All language subtitles for How the Universe Works s03e07 Milky Way.eng

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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