Original subtitles

The milky way...

...an empire of over 200 billion stars.

The earth is our home. The sun is our star.

And the milky way is our galaxy.

It's us. It's our home.

But where did it all come from?

Why do galaxies form at all?

Something has to happen. Something has to mix things up.

So what sparked our galaxy into life?

New research suggests an unlikely hero.

At the center of our galaxy is a massive black hole.

And by massive, i mean really massive.

Even though this thing is terrifying,

our galaxy depends on it.

Could this monster,

the great destroyer of the universe,

actually be a great creator?

Could a black hole have built our home, the milky way?

The black hole may be responsible

for the beginning of our galaxy,

and it'll definitely ultimately be responsible for its death.

look around the universe,

and you'll see galaxies of every kind,

a kaleidoscopic array of unique shapes and sizes.

These grand galactic structures fill the cosmos.

The basic building block of the universe is the galaxy,

and there are hundreds of billions of galaxies

in the universe.

The same way that cells make up your body

or bricks make up a building,

galaxies make up the universe.

We should thank our lucky stars for galaxies.

Galaxies are the only place in the universe

where stars and planets form.

We don't see stars out between the galaxies.

This is the only place

where the hydrogen is brought together,

heated up, and a generation of life can begin.

There are hundreds of billions of galaxies in the universe,

but only one gave birth to us.

I sometimes ask my students to identify where they live.

Well, you would say, for example, "new york city,"

the us of a, "planet earth,"

third planet from the sun.

And then, you would say the milky way galaxy.

The milky way galaxy is our home.

Because we see it from within,

all we see is a band of stars stretched across the sky.

But viewed from outside,

we'd see a spectacular giant spiral galaxy

made up of over 200 billion stars.

Our sun is just a dot within one of its enormous arms.

Our milky way galaxy is huge.

It's actually one of the biggest galaxies in the universe, as a matter of fact.

I'd probably put it in the top 10% certainly.

Massive, magnificent.

Our galaxy has long hidden its secrets at its heart.

The greatest question how did it form?

At stake is not just the answer to the origins of our galaxy,

but the origins of our solar system,

our star, the sun, and ultimately, us.

The milky way's past and the whole story

leads us to where we are now and who we are now.

So what created the milky way,

and how did it grow

into the majestic galaxy we see today?

To answer that,

we have to travel back to the infant universe,

to just after the big bang.

13.6 billion years ago,

There are no stars, no planets...

And no galaxies.

How do we go from that early universe

that's almost perfectly, perfectly featureless

to this complex and very interesting universe

that we see around us today?

The early universe is a thick, uniform soup of gas

with some tiny irregularities.

But it's enough to set gravity to work, pulling gas together.

Gravity keeps on compressing the gas down to a point.

And that's when temperatures rise dramatically

to 50-to 100-million degrees.

At that point, you get ignition.

At that point, hydrogen fuses into helium, and we get a star.

A star is born.

In this theory,

not one, but millions of stars burst into life.

Slowly, gravity brings them together.

After a few million years,

they form a rotating sphere of stars,

and a galaxy is born.

There's a problem, though.

There's too much gravity.

Something other than just the stars

must be holding them together.

But what is it?

Turns out, the answer lies at the center of our own galaxy.

At the very heart of the milky way,

you see stars orbiting something that isn't there.

And if you do the calculations,

the amount of mass needed at the very center

is about four million times the mass of our sun.

So stars are basically orbiting like planets

around this empty object

with four million times the mass of the sun.

This object must be colossal.

It must be unimaginably dense.

It could only be one thing

a black hole,

a supermassive black hole.

if the moon goes around the earth,

and the earth goes around the sun,

then what does the sun go around?

The sun goes around a massive black hole

at the center of the galaxy.

Within the milky way,

scientists find their first supermassive black hole.

But it wasn't the last.

Turns out, they're everywhere.

We know that most big galaxies

have a supermassive black hole right in their center.

That's telling us that these two things are related.

They come as a pair.

Somehow, the black holes and the galaxies

and their origin and evolution are tied together.

