16:57
TEDGlobal 2009

Andrea Ghez: The hunt for a supermassive black hole

Filmed:

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.

- Astronomer
Andrea Ghez is a stargazing detective, tracking the visible and invisible forces lurking in the vastness of interstellar space. Full bio

How do you observe something you can't see?
00:15
This is the basic question of somebody who's interested
00:18
in finding and studying black holes.
00:21
Because black holes are objects
00:23
whose pull of gravity is so intense
00:25
that nothing can escape it, not even light,
00:28
so you can't see it directly.
00:30
So, my story today about black holes
00:32
is about one particular black hole.
00:35
I'm interested in finding whether or not
00:37
there is a really massive, what we like to call
00:40
"supermassive" black hole at the center of our galaxy.
00:43
And the reason this is interesting is that
00:46
it gives us an opportunity to prove
00:49
whether or not these exotic objects really exist.
00:52
And second, it gives us the opportunity
00:56
to understand how these supermassive black holes
00:58
interact with their environment,
01:01
and to understand how they affect the formation and evolution
01:03
of the galaxies which they reside in.
01:06
So, to begin with,
01:09
we need to understand what a black hole is
01:11
so we can understand the proof of a black hole.
01:14
So, what is a black hole?
01:16
Well, in many ways a black hole is an incredibly simple object,
01:18
because there are only three characteristics that you can describe:
01:22
the mass,
01:25
the spin, and the charge.
01:27
And I'm going to only talk about the mass.
01:29
So, in that sense, it's a very simple object.
01:31
But in another sense, it's an incredibly complicated object
01:34
that we need relatively exotic physics to describe,
01:36
and in some sense represents the breakdown of our physical understanding
01:39
of the universe.
01:43
But today, the way I want you to understand a black hole,
01:45
for the proof of a black hole,
01:47
is to think of it as an object
01:49
whose mass is confined to zero volume.
01:51
So, despite the fact that I'm going to talk to you about
01:54
an object that's supermassive,
01:56
and I'm going to get to what that really means in a moment,
01:59
it has no finite size.
02:01
So, this is a little tricky.
02:04
But fortunately there is a finite size that you can see,
02:06
and that's known as the Schwarzschild radius.
02:10
And that's named after the guy who recognized
02:13
why it was such an important radius.
02:15
This is a virtual radius, not reality; the black hole has no size.
02:17
So why is it so important?
02:20
It's important because it tells us
02:22
that any object can become a black hole.
02:24
That means you, your neighbor, your cellphone,
02:28
the auditorium can become a black hole
02:31
if you can figure out how to compress it down
02:33
to the size of the Schwarzschild radius.
02:36
At that point, what's going to happen?
02:38
At that point gravity wins.
02:41
Gravity wins over all other known forces.
02:43
And the object is forced to continue to collapse
02:45
to an infinitely small object.
02:48
And then it's a black hole.
02:50
So, if I were to compress the Earth down to the size of a sugar cube,
02:52
it would become a black hole,
02:57
because the size of a sugar cube is its Schwarzschild radius.
02:59
Now, the key here is to figure out what that Schwarzschild radius is.
03:03
And it turns out that it's actually pretty simple to figure out.
03:06
It depends only on the mass of the object.
03:10
Bigger objects have bigger Schwarzschild radii.
03:12
Smaller objects have smaller Schwarzschild radii.
03:14
So, if I were to take the sun
03:17
and compress it down to the scale of the University of Oxford,
03:19
it would become a black hole.
03:22
So, now we know what a Schwarzschild radius is.
03:25
And it's actually quite a useful concept,
03:28
because it tells us not only
03:30
when a black hole will form,
03:32
but it also gives us the key elements for the proof of a black hole.
03:34
I only need two things.
03:37
I need to understand the mass of the object
03:39
I'm claiming is a black hole,
03:41
and what its Schwarzschild radius is.
03:43
And since the mass determines the Schwarzschild radius,
03:45
there is actually only one thing I really need to know.
03:47
So, my job in convincing you
03:49
that there is a black hole
03:51
is to show that there is some object
03:53
that's confined to within its Schwarzschild radius.
03:55
And your job today is to be skeptical.
03:58
Okay, so, I'm going to talk about no ordinary black hole;
04:01
I'm going to talk about supermassive black holes.
04:05
So, I wanted to say a few words about what an ordinary black hole is,
04:08
as if there could be such a thing as an ordinary black hole.
04:10
An ordinary black hole is thought to be the end state
04:13
of a really massive star's life.
04:16
So, if a star starts its life off
04:18
with much more mass than the mass of the Sun,
