Charles Townes, “The Black Hole at the Center of Our Galaxy” - Ford/MIT Nobel Laureate Lecture Series

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PAUL GRAY: Paul Gray, and it is my privilege to welcome all of you tonight to this second Ford/MIT Nobel Laureate lecture. The Institute is tremendously grateful to the Ford Motor Company for their sponsorship of this lecture series. We started this year and will continue next year, we hope, with three or possibly four such lectures.

Ford is represented tonight by Dr. Christopher Magee, who is in the second row here in the center section. Chris. Chris is a visiting engineer at MIT. He has served this year as a member of the organizing committee for this lecture series. And we're pleased to have him present here tonight with his wife, Joanne.

Our speaker tonight, Dr. Charles H. Townes, received the Nobel Prize in physics in 1964 for, and I quote, "fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle," end quote. Dr. Townes holds the original patent for the maser. For those of you who may not be familiar with the term, my memory is that it stands for molecular amplification through the stimulation of emitted radiation. And he holds jointly, with his brother-in-law Arthur Schawlow, the original laser patent, light amplification through the stimulation of emitted radiation. This work was done largely at Columbia University.

I don't have to tell anyone here about the enormous impact which coherent light and the laser have had on the world. There is hardly anything one can touch without coming in contact with the fruits of those inventions. If you consider the utility of lasers, collimated light, coherent light in instrumentation, scientific and technical instrumentation.

Consider it in medicine, where it is used in all kinds of diagnostic ways. It's used for surgery. In optical communications. We will soon be in an age in which all communication is transmitted, on the earth at least, by optical methods rather than by copper wires. And in all manner of daily conveniences, such as the CD player or the CD-ROM player in your computer, all depend on the laser.

Dr. Townes is no stranger to MIT. He served as provost of the Institute in the administration of Julius Adams Stratton from 1961 to 1965. And that's when I first met him as an assistant professor in electrical engineering. Dr. Townes has also served on the faculty of the University of California in the distinguished position of university professor. And he is now professor in the graduate school at the University of California at Cal Berkeley.

Charlie explained to me before dinner that Cal has this enlightened policy in which emeriti professors, if they wish to continue to do research or to do some teaching, may continue with the title professor in the graduate school. And he explained that he is one of the PIGS, right? Professor in the graduate school.

Dr. Townes has frequently served as advisor to the United States government and indeed to governments elsewhere as well. He was a member and served as vice chairman of the President's Science Advisory Committee in the Johnson administration. He was chairman of the Technical Advisory Committee for the Apollo program through the time of the first lunar landing.

He's been active in the work of the National Academies. He's a member of the Academies of Science and Engineering and has been active in their work with the former Soviet Union on arms control, with China, and with other nations. He has served as a member of the Papal Academy of Sciences and has provided advice in that manner to the pope on arms control and the search for peace.

Dr. Townes has an extraordinary list of honors beyond the Nobel Prize from academies and scientific societies across the world, not just in the US but across the world. After his remarks this evening, he will welcome questions from the audience. His address, as you know, is entitled "The Black Hole at the Center of Our Galaxy."

I should say now and will remind you again at the end that there will be a reception following this lecture in lobby 13, which, if you go through any of these doors and out in that direction, in the northern direction, is just down a set of stairs. There'll be a reception to which you're all invited following the lecture and the questions. Please join me now in welcoming our distinguished speaker, Charles H. Townes.

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CHARLES H. TOWNES: Thank you very much for that very generous introduction. I want to say it's a great pleasure for my wife, Frances, and I to be back here at MIT. We spent five fascinating years here back in the '60s. And it's a pleasure to be back and see some old friends and see all the great things that are happening at MIT.

Yes, I will talk about the black hole in the center of our galaxy. But in doing that, I must also talk about our galaxy very generally, many aspects of it. Now, the work I'm going to discuss will be that of many people. I and my students have done some things on the black hole, yes. But as in much of science, the growth of science and of new ideas is a contribution from many people with many different techniques. And you'll see that illustrated.

First, let me remind you what a galaxy is like, particularly a spiral galaxy. Let's see, I have a pointer here. These are black-and-white pictures of spiral galaxies.

And there's one we're seeing face-on, you might say. This is one that's rather tilted. You'll see that it's almost like a pancake shape. If we tilt it more, it'd be still thinner. And material's going around.