Amazingly, the milky way,

this sparkling expanse of stars,

is all intrinsically linked

to the darkest

and most enigmatic entity in the universe

a supermassive black hole.

It is an object of tremendous fascination and mystery.

How did it get there?

How did it grow to be so large?

Is it gonna continue to grow?

To figure out the origins of our galaxy,

we must first find out how it got its black hole.

In the early universe, the first stars burst into life.

But these stars, they're nothing like our sun.

Those first stars were very, very massive.

And one of the things that happens with massive stars

is they explode quickly.

In just a few hundred million years,

the biggest burn through their hydrogen fuel...

And die.

They would've exploded

as incredibly powerful supernovae, exploding stars.

Their cores would've collapsed to form black holes,

and this may have been

the very first black holes that formed in the universe.

These black holes would start small.

Over billions of years,

one would eat and grow into the monster

that now sits at the heart of our galaxy.

It's a solid theory, but there's a problem.

Astronomers find super-bright lights

in the very early universe.

These aren't stars.

They're called quasars.

Quasars are the bad boys of astronomy.

When we first found them, we were puzzled,

because how can an object emit so much energy?

The energy output is sufficient to light up the entire universe.

These quasars,

though smaller than our solar system,

somehow outshine 100 galaxies put together.

The energy emitted vastly exceeds the energy in a star.

The only process we know

that would produce that kind of energy

is the collapse of huge amounts of matter

into a massive black hole.

We realized, "oh, my god.

These are, in fact, huge, raging black holes."

they're much bigger

than those made at the end of a star's life.

We're not just talking about a stellar mass black hole,

which might have 5 or 10 or 20 times the mass of the sun.

We're talking about a true monster

that has millions or billions of times the mass of the sun.

So where do these black holes come from?

They're way too big to be the result of early exploding stars.

They have to be formed in another way.

The theory that stars formed first,

converging to build galaxies,

needs a radical overhaul.

Instead, does the black hole come first?

is it the mother Of all creation,

giving birth to the milky way, the stars, and us?

The milky way,

our vast, incandescent galaxy, has a heart of darkness.

But which came first the light or the dark?

It's almost sort of like a chicken and an egg.

Which came first, the galaxy or the black hole?

Do you need a black hole to make a galaxy,

or do you need a large galaxy to make a large black hole?

Did the black hole come first?

Or did the stars and the galaxy come first?

In one theory, stars come first.

The biggest die,

creating a black hole during their death throes.

But the discovery of quasars challenges this.

There are supermassive black holes

at the very start of the universe,

far too large to be the remnants of the first stars.

So where do they come from?

And could they go on to create galaxies?

Enter the new theory of direct collapse.

In this theory, in the very early universe,

you have a giant gas cloud

that collapses straight into a black hole.

It's just like the birth of a star,

but the star dies before it's born.

The theory goes like this.

Clouds of gas clump together.

They spiral into a central point,

becoming incredibly dense.

At this point in star formation, the core would ignite.

But here, too much gas and dust is piled in.

The mass of it all is so great

that gravity becomes unstoppable.

It crushes the gas, making it denser and denser,

until it reaches its breaking point.

Finally, the gas collapses...

So violently, it rips through the fabric of space.

A massive black hole is born.

I'm talking about making a black hole that's way bigger

than any kind of black hole

that would form at the end of a star's life.

This could explain how the black holes and quasars

are so huge so early on in the universe.

If true, then it might be black holes come first, before stars.

But for now, it's just a theory.

The jury is still out as to how our galaxy first forms.

The chicken-and-egg question is,

do black holes cause the galaxies

to coil us around them,

or do the galaxies build up and hit some crucial, critical size,

beyond which black holes must form at their center?

And we want to learn about that.

And the only way to learn about

that is to look out in the universe and try and find out.

To prove one of our theories,

we need observational evidence.

And a small dwarf galaxy might provide it.

Henize 2-10 is young.

Many of its stars are just a few million years old.

It might provide us a look back

at our milky way in its infant years.

Henize 2-10 is a very interesting, tiny dwarf galaxy.

