October 16th: Our Growing Galaxy

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Date: October 16, 2009

Title: Our Growing Galaxy

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Podcaster: Sue Ann Heatherly of NRAO

Organization: National Radio Astronomy Observatory (NRAO): http://www.gb.nrao.edu/

Description: Looking at images of the beautiful grand spiral galaxies in our Universe, it’s hard to imagine that they are dynamic – changing – and evolving. Our own Milky Way Galaxy is still growing – swallowing up smaller galaxies and primordial clouds. The details are in the gas – the hydrogen gas. Join us as NRAO astronomer Dr. Jay Lockman tells us more about our dynamic Milky Way (and why the stars in Orion and Ophiuchus are so bright!)

Bio: Sue Ann Heatherly is the Education Officer at the NRAO Green Bank WV site. She comes to astronomy by way of biology (BA in 1981,) and science education (MA in 1985.) She visited the Observatory as a teacher in 1987 and knew she’d found Camelot. She has been employed with the NRAO since 1989.

Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by the National Radio Astronomy Observatory, celebrating Five Decades of Training Young Scientists through summer programs. Explore the hidden universe in radio at www.nrao.edu.

Transcript:

Sue Ann Heatherly: Welcome to this edition of 365 Days of Astronomy podcasts. I’m Sue Ann Heatherly. I’ll be joining you again for this episode. And today we will be talking with Dr. Jay Lockman, and we’re just going to have a conversation about the Milky Way. Is that right, Jay?

Dr. Jay Lockman: That’s right.

Sue Ann: So, why don’t you start off by getting all of us on the same page and tell us what the Milky Way is.

Dr. Lockman: Well, if you go outside on a dark night and look at the sky over you, you’re certain to see a band of stars, a band – almost a cloudy band – that stretches across the sky from one horizon to another, and that’s the Milky Way (it was actually named by the Romans,) a very visible feature of the night sky. Once you get away from the city lights, if you know what to look for, you can see the Milky Way. And what the Milky Way actually is, is our home galaxy. It’s a system of stars – about a hundred billion stars, most of which are kind of like the sun – that’s held together by it’s own gravity and swirls around like a giant pancake. And the Sun is about half way out towards the edge of the Milky Way. The Milky Way doesn’t have a sharp edge; it just kind of trails off. So, the Sun is not quite out in the boondocks, but almost that way.

Sue Ann: So, I know that people have seen pictures of galaxies. I’ve heard our Milky Way called a Grand Spiral Galaxy. Does it look like a pinwheel?

Dr. Lockman: Well, it’s hard for us to know exactly what the Milky Way looks like, because we’re smack-dab in the middle of it. If we could get outside and look back, then we really do expect that we would see it like a giant pinwheel. The brightest parts of the Milky Way – places where new stars are being made and they light up their surroundings – would probably be arranged in a big spiral pattern. We see other galaxies not too far away from us where we can get a good view of these processes, and so we think we know pretty much what the Milky Way looks like, although we’ve never ever really been able to photograph it from the outside.

Sue Ann: What tools do astronomers use to try to study the shape of the Milky Way?

Dr. Lockman: Well, the first thing that people used was star light, because stars are the most visible constituent of the Milky Way – there are a hundred billion of them, as I said – and their light can be seen… with the naked eye, you can see stars that are quite distant. But the big problem there is that, in addition to stars, the Milky Way has an atmosphere, just the way the Earth has an atmosphere. It has gas clouds. It has a gaseous component to it, uh, that fills the space between the stars. And along with this gas, there is little particles of interstellar dust; that’s exactly like the kind of dust we have on Earth. And if you could imagine trying to look through a dust storm, then that’s the situation that astronomers find themselves when they try to look long distances through the Milky Way. There is just so much dust that it stops the star light. So, even the most powerful telescopes trying to look at stars – telescopes like the Hubble Space Telescope – can’t… just can’t see very far when they look smack-dab in the middle of the Milky Way. But we can take advantage of the gas, because it itself can be studied; only this time not by its light, since it only rarely gives off light, but by the radio waves that it naturally gives off. Most of the gas is composed of hydrogen, the simplest atom and the most abundant element in the universe, and hydrogen gives off radio waves at a very specific frequency. So, we can tune our radio receivers on our big radio telescopes into that specific frequency and actually count the number of hydrogen atoms in a given location of the sky. Now, just like radio waves will come through the walls of your house, or into your car, you can just pick up the radio waves from the far parts of the Milky Way that come right through all the gas and dust. So, using radio astronomy, we really can probe to the outer most parts of the Milky Way and study what’s going on there.

Sue Ann: That’s what you do, isn’t that right?

Dr. Lockman: That is what I do, yes.

Sue Ann: Tell us what we’re learning by looking at the hydrogen atoms, the radio waves from the hydrogen in the Milky Way. What have we learned?

Dr. Lockman: Well, one of the big intellectual advances in the past couple of decades in understanding galaxies like the Milky Way is in appreciating that even though they look like beautiful, isolated systems, they’re far from it. One of the first books – written now probably seventy-five years ago – about galaxies, referred to them as island universes, because they do look like splendid isolated systems. But we now know that reality is a lot more messy, and in many ways more interesting. It’s tempting to think of a galaxy having been formed in the distant past, billions of years ago, when a huge cloud of gas collapsed and formed all these stars. But what we realize now is that the process of galaxy formation and evolution is an ongoing one; that galaxies are not closed systems but are constantly interacting with their neighbors, swallowing smaller galaxies in the vicinity, even being bombarded with remnants of material – we don’t even quite know where they are coming from – but in the same way that Earth is bombarded by meteorites and grows by about a hundred tons every night. The Milky Way and other galaxies are in this process of a glomeration and are still growing and making new stars.

