Date: June 19, 2009
Title: A Modest Black Hole
Podcaster: Sue Ann Heatherly
Organization: NRAO: http://www.gb.nrao.edu/
Description: According to UCLA astronomer Mark Morris, the black hole at the center of our Galaxy might be modest, but it isn’t shy! From stars whizzing around it, to flares coming from it, listen as Mark describes some of the ongoing efforts to measure the properties of our very own Galactic Center Black hole. For more information about his research and some really cool movies, visit http://www.astro.ucla.edu/~ghezgroup/gc/pictures/
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 NRAO.
Transcript:
SUE ANN HEATHERLY: Welcome to this addition of 365 Days of Astronomy. I’m Sue Ann Heatherly. I’ll be your host today, and joining me today is Dr. Mark Morris from UCLA. So, welcome to the program today, Mark.
DR. MARK MORRIS: Thank you. I’m glad to be here.
SUE ANN HEATHERLY: So, you study – among other things – the very center of our galaxy. But you can’t see all the way to the Galactic Center, can you?
DR. MARK MORRIS: That’s right. When you look toward Sagittarius, you see the Milky Way, for sure, but the center of our galaxy is hidden by all the dust that resides in our galaxy.
SUE ANN HEATHERLY: So, how do you study the Galactic Center?
DR. MARK MORRIS: Well, some of the light can get through, but not visual light. The light that can get through is infrared light, radio waves, or even x-rays: those can all get through. So, the center of our galaxy wasn’t really appreciated until about the 1960s when radio astronomers and infrared astronomers first peered in our direction and saw that, hey, there is a lot going on there. They’re. . .in fact, the infrared astronomers saw. . .it, at, uh, wavelengths about, uh. . .about five times longer wavelengths than the visual light that we usually look at starlight with; that, uh, there is a concentration of light right at the expected dynamical center of our galaxy, the point about which all of the stars in our galaxy are orbiting.
SUE ANN HEATHERLY: So, they saw a concentration of light there. What did they think it was?
DR. MARK MORRIS: Well, they immediately interpreted that as the central star cluster of our galaxy. As you go closer and closer to the very center of our galaxy, the density of stars gets higher and higher; and so, there. . .there’s a tremendous concentration of stars. And the stars that we see there are dominated by the most luminous stars – the, the Red Giants, the old Red Giant stars – that are a thousand or even 10,000 times as bright as our Sun. But they’re very red, and so, in fact, that. . . in the infrared is the perfect place to look for them, because they’re also putting out a lot of infrared light.
SUE ANN HEATHERLY: And have you been studying these stars near the center of our galaxy?
DR. MARK MORRIS: Yes. And, in fact, with the Keck Telescopes, we have for many years, since about 1995. At UCLA we have a team of people who have been peering right at the center of the galaxy with the highest possible spatial resolutions so we can see deep down inside the central cluster of stars and, and find the stars that are closest to the center. I have to stress that the density of stars there is extremely high. If. . . for every star that you see in our nighttime sky out here in the suburbs of our galaxy where we live, there is about a million stars that you would see in the nighttime sky of a person living at the center of our galaxy.
SUE ANN HEATHERLY: So, what would it look like if we lived at the center of our galaxy and, you know, it became nighttime? I mean, would it be nighttime?
DR. MARK MORRIS: It would be a lot brighter at night. It would be like living in a place with ten or a hundred full moons all. . . always in the sky, the equivalent of that much light. It wouldn’t be like daytime, but it would be awfully bright because of all those stars just shining constantly down on you no matter what time of day it was.
SUE ANN HEATHERLY: That would be kind of neat. Now, you’ve been looking at these stars with an infrared telescope, or a telescope capable of infrared. That’s the Keck Telescope, right?
DR. MARK MORRIS: That’s right.
SUE ANN HEATHERLY: Tell me what’s been going on since 1995 with these stars.
DR. MARK MORRIS: Well, the, the stars that are closest to the center of our galaxy are moving very fast – and we suspected they would – because for some time it has been suspected that there is a super massive black hole at the center of our galaxy. This goes all the way back to 1974, when a very bright radio source was found right at the center of our galaxy right here in Green Bank, West Virginia. And that radio source was suspected to be a black hole, but there wasn’t any way of proving it. And in the 1990s people started gathering more and more evidence that the concentration of mass residing at the center was really there, because everything that got close to it started moving much faster than it otherwise would. In other words, the orbital velocities of things moving around this black hole became very high. That certainly includes the stars, but it also includes the gas.
Well, we were looking at the stars. And the stars that we’re able to follow, we don’t follow their line-of-site velocities, we. . . we look at their, their velocities in the plane of the sky. We see them displace themselves every year a little bit. But at the distance of the Galactic Center, which is 25,000 light years, uh, even a small displacement from year-to-year means a tremendously large velocity. Typically, you can’t see anything move at such a large distance, but because these stars are moving at enormous speeds, uh, we, we can see them move quite a bit.