Originally, i was studying this galaxy

because it has all this star formation going on.

But when i started looking at all of the data,

i was sort of shocked and very excited.

I found a supermassive black hole

at the center of this little galaxy.

Finding a black hole in a galaxy is nothing new,

but the real discovery

is the size of this monster black hole.

Our best estimate for the mass of the black hole in henize 2-10

is a million or two solar masses.

Now, this is comparable to the mass of the black hole

in our own milky way galaxy.

But the milky way is 100,000 light-years across,

whereas henize 2-10 is only a few thousand light-years across.

It's amazing to find a black hole

that is so massive in a small dwarf galaxy.

Before this discovery,

scientists didn't think such a tiny galaxy

could contain such a colossus.

This is completely unexpected.

Usually, supermassive black holes are found

in much larger, much more massive galaxies.

Amy's discovery is groundbreaking.

In henize 2-10,

the black hole is more developed than the galaxy.

It's evidence suggesting the black hole is older,

that it came first.

Could this be the same for other galaxies?

How many dwarf galaxies host massive black holes?

Is henize 2-10 a unique case,

or are there lots of other examples?

We've searched through the sloan digital sky survey

and found over 100 more dwarf galaxies

that have supermassive black holes.

Henize 2-10 could be a blueprint

for how all galaxies first formed,

including our own galaxy, the milky way.

It's fascinating, because it could be the evidence that

the big black holes form first,

and then, the galaxies form around them.

Everything we see in our sky

the stars, our sun,

the planets, our whole galaxy

might all have started as a supermassive black hole.

But how do you go from this

to something as glorious as the milky way we see today?

Where do the stars come from?

13 billion years ago,

The milky way may have started life

as a supermassive black hole,

a huge sphere of black

surrounded by a maelstrom of gas and dust.

This is our galaxy.

But how do you go from this

to the shimmering sweep of stars we see today?

People think of black holes

as being gigantic cosmic vacuum cleaners

that suck everything down.

That's not really true.

If you get too close to one,

yeah, you can fall in, and you'll never get back out.

But they can be a force for creation, as well.

How can a black hole be creative?

One clue black holes aren't just black.

Far from it.

You can think of black holes

as one of the biggest paradoxes in the universe.

They're black, so they don't emit any light.

But they can cause

some of the brightest things in the entire universe.

Quasars prove that these massive black holes

throw out more light than whole galaxies.

Black holes don't just swallow matter.

They also spit it out.

A supermassive black hole is a messy eater.

It's trying to suck matter in,

but it ends up superheating matter and expelling matter,

and sometimes, it will even belch during its meal

and have an outburst.

In the early universe,

the supermassive black hole, the beginnings of the milky way,

is surrounded by gas and dust.

The black holes feast on the matter.

But not all of it is doomed.

When it eats too much too quickly,

it generates so much energy

that even the black hole's gravity can't contain it.

Suddenly, the milky way fires off

highly energized atoms and light from the core...

...pumping out up to a trillion Times more energy than our sun.

If you were to have a close encounter with a supermassive black hole,

you're gonna have to go through a very dangerous environment.

You'd have to survive the intense radiation.

You'd have to survive the jet.

So how do stars form around such violence?

Astronomers find a black hole which might hold the key

to how the milky way got its first stars.

There's a really exciting discovery

of a supermassive black hole,

the kind we normally only find at the hearts of galaxies,

sitting out there by itself with no galaxy around it.

This thing's shining like crazy, so we know it's gobbling up gas.

He0450-2958

sits 5 billion light-years from earth,

a black hole with a huge jet.

This jet is smashing into dust and gas

and its neighboring galaxy.

You'd think it would destroy the galaxy,

but instead, it's helping to build it.

It's next to a big galaxy,

and this big galaxy is forming stars like crazy.

So we think what's going on is, because of the stuff

coming off of the black hole as it's growing,

there are stars being triggered to form

in this galaxy next to it.

The black hole's colossal jet

is the spark needed to create a star factory.

The black hole is emitting radiation.