What you're seeing here is largely stars. And these galaxies have a few billion stars in them, generally. They vary in size, of course. The center of the galaxy would be here.

Now, we ourselves on earth about a third to a half a way out from the center. So we're sort of along about in here. And we try to look into the center. We can't see it with visual light.

Astronomers couldn't see into the center of our galaxy a long time. In fact, it was only in the early part of the last century that people realized we were in a galaxy. And there it was all the time, the Milky Way.

If you look at the Milky Way, that's we looking out towards all of these stars. And the Milky Way is the plane of the galaxy, of a flat, pancake-shaped thing. It was there all the time. But we only realized that, hey, we are in a galaxy. And there are billions of galaxies out there. We are just one of them.

Now, the center of the galaxy wasn't visible because not only are there stars in the galaxy, but there are gas clouds. And these gas clouds have dust particles in them and so on. So if we try to look through the center of the galaxy, we only see the nearest stars. And we can't see very far because of the dust in the clouds. And optical astronomers and visible lights wouldn't get through, and you didn't see them for many years.

The first discovery of the center of the galaxy, actually, was made by an engineer. And this is typical, the accidents that occur in science, people who are curious and investigate and are careful. Engineer Jansky at Bell Telephone Laboratories discovered microwaves coming from a particular position in the sky. And with the help of astronomy, he recognized this is where the center of our galaxy probably is. And it was a very strong microwave source.

That stayed there for some years. People didn't know what to make of it but didn't do too much about it. They didn't know what to do. And only later it began to be studied more and more. And radio waves began to be more and more very important, too, for astronomy.

1968. Jansky's work was in the mid-'30s. 1968, we had for the first time the discovery of infrared waves coming from the center of our galaxy. And that was done by Becklin, a student, and Neugebauer. Neugebauer was a professor, very distinguished professor, invented a lot of infrared astronomy.

And the student, Becklin, said, well, you know, I'm going to be working by myself tonight. Can I look at the center of the galaxy? And Neugebauer said no, I don't think you'll find anything there. Don't bother. But he did anyhow.

Remember that, now. Don't take what your professors say all the time. Question it. He looked. And he discovered this bright infrared source. Wonderful discovery. And of course, they went on to create the science from that. But infrared gets through the dust particles as well as radio waves.

Well, now, I want to show you a picture of our galaxy as we see it from where we are, using a number of different types of waves, because that's what we use. We use all the possible techniques we can. And these are some of the samples. Let me get it right. There we are. These are some of the samples.

Here's radio. And this is the Milky-- well, first let's look at visible light. Here's the visible light. That's the Milky Way. Now, notice these dark patches in there. That's not because they're not stars. It just means there are dark clouds shielding all the stars.

And we didn't wake up to what was going on there for a long time. And finally, we recognized that there are stars there in the dark clouds. And the center of our galaxy is about in there. If we look at it in radio waves, it looks much brighter. It's brighter all through this region. We'll see a more detailed picture later.

Infrared. See, this is most of the infrared energy is sort of centered here. That's the center of our galaxy. X-rays are X-rays coming from the Milky Way. Gamma rays. Look at the gamma rays. They come from special spots in the Milky Way where something very energetic is happening. So that's a kind of a sample of some of our ways of looking at it.

Now let me show you what can be done with an array of telescopes in a radio region. An array of telescopes on the San Agustin plain in New Mexico. About 29 telescopes in all, spaced at varied separation. In this particular one case, they all nod together and point to the sky. It's almost like they're dancing together.

The rays come down. The signals are joined from all these telescopes. And that gives very high angular resolution, as well as good sensitivity. As all of the telescopes working together that's spread over a long distance, that gives very high angular resolution. And so radio waves have obtained angular resolution really better than optical waves so far, even though they're much longer wavelength.

Here is a general picture of the Milky Way taken with radio waves. Whoops. Let me say here it is, taken with radio waves. That's the brightest part in the center that Jansky had found. The brightness is proportional to the intensity of the radio waves.

And this is a kind of a map made with those antennas by-- well, Kassim at Harvard is one of the more important people that's involved in this particular map. And this is the bright center. Now, you'll notice some stripes here also. And people call them threads and snakes. And so there's a snake there, they call it. There's a mouse and so on. Those are just names.

There's supernovae, many supernovae in this area. These are supernovae that have blown up many years ago. But the remnants are still there, and they can be seen. And this is a plane of our galaxy, or the plane of the Milky Way.

