand the motions we see, that we explain by saying there is a component of matter that’s dark.

    If that was all dust, which we know blocks light, which we knows scatters light, which we know absorbs light, all of that dust would be blocking out distant quasars. It would be blocking out distant galaxy clusters. It would basically make it as though we were standing in the middle of a fog bank where we could see a swarm of fireflies near our head, but distant swarms of fireflies their light would be absorbed by the fog.

    We would also see a much stronger infrared signal coming from all around the universe because this dust does absorb and re-emit light from the background. So we not only see this opacity, this basically blocking of background light that is coming in shorter wavelengths…the blue’s, the ultraviolet’s, but we’d also see this constant haze of infrared from the dust emit the dust absorbing and then re-emitting the longer wavelengths of light. We don’t see either of those things.

    Fraser: Right so it’s almost like there’s 10 times as much dark matter as there is regular matter. If you were surrounded by a halo of that, as you said, it would be like you were in a fog. So you probably wouldn’t even see other galaxies. It would just be dust.

    Pamela: Right and so since we do see other galaxies; since we don’t see this swarm of red light in all directions, we have a good sense of pretty much…no, dark matter is some mysterious other thing we still need to work on trying to sort out.

    We do find using careful studies of huge numbers of galaxies and looking at how light from distant quasars is absorbed as it passes through these galaxies, that there is a fairly strong density of gas and dust. There’s about one-thousandth of a solar mass of dust. What we find is in the halo of a galaxy, you can get through the type of dust that we can see in the small magellanic cloud for instance.

    A normal size galaxy might have ten to the seven, or about a million solar masses of dust and given the size of the galaxy in million solar masses of dust isn’t a huge amount of dust to be dealing with.

    Fraser: But there is kind of another mystery which is that even of the regular mass that stars and planets and dust and so on, a good chunk of half of that is missing, right? We can’t account for that, so could that be dust?

    Pamela: This is one of those things is of the more normal stuff, yeah. We’re finding that dust is exists in more than we had previously thought. We’re working to revise or understanding of how much dust there is in, for instance, normal elliptical galaxies.

    When I was first learning astronomy it’s amazing how much it has changed from the early ‘90s. It was elliptical galaxies are old, red, dead, don’t have gas, don’t have dust. What we’re finding now is no they do have a halo of dust around them and it’s changing the color of background quasars that we see through these halos.

    Fraser: So, I guess with dust, is it an annoyance that astronomers are looking to clear out of the way using it for technologies? Or is it a genuine research topic on its own, you know what I mean?

    Pamela: [Laughter]. Yeah. It’s both.

    Fraser: It’s kind of like for some people it’s got to be like “I don’t want dust…I wish there was no dust.” And for other people it’s very much like “well this is my life’s work. This is what my thesis is on. Keep the dust please.”

    Pamela: Now, I have to admit I’m on the “Oh God, dust. That means more math calculations.” There are amazing maps of the sky. There is one called Schlagof, Finkbiner and Davis from 1998.

    It goes through and uses data from space-based telescopes to very carefully map out the distribution of dust within our own Milky Way galaxy. Depending on where you look at the sky, the brightness of an object outside of our galaxy will vary in some cases as much as a magnitude just because of how much dust there is between us and the object within our own galaxy.

    Fraser: Right. And the magnitude is twice as bright, right?

    Pamela: About two and a half. This can be a huge difference. We have to look up the maps, do the calculations, and figure out “okay if I’m observing in this wavelength, that it gets affected this way; if I’m observing in this wavelength that gets affected this way.” Do lots of ugly math. We have software to handle it, but it’s still an annoyance.

    Fraser: Right. It’s like every object you’re trying to look at and know its brightness you’ve got to calculate for these maps on dust to just know how bright it is for real.

    Pamela: Right. But at the same time we know that dust is an important component of how stars form. It’s an important way of you can get really cool, we call them polysysticaromatichydrocarbons –it’s just a cool name for a fancy molecule– that just formed out in space in the interstellar media.