And when this radiation runs into all the gas in the galaxy,

this causes the gas to clump together,

and new stars get made.

Direct evidence that black holes can create stars.

He0450-2958 might be a look back into the milky way's past.

Our galaxy's supermassive black hole's

violent feasting sparks stars into life.

These stars are drawn

by the black hole's huge gravity and orbit,

building the galaxy.

Well, the black hole

could actually stimulate star formation.

So some people believe that the very fact that we have galaxies

is due to the fact

that we have a raging black hole at the center

which helps to initiate star formation.

It's possible that the black hole

could have created many of the stars we see in our sky today,

including the one star we can't live without,

our sun.

It's kind of amazing

that black holes existed as theoretical constructs

that many of the physicists who were involved

in developing those constructs didn't believe in.

Now, we understand that even perhaps

our very existence depends upon them.

They've gone from objects in our imagination

to objects on which our life depends.

Even though this black hole in the center

is terrifying to conceive of,

in fact, our galaxy depends on it.

And our own planet and star

may have formed because of this system.

13 billion years ago,

The first stars of the milky way spark into life.

The galaxy starts to take shape.

The milky way is now big enough to throw its weight around.

And in the early universe, the milky way is not alone.

Its cosmic neighbors become its prey.

The milky way becomes a cannibal.

The young milky way is growing.

It already contains millions of stars.

Now, it's big enough to enter its next stage of evolution.

It's time to get violent.

Our galaxy turns on its cosmic siblings.

Galaxies are gorgeous,

huge pinwheels

spiraling elegantly throughout the universe.

But there's a dark side to these galaxies.

The process of building up galaxies is one of cannibalism.

The galaxies don't form en masse as large objects.

What they do, like many things,

is form by eating smaller objects.

If we could view the infant universe,

we would see a battle raging.

Dwarf galaxies collide and merge.

And in this arena, size matters.

It's a cosmic roller-derby match.

The players represent dwarf galaxies

which populate the early universe.

If you look at a roller-derby match,

you might get a better idea about what galaxy formation's like.

You've got people skating around the middle of a rink.

There's people slamming all over the place.

It's a very violent process, really chaotic.

And it's exactly the same way around the galaxy.

In a galaxy, you've got this middle

that's attracting everything,

and stuff is swimming around it.

Dwarf galaxies smash into one another.

The larger always get the upper hand.

You've got all this stuff slamming together.

Stars are getting thrown all over the place.

They strip mass from each other. They collide.

And if there are any smaller objects in-between,

they get eaten up.

It's billions of years of destructive mayhem.

It's just this crazy, violent dance

that just goes on over and over again.

In the chaos of collisions,

the milky way grows bigger.

Today, our galaxy dominates our part of the universe.

And even now, it's still devouring other galaxies.

There is a galaxy called sagittarius

which has left a huge trail of stars around the milky way

and is essentially in the process of being devoured.

There's a giant stream of stars coming off of it.

So it's totally just being ripped apart

by the milky way itself.

But in this battle,

the milky way doesn't go unscathed.

This collision could've triggered the formation

of the spiral arms of the milky way itself.

So the reason why the milky way is a spiral galaxy

might be because it's eating up sagittarius.

Violence doesn't just build our galaxy.

It sculpts it...

...smashing the milky way Into shape

and rearranging the positions of the stars,

perhaps even our sun.

It's possible that the sun was actually born

much closer to the middle of the galaxy,

and it's migrated out here to the suburbs

over the course of the last couple billion years.

And it's possible that when sagittarius hit the disk,

it created some spiral arms

that then allowed the sun to migrate out.

The sun and our solar system

are now about 26,000 light-years from the galaxy's center.

For life on earth, that's good news.

If you're too close to the big black hole in the center,

there's a lot going on that can actually hurt life.

There's high-energy radiation.

There are bursts of star formation, supernova explosions.

We're in a quieter, kind of outlying suburb of the galaxy.

And things there are much more conducive to life.

Our galaxy's cannibalism

proves essential for life on earth.

Through violence, we're able to live,

and our galaxy continues to grow.