Now let's look at still more detail. That's a little bit more detail. That, in particular, is-- across here is about 1,500 light-years. 1,500 light-years. Now, a light-year, the distance light travels in a year's time, and it's about, oh, 600 billion miles is a light-year. And that's 1,500 of them.

Now let's look in a little more detail still, again with those same antennas, so-called VLA, Very Large Array. This is the bright spot in the very center. And now we're talking about a few hundred light-years across here instead of a few thousand.

Look at these stripes. Look at those stripes. Straight lines. What's doing that? That tells us there's a magnetic field in this direction and radio waves emitted by electrons spiraling around the magnetic fields, and they emit microwaves as they go. And so this tells us something about the direction of the magnetic field and the strength.

The other places, any other striations in here and other stripes that you saw, that's one of the more prominent ones. There's also a very interesting collection of stars up in here at a place called the Pistol, because somebody thought it looked like a pistol. We have to find names for these things.

Now I want to give you still more detail, again with the radio telescopes. We're going look at this region, about this much size, magnified still more. And the first slide will show us that, I hope. We can have the first slide.

Now, these are pictures of various intensities. First, here is that sort of central brightest place. This is, the red represents the brightest part of all. The blue is less bright.

And you see here, there's a kind of an ellipse, almost an ellipse here, as well as a stripe across there. And now up here, we have a little less amplification. And there's a bright spot there. This is another picture of it, a bright spot. But it's elongated along this direction and along this direction.

And now if we go very weak, look at this very, very bright spot still. And this part has gotten blue. We'll call it very dim. In this part, there's a very bright spot there. That bright spot was first found by Balick and Brown in 1974, University of Washington. They did it with different antennas from what I showed you.

These antennas have mapped it out nicely. And that bright spot, very intense in microwave energy. And they couldn't resolve it. They knew it seemed to be a spot, so far as they could tell. And they named it Sagittarius A*. Sagittarius A*.

They didn't know what it was. But it was suggested maybe it's a black hole and things are falling into it. And that'll produce a lot of energy. But there were various other possibilities that could have been. And nobody knew how big it was or just what it was.

But clearly it was a bright spot. And there it is. You see, as we resolve it more and more, there's much radio radiation produced by electrons, high-energy electrons that are going around doing things here. But there's something very peculiar there that is indeed bright and small.

Now, that spot by now has been resolved with radio telescopes. And this was done by a group of people, Backer and Lowe at Berkeley and some others. It was kind of a national cooperative effort through antennas on the East Coast, antennas on the West Coast. And two antennas, east and west, then, the long distance between them was equivalent to a telescope that big, so far as the resolution was concerned.

They did what's known as interferometry. And they found the size of that. They measured it 3 millimeters using short microwaves. 3 millimeters in the baseline of 2,000 miles of separation, being the telescopes are 2,000 miles, basically a telescope that big. And they found it was 1/6 of a thousandth of an arc second in size. 1/6 of a thousandth of an arc second in size.

Now, that's equivalent to looking at an atom at this distance. If we could see an atom, we would be getting the kind of resolution that the radio has gotten. And that object, once they finally resolved it for the first time, found out how big it was. That's the highest resolution anyone's gotten yet, although Bernie Burke, who's here, has achieved something very close to that with the radio astronomy on some other objects.

Now, in addition to that peculiar object, there are many very odd and bright stars in this region. And now if we take that off, I want to show, this is in that so-called Pistol Nebula. And there's a bright star. That's the brightest star we've ever found.

It puts out light which is 10 million times as bright as the sun. It's believed to be about 100 times heavier than the sun. And it's out there where those stripes were, near the galactic center but not at the very center. The brightest star we know.

You'll see as we go along, the galactic center is a wonderful laboratory. The center of our galaxy produces things that we don't see, can't see anywhere else. Conglomerations of matter and matter falling together and energies and things that are going on that we can't produce in a laboratory, we don't see in nearby stars or anything.

It's a laboratory out there for us where we can observe and see what's new and understand it. And if we have to, we can study it in more detail. And it's the best galactic center we have simply because it's the closest. The others are at least 100 times-- the centers of other galaxies are at least 100 times further away.

This center is only about 25,000 light-years away. Well, that's kind of far, but it's the closest one we have. And furthermore, it's close enough for us to get the kind of detail that we're talking about here and I will talk about some more.