Sue Ann: One of the things that was reported on quite extensively, back in 2008 I think, your mapping of a big cloud of hydrogen gas that was heading toward the Milky Way.

Dr. Lockman: Yeah, there’s, there’s sort of two ways that the Milky Way acquires new material, and one is by gobbling up its smaller neighbors. We see an example of that right now where there is a smallish galaxy being torn apart with its interaction with the Milky Way and being made part of the larger Milky Way. But the other way matter can come in is, is much more primitive matter in the form of giant hydrogen clouds. We believe that we see several hydrogen clouds, each of which could make a million stars like the Sun, that are in the process of being drawn into the Milky Way and added to the Milky Way’s gaseous atmosphere. One of these in particular we were able to study with the Green Bank Telescope a few years ago and really see it coming under the first attractions from the Milky Way. It looks like a huge comet, and we can predict that in about thirty million years it’s going to plunge into the Milky Way. It will be pretty, pretty far from the Sun when it does, so there is no need to worry. But in about thirty million years, it’s going to plunge into the Milky Way, bringing fresh gas that is then able to produce many more generations of stars. So, in this way, we can see the process of the Milky Way growth happening.

Sue Ann: So, this material that you’re able to detect with the Green Bank Telescope, and other telescopes around the world, how do you know it’s material that’s primordial or, or not part of our Milky Way? How do you… how can you tell – differentiate – between these exterior clouds of gas in our own galaxy?

Dr. Lockman: One of the easiest ways of differentiating between gas that’s already part of the Milky Way, and gas that’s somewhere else and may be coming in, is by the velocity that it has. The gas in the Milky Way marches around in a very orderly fashion. You can imagine, like, horses on a carousel or a merry go round, everything is pretty much in lock-step (going in the same direction) at about the same speed. But then as we look around with our radio telescopes, we sometimes see these clouds that have a very funny velocity. They’re clearly not part of the rotation of the Milky Way, and yet they’re… they appear to be hanging around in the vicinity of the Milky Way. Those are the ones that we think are perhaps condensing out right now out of very hot gas around the Milky Way, that are gradually being drawn in towards it, and will crash into it some day and add their matter to ours.

Sue Ann: Is anything spectacular supposed to happen when this gas crashes into our Milky Way?

Dr. Lockman: Well, theorists have speculated that when you get one of these big gas clouds crashing into the Milky Way, you could really have some fireworks. You’ve got to imagine that this cloud is coming in quite fast, and its gas is running into the gas in the Milky Way, setting up shockwaves. And in those shockwaves, you could get gas compressed and form a lot of new stars very quickly. So, it’s quite possible that there will be a burst of star formation and a lot of very big, bright stars formed at about the same time when one of these clouds lands in the Milky Way. Now, a very interesting thing is, that astronomers have puzzled for a long time about some of the brightest stars that we see in the night sky. Most of the stars in the Milky Way lie in that disk of the Milky Way – they lie in the band – and the Sun does as well, but the brightest stars in the vicinity of the Sun don’t seem to follow the same rule. Some of the bright stars that are in Orion – for example, the Orion Nebula – is substantially below the Milky Way. And on the other side of the sky you’ve got the stars in Ophiuchus, which are substantially above the Milky Way, and it looks like something triggered a burst of star formation many tens of millions of years ago in the general vicinity of the Sun. And people have theorized – hypothesized – that it was these clouds that plunged into the Milky Way somewhere near the Sun and actually made much of the structure in the night sky that we see with our eyes; that that didn’t arise randomly, but by something like a large cloud hitting those.

Sue Ann: That’s pretty fascinating. Are there any, uh… is there any gaseous evidence, any hydrogen evidence, that’s leftover from something like that? Do you see any structure that follows these bright stars?

Dr. Lockman: Yeah, that’s a great question, because, in fact, when radio astronomers were able to turn their telescopes on the regions of the bright stars, they found hydrogen gas clouds expanding away from us; very gently, but, none the less, a gentle expansion as if it… kind of a big interstellar doughnut. So, it really all does fit – all the little pieces fit – that the area around the Sun was affected by something like a crashing gas cloud.

Sue Ann: Your research is just opening up more and more and more detail about the Milky Way. I kind of thought the Milky Way was finished; that everybody knew everything about the Milky Way.

Dr. Lockman: Oh, no, there are still some great mysteries going on, and I must say that we know from looking at the history of the Milky Way roughly how much gas has to be coming in. And, even now, we can only account for a fraction of it; maybe ten percent. So, somehow the Milky Way is still acquiring matter and we don’t quite know where it’s coming from or how it’s getting here. There is still a lot to be learned about this.

Sue Ann: Great. Well, this has been fun. I’ve enjoyed our little chat today, Jay. Thanks a lot for joining us.

Dr. Lockman: You’re welcome. It was great to talk with you.

Sue Ann: That’s our show for today. For the National Radio Astronomy Observatory and 365 Days of Astronomy podcasts, I’m Sue Ann Heatherly. We’ll talk to you next time.

End of podcast:

365 Days of Astronomy
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About Sue Ann Heatherly

Sue Ann Heatherly is the Education Officer at the NRAO Green Bank WV site. She comes to astronomy by way of biology (BA in 1981), and science education (MA in 1985) She visited the Observatory as a teacher in 1987 and knew she’d found Camelot. She has been employed with the NRAO since 1989.

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