04:20
it's going to end its life by exploding
04:22
and leaving behind these beautiful supernova remnants that we see here.
04:25
And inside that supernova remnant
04:28
is going to be a little black hole
04:30
that has a mass roughly three times the mass of the Sun.
04:32
On an astronomical scale
04:35
that's a very small black hole.
04:37
Now, what I want to talk about are the supermassive black holes.
04:39
And the supermassive black holes are thought to reside at the center of galaxies.
04:42
And this beautiful picture taken with the Hubble Space Telescope
04:46
shows you that galaxies come in all shapes and sizes.
04:49
There are big ones. There are little ones.
04:52
Almost every object in that picture there is a galaxy.
04:54
And there is a very nice spiral up in the upper left.
04:57
And there are a hundred billion stars in that galaxy,
05:00
just to give you a sense of scale.
05:04
And all the light that we see from a typical galaxy,
05:06
which is the kind of galaxies that we're seeing here,
05:08
comes from the light from the stars.
05:10
So, we see the galaxy because of the star light.
05:12
Now, there are a few relatively exotic galaxies.
05:14
I like to call these the prima donna of the galaxy world,
05:18
because they are kind of show offs.
05:21
And we call them active galactic nuclei.
05:23
And we call them that because their nucleus,
05:25
or their center, are very active.
05:27
So, at the center there, that's actually where
05:30
most of the starlight comes out from.
05:32
And yet, what we actually see is light
05:34
that can't be explained by the starlight.
05:36
It's way more energetic.
05:39
In fact, in a few examples it's like the ones that we're seeing here.
05:41
There are also jets emanating out from the center.
05:43
Again, a source of energy that's very difficult to explain
05:46
if you just think that galaxies are composed of stars.
05:50
So, what people have thought is that perhaps
05:52
there are supermassive black holes
05:54
which matter is falling on to.
05:57
So, you can't see the black hole itself,
06:00
but you can convert the gravitational energy of the black hole
06:02
into the light we see.
06:05
So, there is the thought that maybe supermassive black holes
06:07
exist at the center of galaxies.
06:09
But it's a kind of indirect argument.
06:11
Nonetheless, it's given rise to the notion
06:13
that maybe it's not just these prima donnas
06:15
that have these supermassive black holes,
06:18
but rather all galaxies might harbor these
06:20
supermassive black holes at their centers.
06:23
And if that's the case -- and this is an example of a normal galaxy;
06:25
what we see is the star light.
06:28
And if there is a supermassive black hole,
06:30
what we need to assume is that it's a black hole on a diet.
06:32
Because that is the way to suppress the energetic phenomena that we see
06:35
in active galactic nuclei.
06:38
If we're going to look for these stealth black holes
06:41
at the center of galaxies,
06:44
the best place to look is in our own galaxy, our Milky Way.
06:46
And this is a wide field picture
06:50
taken of the center of the Milky Way.
06:52
And what we see is a line of stars.
06:55
And that is because we live in a galaxy which has
06:58
a flattened, disk-like structure.
07:00
And we live in the middle of it, so when we look towards the center,
07:02
we see this plane which defines the plane of the galaxy,
07:04
or line that defines the plane of the galaxy.
07:06
Now, the advantage of studying our own galaxy
07:10
is it's simply the closest example of the center of a galaxy
07:13
that we're ever going to have, because the next closest galaxy
07:16
is 100 times further away.
07:18
So, we can see far more detail in our galaxy
07:21
than anyplace else.
07:23
And as you'll see in a moment, the ability to see detail
07:25
is key to this experiment.
07:27
So, how do astronomers prove that there is a lot of mass
07:30
inside a small volume?
07:33
Which is the job that I have to show you today.
07:35
And the tool that we use is to watch the way
07:38
stars orbit the black hole.
07:40
Stars will orbit the black hole
07:43
in the very same way that planets orbit the sun.
07:45
It's the gravitational pull
07:48
that makes these things orbit.
07:50
If there were no massive objects these things would go flying off,
07:52
or at least go at a much slower rate
07:55
because all that determines how they go around
07:57
is how much mass is inside its orbit.
08:00
So, this is great, because remember my job is to show
08:02
there is a lot of mass inside a small volume.
08:04
So, if I know how fast it goes around, I know the mass.
08:06
And if I know the scale of the orbit I know the radius.
08:09
So, I want to see the stars
08:12
that are as close to the center of the galaxy as possible.
08:14
Because I want to show there is a mass inside as small a region as possible.
08:16
So, this means that I want to see a lot of detail.
08:20
And that's the reason that for this experiment we've used
