TEDGlobal 2009
Andrea Ghez: The hunt for a supermassive black hole
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With new data from the Keck telescopes, Andrea Ghez shows how state-of-the-art adaptive optics are helping astronomers understand our universe's most mysterious objects: black holes. She shares evidence that a supermassive black hole may be lurking at the center of the Milky Way.
Andrea Ghez - Astronomer
Andrea Ghez is a stargazing detective, tracking the visible and invisible forces lurking in the vastness of interstellar space. Full bio
Andrea Ghez is a stargazing detective, tracking the visible and invisible forces lurking in the vastness of interstellar space. Full bio
Double-click the English transcript below to play the video.
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How do you observe something you can't see?
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This is the basic question of somebody who's interested
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in finding and studying black holes.
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Because black holes are objects
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whose pull of gravity is so intense
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that nothing can escape it, not even light,
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so you can't see it directly.
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So, my story today about black holes
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is about one particular black hole.
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I'm interested in finding whether or not
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there is a really massive, what we like to call
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"supermassive" black hole at the center of our galaxy.
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And the reason this is interesting is that
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it gives us an opportunity to prove
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whether or not these exotic objects really exist.
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And second, it gives us the opportunity
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to understand how these supermassive black holes
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interact with their environment,
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and to understand how they affect the formation and evolution
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of the galaxies which they reside in.
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So, to begin with,
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we need to understand what a black hole is
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so we can understand the proof of a black hole.
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So, what is a black hole?
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Well, in many ways a black hole is an incredibly simple object,
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because there are only three characteristics that you can describe:
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the mass,
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the spin, and the charge.
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And I'm going to only talk about the mass.
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So, in that sense, it's a very simple object.
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But in another sense, it's an incredibly complicated object
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that we need relatively exotic physics to describe,
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and in some sense represents the breakdown of our physical understanding
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of the universe.
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But today, the way I want you to understand a black hole,
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for the proof of a black hole,
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is to think of it as an object
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whose mass is confined to zero volume.
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So, despite the fact that I'm going to talk to you about
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an object that's supermassive,
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and I'm going to get to what that really means in a moment,
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it has no finite size.
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So, this is a little tricky.
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But fortunately there is a finite size that you can see,
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and that's known as the Schwarzschild radius.
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And that's named after the guy who recognized
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why it was such an important radius.
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This is a virtual radius, not reality; the black hole has no size.
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So why is it so important?
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It's important because it tells us
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that any object can become a black hole.
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That means you, your neighbor, your cellphone,
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the auditorium can become a black hole
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if you can figure out how to compress it down
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to the size of the Schwarzschild radius.
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At that point, what's going to happen?
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At that point gravity wins.
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Gravity wins over all other known forces.
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And the object is forced to continue to collapse
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to an infinitely small object.
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And then it's a black hole.
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So, if I were to compress the Earth down to the size of a sugar cube,
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it would become a black hole,
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because the size of a sugar cube is its Schwarzschild radius.
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Now, the key here is to figure out what that Schwarzschild radius is.
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And it turns out that it's actually pretty simple to figure out.
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It depends only on the mass of the object.
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Bigger objects have bigger Schwarzschild radii.
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Smaller objects have smaller Schwarzschild radii.
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So, if I were to take the sun
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and compress it down to the scale of the University of Oxford,
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it would become a black hole.
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So, now we know what a Schwarzschild radius is.
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And it's actually quite a useful concept,
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because it tells us not only
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when a black hole will form,
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but it also gives us the key elements for the proof of a black hole.
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I only need two things.
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I need to understand the mass of the object
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I'm claiming is a black hole,
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and what its Schwarzschild radius is.
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And since the mass determines the Schwarzschild radius,
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there is actually only one thing I really need to know.
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So, my job in convincing you
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that there is a black hole
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is to show that there is some object
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that's confined to within its Schwarzschild radius.
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And your job today is to be skeptical.
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Okay, so, I'm going to talk about no ordinary black hole;
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I'm going to talk about supermassive black holes.
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So, I wanted to say a few words about what an ordinary black hole is,
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as if there could be such a thing as an ordinary black hole.
