All language subtitles for How the Universe Works - 5X02 Black Holes, The Secret Origin

Two black holes
circle each other

in a dance of death.

They spiral inwards,

their immense gravities
pulling them ever closer.

When they finally collide,

it's one of the most
powerful events

since the big bang.

This explosive mystery

sends ripples
across the world of science.

But can it also answer

one of the most pressing
questions in cosmology?

How do supermassive
black holes grow so large?

In the known universe,

there are roughly
2,000 billion galaxies.

Each one has a different
shape and size.

But they may all have
one feature in common...

a supermassive black hole
buried at their center.

As its name says,
it is supermassive.

And here, we're talking
about objects

that are millions
or billions of times

the mass of the Sun.

Supermassive black holes

are so big
that we need a special scale

for measuring them.

A solar mass
is the mass of the Sun.

So when we study the universe,

we have to use the tools
that we have in hand.

And what's the most massive
thing that we have around us?

It's the Sun.
And so we refer to things

in multiples
of the mass of the Sun

because it just makes it easier
to wrap our heads around.

However, if you have something
that's 17 billion times

the mass of the Sun,
that's pretty difficult

to wrap your head around anyway.

But we know that those
kinds of black holes

live in the centers of galaxies.

The supermassive black hole

the milky way, is called
Sagittarius "a" -star.

It weighs in
at 4 million solar masses.

But compared to the other
supermassive black holes

out there, it's puny.

This is probably
one of the only contexts

where you would think that
our supermassive black hole

isn't very supermassive.

The supermassive black hole

in our neighboring galaxy,
Andromeda,

is 25 times larger
than Sagittarius "A" -star,

coming in at
100 million solar masses.

But compared
to the largest monsters

out in the universe,
it's a runt.

O.J. 287's primary
supermassive black hole

weighs in
at 18 billion solar masses.

And the black hole
in the core of galaxy NGC 4889

in the coma cluster
weighs 21 billion solar stars.

That's over 5,000 times larger

than Sagittarius "a" -star.

These are incredible things

that are more massive
than some galaxies.

Now astronomers may have ma

a giant, new
supermassive black hole

that's a mind-blowing
30 billion times

the mass of the Sun.

It's a huge puzzle.

And we have simply no idea
how it got so big.

It's a huge mystery how black

we started finding
black holes with millions

and billions of times
the Sun's mass.

No one expected that.

And we have no idea
how they got to be so big.

It's not entirely
clear at this point

how supermassive black holes
can get to be the masses

that they are today.

Regular-sized black holes form

when large stars over 20 times
the mass of our sun

crash and burn.

When a large star
runs out of fuel,

the core stops generating
enough outward force

to counteract the power
of gravity crushing inwards.

As the star collapses,

the outer part explodes
in a supernova.

The inner core shrinks

from a sphere
millions of miles wide

to one just 10 miles across.

It's like shrinking
the earth down

to the size of a golf ball.

This rapid collapse
creates a black hole.

So we now have seen black holes

that are solar-mass black
holes

and black holes that are million

or billion-solar-mass
black holes.

And the question is, how do you
get from one to the other?

Do the giants somehow grow

from a solar-mass black hole?

One of the big puzzles today is,

how do you make one of these
supermassive black holes?

One idea is,
you get there by starting

with a solar-mass black hole,

having it grow through a stage

of being an intermediate-mass
black hole

and then eventually
getting to be

a supermassive black hole.

Theoretically,
intermediate-mass black holes

should be between 100
and 100,000 solar masses.

But we've never seen one.

Part of the mystery
of supermassive black holes

is that black holes seem
to occur in two flavors.

You have ones that are
only a couple times

the mass of the Sun.

And you have ones
that are millions

or billions of times
the mass of the Sun.

So we have small
and extra large.

If we think
of the stellar-mass black hole

as sort of the baby black holes,

and the supermassive black holes
as the grown-up black holes,

we're missing
the teenage black holes.

Where are these black holes
that have masses

that are between stellar mass
and supermassive?

They're sort of like
a holy grail

for black hole hunters.