Other things that were found coming from that region that excited people was gamma rays of half a million electron volts, 0.5111. Now, those of you nuclear physicists will know 0.5111, hey, that's the gamma rays that are created when an electron and a proton come together and annihilate each other. I mean, an electron and a positron come together and annihilate each other. They produce two photons of that energy.

So there's annihilation going up there. And people thought, ah, that, you know, it's probably the black hole. Probably a black hole, and there's a lot of energy around there, things falling in. That's probably what it is. We later found no, it wasn't that.

It's something about half a degree away from the center. When they located where it was really coming from, it's not in the center. About a half a degree-- close by, about a half a degree away. But it's recombining positrons and electrons, about 10 to the 12th tons per second matter being destroyed. And it's now called the Great Annihilator. The greatest annihilator we know, anyhow.

We also have 1.8 million volt gamma rays coming from this region and been discovered. 1.8 million electron volt gamma rays coming from this region. What's that characteristic of? Well, nuclear physicists know what it's characteristic of.

It's aluminum 26, which decays. And as it decays, it produces another atom. And that decays and produces gamma rays. It has a lifetime of about a million years. And so something big, explosive has happened there within the last million years or so.

Supernovae can do that. But this would have to correspond to a lot of supernovae, the intensity is so great. And it's spread around the center, spread around the center. It's in a kind of a volume area. So a lot has happened there within the last million years. Some very explosive things. If supernovae, then there were lots of them, and more than we can generally count or expect at the moment.

Well, now let me try to give you an overall picture here of what we see. Here we quickly get an overall picture. Here is the Milky Way, which is what we see, excepting with a telescope, you see vastly more stars. As I mentioned, our galaxy actually has about 10 billion stars. And so if you look up there with the telescope, you see all these little bright spots of stars.

Now this is the center. And you see, well, we might guess that's a concentrated region. It might be the center. But what is it? We're not really seeing into the center. We're seeing just some of the nearer parts of our galaxy here.

If you look into the center with radio waves, you get this. First place, you see microwaves coming from this region, which is outlined with lines as a sort of an area here. Here is this oval, which I showed you earlier. An oval there, and a stripe going across it.

But in addition to that, there is molecular gas. And this colored matter, the molecular gas. We know that, again, from radio waves, microwaves, which detect the radiation from HCN, the rotation of the molecule HCN. So this is a neutral cloud, an un-ionized cloud, of molecules here.

We can tell the temperature. The temperature is the order of anywhere from 100 degrees down to about 20 degrees. It cools off we get out here. As it cools off, it's so cold that HCN won't even rotate and we don't see the HCN anymore. It's probably still out here, but it's not radiating.

Here it radiates. And as we go in still closer, we get ionized material, which is what you see here in the original radio picture. That's ionized material. This is done with radio spectroscopy, microwave spectroscopy. And there's a cloud of molecules. So we go in from very cold material. It gets warmer as molecular material and gets ionized.

And then in here, there's practically nothing in some places. And this is Sagittarius A*, that point right there. Sagittarius A*. OK, now we take a very, very magnified picture just in this region of Sagittarius A*. It's right there. But we don't see anything much there. This is with infrared with a big telescope.

And this is 2/10 of a parsec. Now, 1 parsec is about 3 light-years. So this is really about 1 light-year across. This is 1 light-year across. This is about 8 light-years across here. This circle here is about 8 light-years in diameter. We're talking about 1 light-year roughly here.

And there is the Sagittarius A*. These are stars of various intensities. We don't see much infrared coming from that region. And yet it has very intense radio waves.

Let me just show you another picture with the Keck telescopes. This is taken by UCLA scientists, University of California Los Angeles, the Keck telescopes. And again, there's Sagittarius A*. And these are stars.

This is very much enlarged. So 5 light-days is what we're seeing here. Very much enlarged. And there just seems to be nothing there.

Now that's surprising, in that black holes-- what is a black hole? A black hole represents a concentration of mass which is so concentrated, the gravitational field is very, very large. And it's so large that nothing can get away, not even light. You shine a flashlight from a black hole, the light goes out a little ways and falls back in again.

Nothing can get away from a black hole, it's such a concentrated mass. Now, it may not be very heavy. It could just have a very small radius and then have a very strong gravitational field.