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An ordinary black hole is thought to be the end state
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of a really massive star's life.
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So, if a star starts its life off
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with much more mass than the mass of the Sun,
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it's going to end its life by exploding
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and leaving behind these beautiful supernova remnants that we see here.
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And inside that supernova remnant
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is going to be a little black hole
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that has a mass roughly three times the mass of the Sun.
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On an astronomical scale
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that's a very small black hole.
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Now, what I want to talk about are the supermassive black holes.
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And the supermassive black holes are thought to reside at the center of galaxies.
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And this beautiful picture taken with the Hubble Space Telescope
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shows you that galaxies come in all shapes and sizes.
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There are big ones. There are little ones.
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Almost every object in that picture there is a galaxy.
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And there is a very nice spiral up in the upper left.
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And there are a hundred billion stars in that galaxy,
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just to give you a sense of scale.
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And all the light that we see from a typical galaxy,
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which is the kind of galaxies that we're seeing here,
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comes from the light from the stars.
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So, we see the galaxy because of the star light.
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Now, there are a few relatively exotic galaxies.
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I like to call these the prima donna of the galaxy world,
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because they are kind of show offs.
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And we call them active galactic nuclei.
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And we call them that because their nucleus,
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or their center, are very active.
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So, at the center there, that's actually where
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most of the starlight comes out from.
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And yet, what we actually see is light
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that can't be explained by the starlight.
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It's way more energetic.
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In fact, in a few examples it's like the ones that we're seeing here.
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There are also jets emanating out from the center.
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Again, a source of energy that's very difficult to explain
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if you just think that galaxies are composed of stars.
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So, what people have thought is that perhaps
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there are supermassive black holes
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which matter is falling on to.
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So, you can't see the black hole itself,
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but you can convert the gravitational energy of the black hole
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into the light we see.
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So, there is the thought that maybe supermassive black holes
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exist at the center of galaxies.
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But it's a kind of indirect argument.
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Nonetheless, it's given rise to the notion
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that maybe it's not just these prima donnas
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that have these supermassive black holes,
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but rather all galaxies might harbor these
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supermassive black holes at their centers.
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And if that's the case -- and this is an example of a normal galaxy;
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what we see is the star light.
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And if there is a supermassive black hole,
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what we need to assume is that it's a black hole on a diet.
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Because that is the way to suppress the energetic phenomena that we see
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in active galactic nuclei.
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If we're going to look for these stealth black holes
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at the center of galaxies,
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the best place to look is in our own galaxy, our Milky Way.
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And this is a wide field picture
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taken of the center of the Milky Way.
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And what we see is a line of stars.
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And that is because we live in a galaxy which has
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a flattened, disk-like structure.
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And we live in the middle of it, so when we look towards the center,
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we see this plane which defines the plane of the galaxy,
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or line that defines the plane of the galaxy.
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Now, the advantage of studying our own galaxy
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is it's simply the closest example of the center of a galaxy
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that we're ever going to have, because the next closest galaxy
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is 100 times further away.
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So, we can see far more detail in our galaxy
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than anyplace else.
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And as you'll see in a moment, the ability to see detail
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is key to this experiment.
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So, how do astronomers prove that there is a lot of mass
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inside a small volume?
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Which is the job that I have to show you today.
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And the tool that we use is to watch the way
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stars orbit the black hole.
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Stars will orbit the black hole
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in the very same way that planets orbit the sun.
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It's the gravitational pull
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that makes these things orbit.
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If there were no massive objects these things would go flying off,
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or at least go at a much slower rate
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because all that determines how they go around
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is how much mass is inside its orbit.
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So, this is great, because remember my job is to show
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there is a lot of mass inside a small volume.
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So, if I know how fast it goes around, I know the mass.
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And if I know the scale of the orbit I know the radius.
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So, I want to see the stars
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that are as close to the center of the galaxy as possible.
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Because I want to show there is a mass inside as small a region as possible.
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So, this means that I want to see a lot of detail.
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And that's the reason that for this experiment we've used
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the world's largest telescope.
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This is the Keck observatory. It hosts two telescopes
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with a mirror 10 meters, which is roughly
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the diameter of a tennis court.