Where are these things?
Where can we find them?

And how do you make them?

Then astronomers caught a break.

They picked up a burst of energy

coming from the NGC 1399 galaxy.

It was the death throes

of a star being eaten
by a black hole.

When they measured its size,
they discovered it was

an elusive intermediate-mass
black hole.

The missing link had been found.

But when scientists did the math

to see if such an
intermediate-mass black hole

could grow into
a supermassive black hole,

they hit a snag.

There hasn't been enough time

since the birth of the universe

for an intermediate-mass
black hole

to eat enough stars

to grow into
a supermassive black hole.

It doesn't seem like
there's enough time

for black holes to get
as big as we see them.

But supermassives
are everywhere we look.

How did they get there?

And how did they grow so huge?

In our universe, we've
detected small black holes.

And we've seen monsters,

supermassive black holes

billions of times
the mass of our sun.

But we'd found
almost none in between.

So how do you get from a small
black hole to a giant one?

One of the most important
outstanding questions

in cosmology is,

how did supermassive black holes

get as big as they are?

And when did that happen?

Black holes are normally S

an all-you-can-eat buffet.

One of the best ideas
for how black holes grow

is that black holes do

what we expect
black holes to do,

and that is eat stuff.

For a black hole,

it's almost as if the universe
is its restaurant.

And on its menu,
you'll find stars, planets,

and clouds of gas and dust.

So is binge-eating the answer

to growing
a supermassive black hole?

Theoretically, black holes
should keep on growing forever

as they consume
more and more food.

But recent discoveries suggest

that the universe
puts them on a diet,

controlling how much they eat.

Black holes are hungry.
They like to eat.

But sometimes,
they eat too much,

and they burp it up.

February 2015.

Astronomers report
something unusual

in the galaxy NGC 2276.

It looked like something
had taken a bite

out of one of its spiral arms.

Sitting alone in the void

was an intermediate-mass
black hole,

about 50,000 times
the mass of the Sun.

One theory
was that the black hole

had eaten everything around it,

creating the dead zone.

But the detection
of a burst of energy

from the black hole suggests

it may have tried
to eat too much

and, in the process,
destroyed its food source,

burping so hard,
its food was blasted away.

Turns out that black holes
are actually very messy

a lot of matter gets thrown
off as it tries to absorb it.

So things move in, gets hot.

But then a lot of it gets
thrown all the way back out.

Black holes
are not vacuums in space.

They do not just eat
everything around them.

And so they are messy.
Some things get in.

And they take that on.
And it grows their mass.

And some things are just
flung out as they're eating.

The enormous gravity of black
holes sucks gas, dust,

and even stars towards them.

Everybody's been to
an all-you-can-eat buffet.

But let's be honest.
There really is a limit

to how much you can eat.

Black holes
are gluttons. They're greedy.

They don't really know
when they've eaten too much.

They just keep on cramming in
more and more food.

It doesn't just fall in.

It has to go down the drain,
more or less.

And so it forms this disk
around the hole.

And as it does that,
there's a lot of turbulence

and magnetic fields

and a witch's brew of forces
going on there

that get it really hot.

As the gas
and dust swirls around,

it heats up,

pushing temperatures to
millions of degrees Fahrenheit.

This swirl, called
the accretion disk,

also generates powerful
magnetic fields.

These fields are dragged
by the SPiN of the black hole

and become focused
above the poles.

As energy builds up,

the magnetic fields
become so compressed

they blast out
super-energized particles.

These beams can actually
be incredibly violent.

Matter is flung out

at a large fraction
of the speed of light.

It's a tremendous wind

that blows very hard away
from the black hole.

The jet hits the gas clouds

surrounding the black hole,

blowing the buffet away.

If they eat too much,

they can basically
blow everything

that's in their vicinity away.

They lose their food supply.
And then they're gonna starve.

They can kind of shoot
themselves in the foot.

With no food available,

the black hole stops growing.

Astronomers think
that's what happened

to the intermediate-mass
black hole

they discovered
in the dead zone.

These burps may
regulate star formation

and stop the black hole
from getting obese.

