TEDxBratislava - Norbert WERNER -- Beating hearts of galaxies


Uploaded by TEDxTalks on 30.06.2011

Transcript:
I grew up in a small town
without too many bright lights disturbing the view of the night sky
so every clear moonless night
I could actually see the Milky Way.
And the view of the dark sky densely sprinkled by stars
was just beautiful.
And because my father was interested in astronomy,
he told me about the finite speed of light.
So I knew that when I look at the moon,
I see the moon the way it looked like one second ago.
That when I look at the sun,
I see the sun the way it looked like eight minutes ago.
And when I look at the stars,
I see them the way they looked like tens
or hundreds or thousands of years ago.
And this was fascinating!
And this was probably the reason,
why I became an astrophysicist.
What I knew a little about was that
how much bigger really the universe is than that.
All the stars that we see on the night sky
are part of our Milky Way galaxy.
If we were able to fly out of our galaxy
and look at it from above,
then it would look something like this galaxy.
And our Sun would be somewhere here in the galactic disk,
two thirds out of the middle.
Our Milky Way galaxy contains two hundred billion stars
out of which we can see with the naked eye
on the night sky only few thousand.
And the highest density of the stars in our Milky Way galaxy
is here in the middle, in the galactic bulge.
And in the middle of the galactic bulge,
in the center, in the heart of our galaxy
is a supermassive black hole.
Now, black holes are truly exotic and extreme objects.
All their mass is compressed into an infinitely small space,
which is surrounded by a boundary called the event horizon.
And nothing,
absolutely nothing, can escape this event horizon.
Everything that crosses the event horizon
ends up in the black hole forever.
Now, if you could compress the Earth into a volume
as big as a grape
it would become a black hole
and its event horizon would be of size of a grape.
Astrophysicists already for some long time suspected
that the largest stars in the universe
end their life by imploding into black holes.
The mass of these black holes
is few times to few tens of times the mass of the Sun.
But the mass of the black hole in the center of our Galaxy
is four million times the mass of the Sun.
So it is a truly supermassive black hole.
Now, black holes are not just holes in the space-time.
They are not undetectable.
Huge gravity drives really energetic processes.
So as the gas from the surrounding galaxy
is falling towards the black hole,
it forms a disk,
which we call an accretion disk.
And the gas is spiraling down to the black hole
in this accretion disk
and the friction heats the gas up.
And so this disc starts to glow.
Very close to the black hole,
the temperatures in the disc are so high,
that it emits X-rays.
And the magnetic fields in the gas
close to the black hole drive winds
and powerful jets,
which are perpendicular to the disc.
So while nothing,
absolutely nothing,
can escape the event horizon,
which is indicated here as this black sphere,
the black holes function as machines,
which are converting the mass of the infalling matter into heat,
into radiation, into outflows.
And interestingly,
the gravity of the black holes is more efficient
in converting mass into energy
than the thermonuclear reactions powering the stars.
Now, the best evidence that we have
that there is indeed a supermassive black hole
in the center of our Galaxy
comes from the observations of the movements of stars
in the center of our Galaxy.
Here, you can see the orbits of stars
in the very very center of our Galaxy
between 1995 and 2010.
And if you look at these stars,
star here marked as SO2,
it made a complete orbit in this time.
And this other star here,
made this really eccentric orbit.
Now the orbits of the stars in the very center of our Galaxy
prove completely unambiguously
that there is a mass of 400 billion times bigger
than the mass of the Sun
compressed into a volume of space
with a radius smaller than the orbit of Neptune.
And the only object,
which can have these properties
is a supermassive black hole.
There were two teams of astronomers working on this,
observing the motions of the stars
around the center of our Galaxy:
One was a European team working with the very large telescopes in Chile.
And the other was an American team working with the Keck telescopes in
Hawaii.
And they produced this image.
Now, our galaxy is not the only galaxy in the universe.
Astronomers working with the Hubble space telescope
pointed this space observatory into seemingly empty spot in the sky.
The surface of this area in the sky was 100 times smaller
than the surface area covered by the full moon.
And they let the space telescope exposed four weeks.
And so, we obtained the most remarkable image,
which I believe is the most remarkable image that humanity ever obtained.
On this image we can see 10 thousands galaxies.
10 thousands galaxies.
And in the heart of each of these galaxies
is a supermassive black hole.
And interestingly, the bigger the galaxy,
the bigger the black hole at its heart.
So these black holes and the galaxies somehow evolve hand in hand.
They influence each other’s growth.
Now, when we look at this image,
we are truly looking into the past.
We are looking to the edge of the observable universe.
From the furthest galaxy on this image,
the light was travelling to us for more than 13 billion years.
And if we scale this image to the whole sky
we get that the observable universe contains 200 billion galaxies.
But these galaxies are not uniformly distributed across the universe.
This is a galaxy map obtained by the Sloan digital sky survey.
And every dot on this image represents a galaxy.
And you can see
that the galaxies form filamentary structure in the universe,
which we call 'the cosmic Web'.