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Now, this is wonderful,
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because the campaign promise
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of large telescopes is that is that the bigger the telescope,
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the smaller the detail that we can see.
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But it turns out these telescopes, or any telescope on the ground
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has had a little bit of a challenge living up to this campaign promise.
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And that is because of the atmosphere.
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Atmosphere is great for us; it allows us
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to survive here on Earth.
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But it's relatively challenging for astronomers
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who want to look through the atmosphere to astronomical sources.
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So, to give you a sense of what this is like,
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it's actually like looking at a pebble
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at the bottom of a stream.
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Looking at the pebble on the bottom of the stream,
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the stream is continuously moving and turbulent,
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and that makes it very difficult to see the pebble on the bottom of the stream.
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Very much in the same way, it's very difficult
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to see astronomical sources, because of the
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atmosphere that's continuously moving by.
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So, I've spent a lot of my career working on ways
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to correct for the atmosphere, to give us a cleaner view.
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And that buys us about a factor of 20.
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And I think all of you can agree that if you can
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figure out how to improve life by a factor of 20,
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you've probably improved your lifestyle by a lot,
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say your salary, you'd notice, or your kids, you'd notice.
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And this animation here shows you one example of
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the techniques that we use, called adaptive optics.
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You're seeing an animation that goes between
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an example of what you would see if you don't use this technique --
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in other words, just a picture that shows the stars --
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and the box is centered on the center of the galaxy,
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where we think the black hole is.
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So, without this technology you can't see the stars.
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With this technology all of a sudden you can see it.
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This technology works by introducing a mirror
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into the telescope optics system
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that's continuously changing to counteract what the atmosphere is doing to you.
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So, it's kind of like very fancy eyeglasses for your telescope.
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Now, in the next few slides I'm just going to focus on
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that little square there.
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So, we're only going to look at the stars inside that small square,
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although we've looked at all of them.
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So, I want to see how these things have moved.
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And over the course of this experiment, these stars
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have moved a tremendous amount.
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So, we've been doing this experiment for 15 years,
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and we see the stars go all the way around.
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Now, most astronomers have a favorite star,
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and mine today is a star that's labeled up there, SO-2.
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Absolutely my favorite star in the world.
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And that's because it goes around in only 15 years.
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And to give you a sense of how short that is,
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the sun takes 200 million years to go around the center of the galaxy.
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Stars that we knew about before, that were as close to the center of the galaxy
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as possible, take 500 years.
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And this one, this one goes around in a human lifetime.
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That's kind of profound, in a way.
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But it's the key to this experiment. The orbit tells me
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how much mass is inside a very small radius.
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So, next we see a picture here that shows you
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before this experiment the size to which we could
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confine the mass of the center of the galaxy.
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What we knew before is that there was four million
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times the mass of the sun inside that circle.
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And as you can see, there was a lot of other stuff inside that circle.
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You can see a lot of stars.
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So, there was actually lots of alternatives
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to the idea that there was a supermassive black hole at the center of the galaxy,
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because you could put a lot of stuff in there.
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But with this experiment, we've confined
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that same mass to a much smaller volume
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that's 10,000 times smaller.
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And because of that, we've been able to show
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that there is a supermassive black hole there.
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To give you a sense of how small that size is,
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that's the size of our solar system.
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So, we're cramming four million times the mass of the sun
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into that small volume.
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Now, truth in advertising. Right?
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I have told you my job is to get it down to the Schwarzchild radius.
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And the truth is, I'm not quite there.
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But we actually have no alternative today
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to explaining this concentration of mass.
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And, in fact, it's the best evidence we have to date
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for not only existence of a supermassive black hole
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at the center of our own galaxy, but any in our universe.
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So, what next? I actually think
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this is about as good as we're going to do with today's technology,
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so let's move on with the problem.
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So, what I want to tell you, very briefly,
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is a few examples
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of the excitement of what we can do today
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at the center of the galaxy, now that we know that there is,
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or at least we believe,
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that there is a supermassive black hole there.
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And the fun phase of this experiment
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is, while we've tested some of our ideas
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about the consequences of a supermassive black hole
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being at the center of our galaxy,
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almost every single one
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has been inconsistent with what we actually see.