But typically, what we know of black holes, what we think we know, they are bigger objects, like at least as heavy as a star. And we'll see that this one is much heavier. And still a small enough radius that the gravitational fields are enormously large.

If you fall in, you're gone. You'll never get out. And you're more or less indescribable from then on. It's just all matter is all the same in there. That's a black hole.

And now, we have to expect there should be some black holes somewhere if our theoretical ideas are right. If things can become so condensed, I'd expect, yes, there'll be some black holes. And where would we expect to find them? Well, the center of our galaxy is a natural place.

Everything is attract-- things are being attracted to the center of our galaxy. We're spinning around them slowly. If we slow down, we'll start falling in. Centrifugal force will decrease. We'll fall in.

And so things fall in. Things should accumulate there. That's where matter should accumulate. And we should have a very high mass, and then maybe a black hole will develop.

There is a still more detailed picture of that central region we showed. And we'll just see it again. There's Sagittarius A*. The so-called Western Arc, which is part of this ellipse, I'll call it-- and you'll see more about the ellipse here. And this thing is sort of at the center of this ellipse.

And then there's this molecular gas around it, which then gets cold as it goes on out there. And then there's a northern arm pushing material that seems to be just coming in towards the center right here. The northern arm. And then part of the ellipse that goes around like this.

OK. Now let me show you another tool which is used. And the next slide will show another important tool in the study of the galactic center, which is an airplane. This is an airplane that flies above most of the atmosphere. It flies at around 40,000 feet.

Why is that? That's because far infrared radiation doesn't get to our atmosphere very well, particularly water vapor, but other parts of the atmosphere. So you want to get above the atmosphere as much as possible.

And this is so-called Kuiper Observatory built by NASA. You get up about 40,000 feet. And then you open up this thing here. We're back in the middle of the plane, closed off because the pressure is too low here for us to be in that section. It's separated off here. And the telescope is out there. The telescope can be operated lookout and study waves that can't get through to the ground.

Now I want to show next something, a result obtained in that airplane by Gordon Stacey, who worked with us at Berkeley for a while. But he did this particular thing with students at Cornell. He took the mapped-out, mid or far-- well, far infrared, generally, we would call it-- in this region. And this is what he found.

This is the ellipse that you've been seeing, which you can see it also seems to go all the way around almost and lips around there. Sagittarius A* is right in here. This is at 32 microns wavelength. 32 microns is what?

Well, 1 micron is 10 to the minus 4 centimeters, or 1 thousandth of a millimeter. So this is about a 30th of a millimeter wavelength. Far infrared, this is 38 microns. And you see they're a little different. They depend a little differently on temperature.

But you can see it's pretty clearly an elliptical thing here and something bright coming down there and across here. And then the other warm things around. And this is a heat radiation from something that is warm, but it's not very warm in order to put out these longer wavelengths. That's about three times the peak wavelength put out by the Earth at its temperature.

All right. Now, we at Berkeley started studying the galactic center fairly early, not so long after it had been detected in the infrared at Caltech. And the next slide will show us a couple of students and I down in Chile. Chile is the best place we know on Earth to observe in telescopes because the atmosphere is so calm and non-turbulent.

And there is Tom Duvall and myself and John Lacy. Tom is now out in Hawaii at the University of Hawaii. And Lacy is at the University of Texas. And we went down to Chile partly because of the good atmosphere, but partly because the galactic center goes right overhead there.

For us here, we have to look fairly far south to see the galactic center. The galactic center goes right overhead. And there was a nice telescope there. And so we went down there to examine it.

And we decided to examine the gas because we didn't know anything much about the center. Much of what I showed you hadn't yet been measured. And we felt if we measured the gas, the type of gas and its velocity, maybe we could see whether there's a black hole there, whether things were going on around a black hole.

Now, what gases would we measure? Well, the radio people, seeing waves, knew that the gas was ionized in the central region. And so we would look for ions. We could look for ions of argon or neon or sulfur and the spectral lines and work in the infrared, which would get through the dust.

We worked at the 10 microns. 10 microns, the mid infrared. Get through the dust, look into there, and see. And by picking those ions, we could tell what's the energy of excitation, because each one had a different energy of excitation. What's the average energy of excitation?

And we found neon was very plentiful. Argon was not so plentiful. Sulfur wasn't there at all. That meant that the stars producing ultraviolet there had temperatures up to about 30,000 or 35,000 degrees. Our sun is around 5,000 degrees, you may know. Well, these stars are bright and hot compared to the sun, but they're not the hottest type. So we get fairly hot stars but not very hot stars in the galactic center.