But can anything stop

the juggernaut of our cannibal galaxy?

Looking out at the milky way,

astronomers find hardly any new stars.

Turns out, something is shutting our galaxy's growth down,

the biggest flamethrower in the universe.

The milky way started small.

Over billions of years, it has grown huge,

spawning over 200 billion stars and counting.

But the count is slowing.

So, in the milky way right now,

there are stars that are being born.

And there's about one star per year

somewhere in our giant galaxy that's being born.

Star formation's not done in the milky way,

but it's settled down.

In its past, the milky way

was bursting with star formation.

So what's changed?

So, one of the big questions in galaxy formation today is,

why isn't more gas turning into stars?

Well, one clue is that black holes

actually might be limiting this process.

In the early universe,

our black hole may have sparked stars into life.

Now, it might be stopping stars from forming.

To find out why, we need to look

at the milky way's supermassive black hole in detail.

And for the first time, we can,

thanks to one of nasa's newest space telescopes, nustar.

Fiona harrison runs the nustar mission.

Its first target

the black hole at the center of our galaxy.

Nustar can see the very highest-energy x-rays

that can penetrate through dust and gas.

It enables us to have this view of this black hole.

Nustar's x-ray vision

sees only the most violent events.

Black hole tantrums are rare, but nustar got lucky.

We looked.

And about six hours after we looked,

we saw the black hole get a hundred times brighter.

How long did that last? Only a few hours.

Then it faded away back into oblivion.

But this event was what we were looking for.

We were all just amazed.

There were cheers in the room.

It was just one of the most exciting moments,

and so early on in the mission, too.

It's direct evidence

our black hole is still active

and still has the muscle to control the galaxy.

The black hole's power is revealed

when it lights up a disk of gas and dust which spins around it.

As this material is swirling around the black hole in a disk,

it rubs against each other.

And there's also magnetic fields and other forces.

All of this heats that disk to much hotter, even, than the sun.

Nustar detects that gas around the black hole

is heating up to 180 million degrees fahrenheit.

That's 18,000 times hotter than the surface of the sun.

This superheated gas is bad news for star formation.

The gas has to get cold in order for it to eventually form stars.

And that's because the gas has to get very, very dense

so that, eventually, the gas can collapse

into something that's gonna have nuclear fusion in its core.

So in regions around black holes,

because they're so hot,

they heat up the gas around them.

And that totally limits the ability

for that gas to turn into stars.

Massive amounts of energy are emitted.

And that actually can not only destroy stars,

it can blow the gas away that would later on form stars.

Star birth shuts down.

Over our galaxy's life,

our supermassive black hole drags gas and dust towards it,

the ingredients needed for star formation.

In its infancy,

its power slams these gas clouds together,

sparking stars into life.

Now, it blows them apart with its extreme heat,

regulating the population of stars in the galaxy.

The black hole in the center acts a little bit like a valve,

controlling how stars form in the galaxy itself.

There is a remarkable symbiotic relationship

between black holes and galaxies.

Black holes act like cosmic regulators,

increasing, at certain times, star formation

and governing the rate at which galaxies evolve.

We're not sure why our black hole

stops some star formation and sparks others.

All we do know

is that this regulation might be essential for us.

When it made stars,

the black hole might have also helped create our sun.

Now, it limits star formation,

which could bring lethal radiation near planet earth.

If we were living in an area where there were

lots of young stars and supernovae blowing up,

that would not be so good for life on earth.

Now the conditions for life are perfect.

Looking up at our night sky, it looks unchanging, eternal.

But in the universe, nothing lasts forever.

Our galaxy is gearing up for its next big change.

So what does the future hold?

The answer is that we won't be a spiral galaxy for much longer.

Our lifetime as a spiral galaxy

is about two-thirds of the way into its final death throes.

The milky way has a giant sister out there,

too close for comfort.

Their sibling rivalry will set the night on fire

and pit two of the biggest heavyweights in the cosmos

in a fight to the death.

Around 13 billion years ago,

the milky way forms around a supermassive black hole.