SUE ANN HEATHERLY: So, they’re covering some ground.
DR. MARK MORRIS: That’s right. They’re, they’re moving at speeds that would. . .the fastest star we’ve seen is over 10,000 kilometers per second. Imagine that? You could go from, uh. . . uh, Los Angeles to Europe in about a. . . a second.
SUE ANN HEATHERLY: That would be great.
DR. MARK MORRIS: But this is a whole star moving at that speed, and it. . .and it’s moving at that speed because the mass of the central black hole is so enormous. And in order to stay in orbit, the star has to move that fast. Now, these orbits that the stars are following as they orbit the black hole are just like the orbits of the planets around the Sun in our solar system. They’re very normal, elliptical orbits. That’s been very interesting, too, because the. . .our local laws of physics work in that extreme environment just as well. Eventually, we, we hope to start seeing alterations of these orbits that are caused by, uh, the fact that old Newtonian Physics isn’t quite accurate in such an extreme environment. We have to use general relativity, and that makes the orbits behave a little differently. Instead of just looping around in an elliptical orbit, these orbits are going to précis and form rosette orbits that make the long axis of the orbit rotate with time; and we can attribute that to effects like general relativity, or maybe to the presence of dark matter, if there is any of that at our Galactic Center. We haven’t seen it yet, but that’s the Holy Grail; that’s what we’re looking for. But, in the meantime, we’re having a great time following these stars.
The star with the shortest-period orbit has a fifteen-year orbit – we call it SO2 – and this, this star is a very luminous young star; and it just happens to be near the center of the black hole, so it’s easy to follow because it’s so luminous; and it comes within about a hundred astronomical units of the black hole. An astronomical unit is the distance between the Earth and the Sun. So, it’s point of closest approach to the black hole is about the size of our solar system, and that’s pretty small on. . .on the scale of the galaxy. So, that, that’s quite close. And even at that relatively large distance of a hundred astronomical units, it is really hauling at, at, uh, these high speeds approaching 10,000 kilometers per second.
SUE ANN HEATHERLY: So, if you’ve been able to see this since 1995 and it has a fifteen-year orbit, you’ve seen it go around once.
DR. MARK MORRIS: That’s right. It’s now completing its first orbit. And, uh, in all, in all this time, we are getting more and more sensitive all the time and are able to see more and more stars. Now, keep in mind that the star field here is tremendously densely packed with stars, and, so, confusion is a, is an issue. And we really need the highest possible spatial resolution. And that’s what the Keck Telescope offers us, using this technique of adaptive optics, where the blurring that the atmosphere usually causes is compensated for by following what the atmosphere is doing with a, with a high-powered laser, and then correcting the surface of one of the mirrors in the optical path to compensate for the deformation of the wave-front of light coming from the star; and that makes it as if the atmosphere is almost not there, and we can see with a spatial resolution that was previously unattainable. That’s what we need to follow these orbits. And now we’re able to follow the orbits of many dozens of stars. And over time, as these orbits get better and better, we’re really improving our knowledge of the mass of the black hole, the distance of the black hole, and, and, and what’s in it’s immediate environment: is. . .are there, are there little black holes also orbiting the big black hole; are. . .is there dark matter; is, is. . .are there stellar remnants, like, neutron stars, lurking in this environment that manifests themselves only by their mass?
SUE ANN HEATHERLY: And they would perturb the orbits of these stars that you’re measuring? Is that the idea?
DR. MARK MORRIS: That’s right. They. . .
SUE ANN HEATHERLY: Uh-huh.
DR. MARK MORRIS: They would cause these orbits to, uh. . .uh, be altered from purely elliptical orbits, and that. . .that has to be sorted out. Because, as I mentioned, we have these effects due to relativity that cause the orbits to behave in one way, and the effects of a, of a cluster of dark objects would cause the orbits to behave in a. . . in yet a different way, and it’s going to be a lot of work to sort that out. But I’d love to be able to sort that out, as soon as we start seeing deviations from these purely classical Kaplarian orbits.
SUE ANN HEATHERLY: It’s just so amazing to think that you’re able to measure something like this on such a short timescale. You know, you think about the universe as just kind of very slowly changing over time, but yet you’re seeing these stars orbiting this, this object in. . .in almost, you know. . . in real time, in a sense.
DR. MARK MORRIS: That’s right. Not much usually happens in an astronomer’s lifetime. If you want. . .if you want to see something happen, then you have to go study binary stars or something. But here we’re seeing things happen from year-to-year and it keeps us really interested in what we’re doing.