08:23
the world's largest telescope.
08:25
This is the Keck observatory. It hosts two telescopes
08:27
with a mirror 10 meters, which is roughly
08:30
the diameter of a tennis court.
08:32
Now, this is wonderful,
08:34
because the campaign promise
08:36
of large telescopes is that is that the bigger the telescope,
08:38
the smaller the detail that we can see.
08:41
But it turns out these telescopes, or any telescope on the ground
08:45
has had a little bit of a challenge living up to this campaign promise.
08:48
And that is because of the atmosphere.
08:52
Atmosphere is great for us; it allows us
08:54
to survive here on Earth.
08:56
But it's relatively challenging for astronomers
08:58
who want to look through the atmosphere to astronomical sources.
09:01
So, to give you a sense of what this is like,
09:05
it's actually like looking at a pebble
09:07
at the bottom of a stream.
09:09
Looking at the pebble on the bottom of the stream,
09:11
the stream is continuously moving and turbulent,
09:13
and that makes it very difficult to see the pebble on the bottom of the stream.
09:16
Very much in the same way, it's very difficult
09:20
to see astronomical sources, because of the
09:22
atmosphere that's continuously moving by.
09:24
So, I've spent a lot of my career working on ways
09:26
to correct for the atmosphere, to give us a cleaner view.
09:29
And that buys us about a factor of 20.
09:32
And I think all of you can agree that if you can
09:35
figure out how to improve life by a factor of 20,
09:37
you've probably improved your lifestyle by a lot,
09:40
say your salary, you'd notice, or your kids, you'd notice.
09:42
And this animation here shows you one example of
09:47
the techniques that we use, called adaptive optics.
09:49
You're seeing an animation that goes between
09:52
an example of what you would see if you don't use this technique --
09:54
in other words, just a picture that shows the stars --
09:57
and the box is centered on the center of the galaxy,
10:00
where we think the black hole is.
10:02
So, without this technology you can't see the stars.
10:04
With this technology all of a sudden you can see it.
10:07
This technology works by introducing a mirror
10:09
into the telescope optics system
10:11
that's continuously changing to counteract what the atmosphere is doing to you.
10:13
So, it's kind of like very fancy eyeglasses for your telescope.
10:18
Now, in the next few slides I'm just going to focus on
10:22
that little square there.
10:24
So, we're only going to look at the stars inside that small square,
10:26
although we've looked at all of them.
10:28
So, I want to see how these things have moved.
10:30
And over the course of this experiment, these stars
10:32
have moved a tremendous amount.
10:34
So, we've been doing this experiment for 15 years,
10:36
and we see the stars go all the way around.
10:38
Now, most astronomers have a favorite star,
10:40
and mine today is a star that's labeled up there, SO-2.
10:43
Absolutely my favorite star in the world.
10:47
And that's because it goes around in only 15 years.
10:49
And to give you a sense of how short that is,
10:52
the sun takes 200 million years to go around the center of the galaxy.
10:54
Stars that we knew about before, that were as close to the center of the galaxy
10:59
as possible, take 500 years.
11:02
And this one, this one goes around in a human lifetime.
11:04
That's kind of profound, in a way.
11:08
But it's the key to this experiment. The orbit tells me
11:10
how much mass is inside a very small radius.
11:12
So, next we see a picture here that shows you
11:16
before this experiment the size to which we could
11:19
confine the mass of the center of the galaxy.
11:21
What we knew before is that there was four million
11:24
times the mass of the sun inside that circle.
11:26
And as you can see, there was a lot of other stuff inside that circle.
11:29
You can see a lot of stars.
11:31
So, there was actually lots of alternatives
11:33
to the idea that there was a supermassive black hole at the center of the galaxy,
11:35
because you could put a lot of stuff in there.
11:38
But with this experiment, we've confined
11:40
that same mass to a much smaller volume
11:42
that's 10,000 times smaller.
11:45
And because of that, we've been able to show
11:49
that there is a supermassive black hole there.
11:51
To give you a sense of how small that size is,
11:53
that's the size of our solar system.
11:55
So, we're cramming four million times the mass of the sun
11:57
into that small volume.
12:01
Now, truth in advertising. Right?
12:03
I have told you my job is to get it down to the Schwarzchild radius.
12:06
And the truth is, I'm not quite there.
12:09
But we actually have no alternative today
12:11
to explaining this concentration of mass.
12:13
And, in fact, it's the best evidence we have to date
12:16
for not only existence of a supermassive black hole
12:19
at the center of our own galaxy, but any in our universe.
12:21
So, what next? I actually think