It adds hundreds of billions of stars,

settles into a flat disk and is sculpted into a spiral.

Our galaxy has constantly evolved.

Its future is no different.

And it's going to get violent.

Go out tonight and look at the night sky

with a pair of binoculars,

and you can see the andromeda galaxy.

That is our future.

Perhaps 4 or 5 billion years from now,

we will be on a collision course with our next-door neighbor.

And it could be like a hostile takeover.

Andromeda is heading straight for us.

Collisions are, of course, nothing new.

In its infancy,

our galaxy grew by colliding and eating other galaxies.

But this time, it's different.

The original schoolyard bully is going to meet its match.

The milky way has always been the biggest thing around.

So any little dwarf galaxy that's gotten near

has gotten torn apart,

but the milky way just keeps right on going.

Now, there's another really big galaxy

that's actually headed right for us right now.

That's andromeda. It's another disk.

And when these two big disks come together,

there's not gonna be a disk left.

Neither of those disks is gonna win.

As the collision nears,

our night sky will change completely.

Today, if you look out when it's really dark,

you see the big band of the milky way.

It's a beautiful thing.

A few billion years from now,

what you would see is not just one band of stars,

but another band of stars that crisscrosses like this.

As it nears,

andromeda grows larger and larger in our sky.

Finally, the galaxies smash into one another.

Stars are torn from their orbits.

The stars don't actually collide.

Stars are extremely small

compared to the space in-between them.

But that's not true for gas clouds.

Gas clouds are very large.

They can actually slam into each other.

When they collide, that creates new star formation.

This gas and dust is gonna get set on fire.

There will be a crazy thing going on,

and maybe even begin to look like fireworks in the sky

as stars are born.

Huge gas clouds blazing out light

from the mass of stars forming in them.

It would be magnificent.

This is our swan song.

This burst of star formation marks the end for our galaxy.

The milky way and andromeda rip each other to shreds.

When these two beautiful structured spirals

smack into each other,

that really orderly shape is going to be destroyed.

And what's probably gonna be left is sort

of a big blob of stars that's called an elliptical galaxy.

Those two galaxies are gonna turn into a ball of stars.

You won't see any bands at all.

It'll just be stars spread across the sky.

The milky way and andromeda are gone.

In their place, a new galaxy, milkomeda.

But it's not over.

Their two supermassive black holes hurtle towards each other.

Those black holes are gonna be hunting for each other.

So you've got two giant black holes,

both more than a million times the mass of the sun,

spiraling in towards each other.

As this is happening,

both of them will probably start gobbling up gas

that happens to be around them.

They're both trying to eat all the gas that's around them,

and they're gonna get bright, so it's gonna be a crazy event.

It will be fantastic.

Two fireballs rotating around each other

until the black holes at the center of them

finally coalesce.

The black holes merge,

forming an even larger supermassive black hole,

a new king to rule over a new galaxy.

But this new galaxy is already dying.

Over billions of years, the stars slowly die out.

There's no fuel left to create new stars and replace them.

What you're left with is basically a dark galaxy.

It's not generating any energy, any heat, any light.

It's just black.

One hundred trillion years

after it was formed in the darkness of the early universe,

the voracious black hole returns to darkness.

Here, it's left to feast on the galaxy it built,

eating the dead remains of stars and planets.

The orbits of the stars decay,

and they fall in toward the supermassive black hole.

And it ultimately it's thought galaxies like the milky way

will just form one supermassive black hole.

In literature,

beginnings and endings are always tied together,

but the same is true for our galaxy's black hole.

It is quite possible that,

without the formation of the black hole at its center,

our galaxy would not have coalesced around it

and have the properties it has.

But the ultimate future of our galaxy

is to collapse into a massive black hole.

So, in that sense, the black hole may be responsible

for the beginning of our galaxy,

and it'll definitely ultimately be responsible for its death.

Our galaxy is magnificent.

All this, everything we see in our night sky,

could be the result

of one of the most fearsome objects in the universe

a supermassive black hole that could've been our creator

and will be the destroyer of our galaxy

and all the galaxies in the universe.

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