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And that's the fun.
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So, let me give you the two examples.
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You can ask, "What do you expect
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for the old stars, stars that have been around the center of the galaxy
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for a long time, they've had plenty of time to interact with the black hole."
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What you expect there is that old stars
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should be very clustered around the black hole.
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You should see a lot of old stars next to that black hole.
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Likewise, for the young stars, or in contrast, the young stars,
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they just should not be there.
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A black hole does not make a kind neighbor to a stellar nursery.
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To get a star to form, you need a big ball of gas and dust to collapse.
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And it's a very fragile entity.
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And what does the big black hole do?
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It strips that gas cloud apart.
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It pulls much stronger on one side than the other
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and the cloud is stripped apart.
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In fact, we anticipated that star formation shouldn't proceed in that environment.
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So, you shouldn't see young stars.
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So, what do we see?
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Using observations that are not the ones I've shown you today,
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we can actually figure out which ones are old and which ones are young.
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The old ones are red.
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The young ones are blue. And the yellow ones, we don't know yet.
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So, you can already see the surprise.
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There is a dearth of old stars.
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There is an abundance of young stars, so it's the exact opposite of the prediction.
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So, this is the fun part.
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And in fact, today, this is what we're trying to figure out,
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this mystery of how do you get --
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how do you resolve this contradiction.
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So, in fact, my graduate students
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are, at this very moment, today, at the telescope,
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in Hawaii, making observations to get us
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hopefully to the next stage,
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where we can address this question
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of why are there so many young stars,
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and so few old stars.
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To make further progress we really need to look at the orbits
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of stars that are much further away.
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To do that we'll probably need much more
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sophisticated technology than we have today.
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Because, in truth, while I said we're correcting
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for the Earth's atmosphere, we actually only
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correct for half the errors that are introduced.
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We do this by shooting a laser up into the atmosphere,
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and what we think we can do is if we
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shine a few more that we can correct the rest.
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So this is what we hope to do in the next few years.
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And on a much longer time scale,
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what we hope to do is build even larger telescopes,
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because, remember, bigger is better in astronomy.
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So, we want to build a 30 meter telescope.
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And with this telescope we should be able to see
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stars that are even closer to the center of the galaxy.
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And we hope to be able to test some of
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Einstein's theories of general relativity,
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some ideas in cosmology about how galaxies form.
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So, we think the future of this experiment
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is quite exciting.
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So, in conclusion, I'm going to show you an animation
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that basically shows you how these
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orbits have been moving, in three dimensions.
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And I hope, if nothing else,
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I've convinced you that, one, we do in fact
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have a supermassive black hole at the center of the galaxy.
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And this means that these things do exist in our universe,
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and we have to contend with this, we have to explain
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how you can get these objects in our physical world.
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Second, we've been able to look at that interaction
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of how supermassive black holes interact,
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and understand, maybe, the role in which they play
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in shaping what galaxies are, and how they work.
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And last but not least,
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none of this would have happened
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without the advent of the tremendous progress
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that's been made on the technology front.
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And we think that this is a field that is moving incredibly fast,
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and holds a lot in store for the future.
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Thanks very much.
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(Applause)
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ABOUT THE SPEAKER
Andrea Ghez - AstronomerAndrea Ghez is a stargazing detective, tracking the visible and invisible forces lurking in the vastness of interstellar space.
Why you should listen
Seeing the unseen (from 26,000 light-years away) is a specialty of UCLA astronomer Andrea Ghez. From the highest and coldest mountaintop of Hawaii, home of the Keck Observatory telescopes, using bleeding-edge deep-space-scrying technology, Ghez handily confirmed 30 years of suspicions of what lies at the heart of the Milky Way galaxy -- a supermassive black hole, which sends its satellite stars spinning in orbits approaching the speed of light.
Ghez received a MacArthur "genius grant" in 2008 for her work in surmounting the limitations of earthbound telescopes. Early in her career, she developed a technique known as speckle imaging, which combined many short exposures from a telescope into one much-crisper image. Lately she's been using adaptive optics to further sharpen our view from here -- and compile evidence of young stars at the center of the universe.
Andrea Ghez | Speaker | TED.com