Now, one of the things that’s actively changing is the black hole itself; it isn’t just black.
SUE ANN HEATHERLY: Let’s make sure that everybody realizes a black hole is not a hole – it’s an object – and you are measuring it’s mass by measuring how fast these stars orbit around it. And how big is this black hole?
DR. MARK MORRIS: The mass of this black hole is now figured to be four million times the mass of our Sun. That’s a pretty respectable black hole. That’s why we call it a supermassive black hole, but it’s not the most massive one. Quasars apparently have supermassive black holes that can be even a thousand times bigger than that, so our galaxy has a fairly modest black hole.
SUE ANN HEATHERLY: But as you were getting ready to say – before I so rudely interrupted you – it’s adding to itself a little bit, right?
DR. MARK MORRIS: It’s constantly eating. That’s right. And that’s what we can actually see evidence for. As it. . . as it absorbs material that happens to wander in its path, it puts on a little light show for us. Usually, what it’s eating is gas that’s in its vicinity. There is gas everywhere throughout the interstellar medium at the center of the galaxy. But the, the black hole at the center of our galaxy happens to be in the presence of a lot of windy, young stars that are belting out strong winds, and this black hole just absorbs the matter – the gas – in these winds; and, uh. . .the, the matter spirals into the black hole. And as it spirals in, it heats up, radiates light – or infrared radiation – that we can detect. We also see radio waves, and that’s what caused the radio waves that were first detected here in Green Bank in 1974, and it, it emits x-rays. And so, it’s putting on a show. And it’s not just a constant source of luminous energy. It fluctuates.
SUE ANN HEATHERLY: Hmm.
DR. MARK MORRIS: And so, it really is fascinating to watch during the night, the. . .the black hole – Sagittarius A Star we call it – is, uh, pulsating in its, uh, luminous output relative to the stars around it, which are rock solid in their light output. The, the black hole is sitting there literally pulsating, or flaring.
SUE ANN HEATHERLY: On any given occasion, you would see this fluctuation during your observations?
DR. MARK MORRIS: Yes, it’s almost always doing this. And uh, there, there have been nights where it is relatively dull, and other nights when it’s really hopping. Now, the, the black hole isn’t growing very rapidly. It doesn’t take much gas to cause it to, to be luminous. In fact, it’s consuming a, a very small amount of gas, uh, each year. And uh, if that was all it ever ate, it would never grow very much. But,uh, over time there have been episodes in which dense clouds of gas come nearby, and uh, then the power output of our black hole would make our galaxy look, uh, almost like a quasar. It can – and has, probably –flared up tremendously – millions of times more energetic at various times in the history of our galaxy than it is right now.
SUE ANN HEATHERLY: Don’t quasars or radio galaxies have jets coming from their black holes? Does the Milky Way have a jet coming out of its black hole?
DR. MARK MORRIS: Well, we don’t right now have strong radio jets, or jets of, of, uh, hot matter being spewed out from the center; at least, none that we can detect. Uh, there are discussions of whether our black hole is putting out mild jets right now that we simply can’t detect. But when. . .when it’s fueled, uh, much more than it’s currently fueled, or when it swallows a hole star, it probably does, uh, temporarily put out some rather strong jets, just like quasars do. It can, it can be a spectacular phenomenon. Uh. . .people have , uh, asked me whether we should worry about that – is, is. . .is the, is the, uh, activity at the center of our galaxy, is the black hole threatening us in any way. Well, fortunately, at a distance of 25,000 light years, we’re pretty safe, even it if were to go through one of its more extreme states. Oh, we would, we would see a. . .it’s a raised level of luminous energy coming from it. And even the amount of x-rays and gamma rays coming from it would go up, but it wouldn’t be much of an increment over the natural background of x-rays and gamma rays that we have on Earth anyway.
SUE ANN HEATHERLY: Well, that’s good to know, and that. . .and also, that we’re not going to fall in at some time in the future, are we?
DR. MARK MORRIS: Absolutely not. And, in fact, I, I also get asked that a lot. And even in the distant future of our galaxy, uh, that’s not going to happen. The, the black hole will continue to eat anything that comes in its path, but because most things in the galaxy are happily orbiting at some large distance, they’re going to stay in their orbits and not migrate inward toward the central black hole. And so, that’s true for us as well. We’re not going to get anywhere near it. The black hole has a strong effect on everything within about one or two light years of it. Well, we’re 25,000 light years away, so. . .
SUE ANN HEATHERLY: There you go. You’ve been reassured folks by an astronomer from UCLA. He knows what he is talking about. And your research is truly fascinating. Thank you so much for talking with me this afternoon. I really appreciate it.
DR. MARK MORRIS: It’s my pleasure. Good talking with you.
End of podcast:
365 Days of Astronomy
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