Now also, this allowed one to tell the density of the gas. The density of the gas is around 10 to the 4th or 10 to the 5th particles per cubic centimeter. Now, this air we're breathing is about 10 to the 19th particles per cubic centimeter. So that's very rarefied, about the best vacuum we can get on Earth. But nevertheless, for interstellar space that's a fairly high density, 10 to the 4th, 10 to the 5th.

Material is ionized. We could see what it was. Also, we could measure the velocity using Doppler shifts. Using the Doppler effect, we could measure velocity. We found, sure enough, some pretty high velocities. And further study of that showed a good deal about what might be there.

And this was followed up by my students as they moved on to other places, which I was pleased about. [INAUDIBLE] John Lacy and Gene Serabyn. John Lacy was in Caltech and then Texas, and Serabyn was down at Caltech.

And here is the ellipse. And these are the points that they measured. They measured the velocity of the gas at all of these points and along these points, which we call the northern arm. This is the ellipse. This is the northern arm. Those are points that they measured.

And now, what did they find? Let's look along the ellipse first. And this is what they found. As you go from the bottom to the top, the velocity varied. From the bottom of the ellipse, minus 100 kilometers per second to plus 100 kilometers per second.

Now, minus 100 kilometers per second, if the sun was used minus, you'll say it's coming toward you. Minus 100 kilometers means it's coming toward you at 100 kilometers per second. Plus 100 means it's going away from you.

Well, that's just what you would expect of something going around in a circular fashion. When you look at it edge-on, you can work out the arithmetic, is what's the velocity towards or away from you? It goes exactly linearly with distance up the ellipse. It was exactly linear. That's just what, almost what it was doing.

These variations, however, are real. Those variations from linearity are real. So it's almost a steadily rotating circle of material, but not quite steady.

Now, that says that-- you notice around Sagittarius A*, there was some gas, yes. But there was evacuated material, and then there was gas a little further out at the ellipse. That says something big happened sometime in the past. This material was blown out.

How far in the past? Well, with these differences in velocities, one could say that this ring of material, different parts would bump into each other in about 50,000 years. And they would even out their velocity, all go around the same velocity. They weren't going quite the same velocity. So something big has happened within the last 50,000 years there.

In addition, with the velocity, one could say, well, how much mass is inside of there? This is like the Earth circling around the sun. We know how fast it moves around the sun and how far away it is. We can say, well, what's the mass of the sun, because the gravity is attracting it, and the centrifugal force has to equal that attraction.

So the mass in there was about 4 million times the mass of the sun. Now, this is only-- that ellipse has a radius of about 4 light-years. And about 4 light-years is the distance to the nearest star to us. Well, you have mass of 4 million stars within that distance, within that ellipse or that circulating material.

Serabyn interpreted the so-called northern arm. And he found that was equivalent to another, a very-- not a circle, but an elliptical thing. And that's the position of Sagittarius A*. There's his points. And they fit very nicely in velocity and position within the ellipse.

And the velocity, you could say yes, that looks like it has about 3 or 3 and 1/2 million times the mass of the sun. And he comes in fairly close, in this case several times closer than the ellipse was. So it has to be in fairly close. Within that distance, there's still about 4 million times the mass of the sun.

All right. Now I want to show another kind of overall picture, a diagram, to remind you of some of the things that we're seeing. This has where Sagittarius A* is. There's the ellipse. This is the western arc of the ellipse.

Just outside of that is a molecular material. And the northern arm coming down there. We find here's a batch of atomic material, again seen from its spectra. And Reinhard Genzel, who was at Berkeley, now largely in Munich, did much of that work.

And there are other things that various people found. Radio astronomers, especially, found the star here, with material kind of streaming out behind it, as if wind or something is blowing that material. Furthermore, there's a kind of a cavity here, as if something has cleared that out.

The thought is, well, maybe that was over close to the Sagittarius A*, and Sagittarius A* had a big wind and blew that out. And maybe it's blowing this out. It's exactly in the direction going from Sagittarius A* to this star where these tails are blown out there.

Now if we go a little further, make a map of a bigger area, here is the striations corresponding with magnetic fields. We have other striations in here. Here's the Great Annihilator down here, annihilating positrons and electrons at a great rate. So those are some of the many things that a wide number of people discovered. And I give you a general overall picture.