And if you look carefully,
you see that at the intersections of the individual filaments,
the density of the dots is very large.
These are the densest regions of the universe
and we call them clusters of galaxies.
If you look at the galaxy cluster in optical telescope,
you see many many galaxies compressed very close to each other.
But all these visible galaxies are just the tip of the iceberg.
Most of the normal matter, matter made of atoms,
in these cluster of galaxies is in the form of hot gas,
so hot that is emits X-rays.
This is a composite image.
And we see on it the light emitted by the individual galaxy cluster
and this pinkish, purplish hue is the X-ray emission from the hot gas,
which fills the space between the galaxies.
And if you look at the previous images,
you might have thought,
the universe must be really really efficient in forming galaxies
because there are so many.
But it's not true.
Only tenth of the normal matter in the universe
is in the stars and the galaxies.
The rest is in the form of gas filling the space between the galaxies.
So the universe is not very efficient in forming stars.
And if you look at the X-ray image of this galaxy cluster,
you see that the center is very bright.
It's bright because the density of the gas there is relatively high.
And based on simple physical arguments
we would expect this gas to be forming the stars
at the rate of thousand stars per year.
And while the biggest galaxies in the universe
like M87 here at the center of the Virgo cluster
are at the centers of galaxy clusters,
they are still ten times smaller than what we would expect
and they are certainly not forming stars
at the rate of thousands stars per year.
Now, why is that so?
We can get the glimpse of the answer,
if we look into the very center,
into the heart of this galaxy.
And we see there this jet.
This jet coming from the central supermassive black hole
is 4500 light years long.
And it's extremely energetic.
It's composed of superhot plasma,
the particles of which are moving very close to the speed of light.
And now I'll show you a computer animation
that shows us how this jet is interacting with the surrounding gas.
And you can see that this jet
is blowing bubbles into the surrounding gas
and as it is blowing these bubbles
it is driving blast waves into the surrounding environment.
This is an X-ray image of the hot gas in the center of the Virgo cluster.
This is the gas with which we would expect to form stars.
And if you look at this image you see these dark bubbles.
They are dark because the superhot plasma from the jets
pushed the gas out from these bubbles.
And because they are filled with superhot plasma,
these bubbles would buoyantly rise in the cluster atmosphere.
And as they will rise,
they will displace the gas.
The gas, which if left undisturbed
would start to form stars.
If they displace the gas and
they will uplift the densest gas from the center of the cluster,
from the center of the biggest galaxy in the center of the cluster.
And you can see that there are these bubbles
but there are also these bubbles
so there are two different generations of bubbles.
And if you look very carefully
then you might even see ripples on this image.
These ripples are from the blast waves,
which were driven by the jet
while it was inflating these bubbles.
And these blast waves are heating the gas.
So, you can see that these outbursts are a repeated process.
We see two generations of bubbles
and several generations of these ripples.
So this is a repeated process,
so these supermassive black holes are the beating hearts
in the centers of galaxies regulating their growth.
And other very nice example of this process
can be observed in the galaxy M87.
If you look here at the right hand side,
this is a radio image,
and it shows us the superhot plasma from the jets.
And look at this mushroom cloud.
This is the superhot plasma rising in the atmosphere of the cluster.
And as it is rising in the atmosphere of the cluster,
on the left hand side,
you can see an X-ray image,
which shows the gas in the galaxy cluster.
You see that this mushroom cloud
is dragging up the gas from the galaxy M87,
the gas, which if left undisturbed
would be forming stars in the center of that cluster galaxy.
Now, the physics of this process is quite similar
to some of the physics observed here on Earth.
This is the unpronounceable Icelandic volcano
and what we see here is that
as the hot gas blasts through the lava
it drives blast waves into the surrounding environment.
And afterwards, it rises buoyantly
and it uplifts the dark ashes from the bottom of the volcano
into the atmosphere.
Now, you will be able to see it again,
you see the blast waves,
look carefully,
you can see the blast waves
followed by the buoyant rise of the hot gas,
which is uplifting the dark ashes from the volcano
into the atmosphere forming this mushroom cloud.
But the processes, which we see in the Perseus cluster
or in the Virgo cluster
are by no means extreme.
Astronomers recently discovered a galaxy cluster,
which has bubbles 600 000 light years big.
600 000 light years is six time bigger
than the disc of our Milky Way galaxy.
And the energy in this structure
is as big as ten billion supernovae exploding at once.
Ten billion stars exploding at once.
What you can see on this image in blue
is the gas in the galaxy cluster
and in red you see the superhot plasma from the jets on the black hole.
So, if you think about it:
a black hole, the size of which,
in comparison to the size of the galaxy cluster
is like a blueberry in comparison to the size of the Earth
controls the growth and the evolution of the galaxy cluster.
It's amazing.
So these black holes are the beating hearts of the galaxies
controlling their life.
And these hearts do not beat very often,
only once in a 100 million years or so,
but they beat really hard
and these heartbeats are being heard across the universe.
Thank you.