12:24
this is about as good as we're going to do with today's technology,
12:27
so let's move on with the problem.
12:29
So, what I want to tell you, very briefly,
12:31
is a few examples
12:33
of the excitement of what we can do today
12:35
at the center of the galaxy, now that we know that there is,
12:37
or at least we believe,
12:39
that there is a supermassive black hole there.
12:41
And the fun phase of this experiment
12:43
is, while we've tested some of our ideas
12:45
about the consequences of a supermassive black hole
12:48
being at the center of our galaxy,
12:50
almost every single one
12:52
has been inconsistent with what we actually see.
12:54
And that's the fun.
12:56
So, let me give you the two examples.
12:58
You can ask, "What do you expect
13:00
for the old stars, stars that have been around the center of the galaxy
13:02
for a long time, they've had plenty of time to interact with the black hole."
13:04
What you expect there is that old stars
13:08
should be very clustered around the black hole.
13:10
You should see a lot of old stars next to that black hole.
13:12
Likewise, for the young stars, or in contrast, the young stars,
13:16
they just should not be there.
13:20
A black hole does not make a kind neighbor to a stellar nursery.
13:22
To get a star to form, you need a big ball of gas and dust to collapse.
13:26
And it's a very fragile entity.
13:30
And what does the big black hole do?
13:32
It strips that gas cloud apart.
13:34
It pulls much stronger on one side than the other
13:36
and the cloud is stripped apart.
13:38
In fact, we anticipated that star formation shouldn't proceed in that environment.
13:40
So, you shouldn't see young stars.
13:43
So, what do we see?
13:45
Using observations that are not the ones I've shown you today,
13:47
we can actually figure out which ones are old and which ones are young.
13:49
The old ones are red.
13:52
The young ones are blue. And the yellow ones, we don't know yet.
13:54
So, you can already see the surprise.
13:57
There is a dearth of old stars.
13:59
There is an abundance of young stars, so it's the exact opposite of the prediction.
14:01
So, this is the fun part.
14:05
And in fact, today, this is what we're trying to figure out,
14:07
this mystery of how do you get --
14:09
how do you resolve this contradiction.
14:11
So, in fact, my graduate students
14:13
are, at this very moment, today, at the telescope,
14:15
in Hawaii, making observations to get us
14:19
hopefully to the next stage,
14:22
where we can address this question
14:24
of why are there so many young stars,
14:26
and so few old stars.
14:28
To make further progress we really need to look at the orbits
14:30
of stars that are much further away.
14:32
To do that we'll probably need much more
14:34
sophisticated technology than we have today.
14:36
Because, in truth, while I said we're correcting
14:38
for the Earth's atmosphere, we actually only
14:40
correct for half the errors that are introduced.
14:42
We do this by shooting a laser up into the atmosphere,
14:44
and what we think we can do is if we
14:47
shine a few more that we can correct the rest.
14:50
So this is what we hope to do in the next few years.
14:52
And on a much longer time scale,
14:54
what we hope to do is build even larger telescopes,
14:56
because, remember, bigger is better in astronomy.
14:59
So, we want to build a 30 meter telescope.
15:02
And with this telescope we should be able to see
15:04
stars that are even closer to the center of the galaxy.
15:06
And we hope to be able to test some of
15:09
Einstein's theories of general relativity,
15:11
some ideas in cosmology about how galaxies form.
15:14
So, we think the future of this experiment
15:17
is quite exciting.
15:19
So, in conclusion, I'm going to show you an animation
15:22
that basically shows you how these
15:24
orbits have been moving, in three dimensions.
15:26
And I hope, if nothing else,
15:29
I've convinced you that, one, we do in fact
15:31
have a supermassive black hole at the center of the galaxy.
15:33
And this means that these things do exist in our universe,
15:36
and we have to contend with this, we have to explain
15:39
how you can get these objects in our physical world.
15:41
Second, we've been able to look at that interaction
15:44
of how supermassive black holes interact,
15:47
and understand, maybe, the role in which they play
15:50
in shaping what galaxies are, and how they work.
15:54
And last but not least,
15:57
none of this would have happened
15:59
without the advent of the tremendous progress
16:01
that's been made on the technology front.
16:04
And we think that this is a field that is moving incredibly fast,
16:06
and holds a lot in store for the future.
16:10
Thanks very much.
16:13
(Applause)
16:15

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About the Speaker:

Andrea Ghez - Astronomer
Andrea 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.

More profile about the speaker
Andrea Ghez | Speaker | TED.com