Now, we thought we could argue fairly strongly that there had to be a black hole there because of the very high velocities of gas, because the patterns of motion of the gas. But not everybody wanted to believe that. Why? Because a black hole attracts material.

Material will fall in it. We know there's a lot of gas here. We could measure the gas. A lot of gas there, a lot of stars putting off gas. We could measure the gas. We know there must be material falling in.

And to calculate what that material falling in would do, it would fall in towards the black hole. It would circle around it, get ionized, and then circle around and emit a lot of radiation. Emit too much radiation. And people calculated it at about 1,000 times more than what we're seeing.

So it couldn't be a black hole. Couldn't be a black hole of that much mass, anyhow. That was the current theory of the day, that it just can't be that kind of mass. There's something wrong there, because material falling in has got to get accelerated and then ionized. And it'll radiate. And it'll radiate far too much.

Now, that argument went on for some time. We felt the evidence was pretty good. But people said, well, maybe the gas is being blown by a wind. That's what gives it its high velocity, some kind of a wind. Or maybe there's a magnetic field that's accelerated the gas. After all, we know the magnetic field's there.

Well, we could measure the magnetic field, however, from Zeeman effects in the fine structure of atoms. Magnetic fields are not very big. The winds, we knew something about those. Those weren't very big.

Nevertheless, everybody was looking hard for some other explanation. And now, this is a not an uncommon phenomenon. Sometimes you're right. Sometimes you're wrong. But you find some evidence. And yes, it might be this. It might be that. And you think the evidence is pretty strong but doesn't agree with somebody else.

And so you've got to struggle over it. And this debate among scientists is an important part of the process. And it was debated for some time. Now I think it's been more or less settled.

And what really convinced the remaining astronomers was the measurement of stellar motion, because they felt, well, the gas, somehow that can be blown around, and we can't rely on gas velocity too much. Stars, on the other hand, are kind of hard, firm objects. And you can't do too much fancy with them.

And here are some stars measured by Ghez and her colleagues down at UCLA using a Keck telescope. Here's 1995, a group of stars very close to Sagittarius A*. Look at them three years later. You see the difference in position?

Look, this angle, the angle between these two, has changed. The distance between these two and the position has changed, and so on. Even one star which was there has disappeared, changed intensity. So the stars definitely have moved.

Now, we know roughly how far that is. We know the angle they moved, so we can tell their velocity. Yes, their velocities are very much like what was being found with the gas velocities. And that was watched over several years.

The first person to see that motion actually was Genzel and Eckert in Munich. But Ghez and others at UCLA have done a lot of the measurements too. And they agree now.

Here's Eckert and Genzel's work. They plotted the position of stars as a function of year, from '92, you see, to about 2000. You can see these are the positions, and clearly they have moved in angle. And so from that, you can determine the velocities. And you get the velocities of stars.

Here is a picture of the very stars and their relative motions. Well, this is a scale of velocity. That's 500 kilometers per second. And another thing that using the stars allowed was to get very close into the center.

The center is about in here. And some of these stars are very close, so they were getting 500 kilometers a second. The gas that we saw was up to about 250 or 300 kilometers per second. This is still faster. And some of them faster than that, even. And here is a picture very close in with the various directions and magnitudes of velocities as found.

Recently, the UCLA group thinks they have even seen some curvature in the motion, saying now they see them circling around. And this is a plot. There's Sagittarius A*. This is a plot of the positions.

Now, there's some curvature there. You might doubt how certain that is, that curvature. But they say, well, you can plot it in various ways. There are various sorts of ellipses that this might be forming.

But here's the best one. They think it's going around an ellipse like this. Give them another 5 or 10 years and they'll be around here. And sure enough, they'll be able to tell exactly the orbits.

And now furthermore, some of these are very close. This one is very close in. See, that seems to have some curvature. And so we can plot these ellipses.

Given another decade, we'll have very detailed information about these orbits, exactly what mass is there, what the distances are. And we'll know much more. But already we know enough to be pretty convincing.

Now let's look at a batch of stars interacting with each other. And how should they behave? Well, they're very much like molecules. They bump into each other and bump into each other and share energy.

They should all have the same energy, just like molecules bumping into each other have the same temperature. They have the same kinetic energy. So when stars are just mashed together in a galaxy, nothing else there. They're bumping into each other and sharing energy. They should all have the same velocity.