    These are complex molecules often rich in carbons and organics. Just knowing that you can get all of these different things forming out in space is really neat chemistry that we can’t do here on the planet Earth.

    We don’t have the long periods of time for these very slow chemical reactions to take place to form molecules in the same ways that we see in the inner stellar clouds. We also get some of the crazy molecules in space that we get on the planet Earth

    We get sheets of graphene which is basically sheets of carbon atoms that are just layer, upon layer, upon layer of molecule that basically line up on top of each other much like looking at layers of rock in a cliff face.

    All of these things can form out in space. Just trying to understand all these different things that form is really cool and that’s all there is to it.

    Fraser: One of the other things that I find quite funny is that we take for granted the way the solar system forms. The form of a nebula of gas and dust, collapse down and rotates, you get a protostar and it’s shrouded in gas and dust. You get this protoplanetary disk around it.

    But it is the protoplanetary theory that although astronomers have found some potential protoplanetary disks, it’s really hard to prove this theory because at the moment of when all the stuff is happening, it has been shrouded in dust.

    Pamela: This is where looking at the sky in radio, looking at it in millimeter wavelengths, looking at it in infrared; we can see the first seeds of someplace a star is going to form by looking out in the millimeter.

    As the gas and dust collapses, it heats up. It is this heat light escaping through the gas and dust that we see as “Whoo, there’s going to be a star there.” Then over time, as it gets warmer, and warmer, and warmer, we see the amount of light coming out and going out; we see the color of the light changing. It is only by looking through the dust in these longer wavelengths that we are able to get a sense of what is probably happening in this completely enshrouded area.

    As we look out in some of the richest star forming regions, we see looking out in the great nebula, looking out in the Orion star forming region, we see what are called proplets. These are little cocoon-shaped blobs of dust and gas that have a seed of the star in the center that is in the process of collapsing down, heating up, and eventually starting nuclear reactions.

    But they’re not quite to that final step yet. It is strictly infrared light passing through warming up and heating this blob of gas and dust. That is all we see for a long time.

    Fraser: Yeah. So hopefully in the next few years we can get better and better images of actual protoplanetary disks in formation and see the planets forming. But that’s going to take some pretty big telescopes.

    Pamela: This is where ALMA, which is a brand new large radio ray that is getting built down in the Atacama Desert, is going to be so important. This is working out in the millimeter, working out in the radio-type wavelengths.

    As they get more and more disks up, more and more dishes out there, it is going to work sort of like the Very Large Array except in a different set of wavelengths working more in the millimeter.

    These combined dishes spread out over a huge area of land are going to allow us very high resolution images through the depths to see what is going on in the course of these proplets.

    Fraser: And what is the state of ALMA? Parts of her are already complete right?

    Pamela: There are a few dishes that are already up and running. If you want to learn more about ALMA, we actually have a February 4 episode from 365 Days of Astronomy, which is at 365daysofastronomy.org. You can go learn more about this new telescope that is working out in the millimeters and sub millimeter.

    It is a joint program between the European Southern Observatory, the National Science Foundation, Canada, Japan, Taiwan, Chile, all working to get all of these dishes spread out and it’s hopefully going to be done around 2011.

    The first dish was delivered back in 2007. There were getting them online one at a time. These are 12 meter in diameter dishes and there is going to be probably more than 50 of them by the time they are all done spread out in the desert.

    It’s a great program and it in collaboration with the James Webb space telescope that works in the infrared; it’s by looking through all these different wavelengths that will allow us to see different amounts through the dust. We are able to fully figure out how our universe is working and fully map out where the dust is located everywhere. Everywhere in the universe.

    Fraser: And finally that dust problem will be solved.

    Pamela: [Laughter] Or at least we’ll understand the dust a little bit better.

    Fraser: Except for those or you who research on it, then we respect the dust. We love it. Alright, well thanks, Pamela. That was great. We’ll talk to you on our next question show.

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