If, on the other hand, you have a heavy mass in the center of the galaxy, then the stars not only bump into each other, they go around it. And as they go around, the velocity should increase like 1 over the square root of the distance. As they get closer and closer, they get faster and faster. Just like Mercury's closer to the sun than we are. It goes faster.

Some of you students might work these things out and see just exactly how they behave, how it gets faster and faster as you get closer. And sure enough, the velocities got faster and faster. The gas velocities and the stellar velocities got faster and faster.

And so you could plot, then, the mass inside of a given radius as a function of radius. And this is one of the latest plots now, using all the data. The gas information gave us points up to about in here. But the stellar information gives us points really right on much, much closer.

How close? Well, up to about 0.02 parsecs. Now, 1 parsec is 3 light-years. So that's 0.06 light-years, getting awfully close. And inside of that, there has to be a mass, still, of about 3 times 3 million solar masses inside of 1/16 of a light-year, even.

It has to be a mass. And you see the mass is pretty constant. This plots the mass as a function of mass inside of a circle is a function of radius of the circle. And it's pretty constant. So clearly there's a central mass inside of a 1/16 of a light-year.

And, well, that's pretty concentrated for 3 million stars. And people say, well, yeah, that has to be a black hole. It has to be a black hole, a black hole of about 3 million solar masses.

Now, that was satisfying in a way, to come out with an answer. Everybody is pretty much agreement now. Yes. However, what about the problem of radiation?

It radiates radio waves, a good, pretty intense radio waves. Very, very little infrared. It's doubtful whether we've really seen any infrared radiation. We won't see visible radiation anyhow because it won't get out through the dust clouds. But we should see infrared radiation.

How much should we see? We know how much material is falling in. Calculations say we should see at least 1,000 times more than we've seen. So theorists have been struggling with, well, how can we explain this? How can we explain this? How can we get away from it?

Now, as a matter of fact, by the time this came out, by the time all this got settled over a measuring period of about 25 or 30 years, more and more data kept coming in. But also, other galaxies were being examined, and velocities of materials around the other galaxies. And people were finding heavy black holes, they felt, in other galaxies, some of them as large as a billion solar masses. So things were going around very fast.

Now, some of those are radiating very intensely. Just what was expected. They were radiating intensely. But about 40% of them were not.

Now, in the case with a distant galaxy, you can cast that aside and say, well, we don't know how much material is very close in. We can't see. There may not be much material close in. Maybe nothing is falling in right now.

OK. So those distant galaxies, even though a lot of them are not radiating the way you'd think, maybe there's nothing falling in. What about our galaxy? Now, we can calculate how much gas is falling in. We also know that a star ought to fall in every once in a while.

When a star gets as close as the Earth is to the sun, it'll be sort of torn apart, torn apart and [INAUDIBLE] gas. And it'll rotate and fall on in. So a lot of stars would fall in. Every few thousand years, there ought to be another star falling in. That ought to make a big burst of energy, enormous.

But we know there's gas falling in all the time. And why not getting some energy? Now, the Russians have used the Granat spacecraft, which measured X-rays. And the Russians have looked at the galactic center and near it and all around the galaxy to see if, could they find any X-rays due to this very energetic source, or what's supposed to be a very energetic source.

Well, they find some X-rays coming from the galactic center. Not a large amount. But again, roughly 1,000 times too small. Now, as they moved out in the galaxy, said well, if we find X-rays out here, those might have come from the center of the galaxy having a big explosion. X-rays go out, and they hit some gas out here and get scattered towards us.

So if you go 100 light-years away from the center and see is anything being scattered towards us, you can say did something happen 100 years ago in the galactic center? What a wonderful timing device. You can go 500 years out from the center and say, well, did anything happen 500 years ago? And so on.

And they had enough sensitivity, they felt, to go out about 1,000 years. Nothing happened. Nothing happened that was big in the last 1,000 years there. Well, again, that always gets pretty serious, the difficulty.

And let me summarize now by saying what we found and what we understand and what we don't understand. Well, you can see, we understand a lot of things here. It's a wonderful laboratory, unusual things happening.

We find the black hole. It's L2.9 million solar masses is sort of the best number. But that's plus or minus 10%. With about 3 million solar masses, there's a black hole. I think almost everybody is pretty convinced.