But over time,

the black hole
will start eating again

as gas falls back towards it.

But can an intermediate-mass
black hole eat enough

to become
a supermassive black hole

weighing billions
of solar masses?

Could that black hole
become so obese by eating?

That's a really
interesting question.

You'd have to eat
a heck of a lot

to get that fat.

When you think about it, if
you imagine an average galaxy

has 100 billion stars,

the black hole
would have to eat one

in every five stars
in the galaxy.

The universe is old.
But is it really old enough

that black holes have had time
to consume billions of stars?

That seems kind of unlikely.

It doesn't seem to add up.

We need some other way

to make these
supermassive black holes.

And the question is,
what is that?

Maybe we've been making
this all too complicated.

Maybe to get a big black hole is
to start big in the first place.

So how can black
holes start big?

To answer that question,
scientists had to journey back

to the very start
of the universe,

to a mysterious time
called the dark ages.

As we look out
into the universe,

we're seeing farther
and farther back in time.

We have now looked back

over 12 billion years

to the time when the cosmos
was still an infant.

And what we found
was a huge surprise.

We had made the assumption

that as you look farther
out into the universe,

the black holes
would be smaller.

They haven't had
much time to grow.

But now we've found
a 12-billion-solar-mass

black hole that's actually less

than a billion years
into the universe.

How did this thing
form so early?

How did it grow so fast?

This is like walking
into a delivery room

and finding a 100-pound baby.

I mean, how does that
even happen?

It doesn't make any sense.

Physics tells us no black hole
could swallow enough stuff

to get that big that quickly.

There really wasn't enough
time between the big bang

and when we're
studying these things

for them to grow
to such large sizes

just by eating
matter around them.

So if there's not enough time

maybe they're born supermassive.

To understand how, we have
to travel back even farther,

to not long after
the birth of the universe.

The early universe
was definitely

a much more compact

and richer place for material.

It was smaller,
and it was denser.

Things were much closer.
It was hotter.

It was just a much
more intense place to be.

Clouds of hydrogen and
helium gas clumped together.

As the clouds grew,
so did their gravity,

sucking in more and more gas.

Eventually, the ball of gas

became so dense, it collapsed,

triggering nuclear fusion.

A star was born.

These massive first stars

are called population III stars.

Because there was
so much food around,

these stars were huge,

many times bigger than
any stars that exist today.

We think a lot of these
population III stars

probably were
incredibly massive,

incredibly short-lived,
and just blew up right away.

They would've left massive
black holes behind.

With so much food available,

these young, ravenous
black holes, called quasars,

started binge-eating

and became incredibly bright.

Billions of years later, we
can still see their gluttony.

The most luminous,
bright objects in the universe

are things called quasars.

And it may seem kind of ironic.

But what these really are
are supermassive black holes.

There's so much stuff
trying to cram itself down

the black hole that everything
gets very hot, very energetic.

And you can see them
clear across the universe.

But when we measured
the size of the young quasars,

we discovered they were already
billions of solar masses.

There's not enough time,

a billion years after
the universe was created,

for them to get to
a billion solar masses in...

it's just too short a time.

So the question becomes,

that are this big
in that small amount of time?

We need some other way

of growing these
supermassive black holes.

There needs to be
some other mechanism

that allows them
to get that massive so early.

But what is that?

A clue can be found
in the very early universe.

The early universe is still
so much of a mystery to us.

We know that conditions
were very different.

It was denser.
There was a lot more material.

This period
is called the dark ages.

During the dark age, we know

that there was
basically nothing happening.

Matter existed.

We think that there
was hydrogen and helium gas

but really not much else.

There were a few stars around,

but nothing large enough
to form giant black holes.

But there were
huge clouds of gas.

And because the universe
was much smaller and denser,

the clouds were much thicker.

The idea is that
from these basic ingredients,

gravity and gas,

the cosmos built
massive black holes.

Somehow, the universe
has created a shortcut

to the black hole.

We've typically
thought of it as,

cloud of gas
collapses into a star,

star evolves, star dies,
leaves behind a black hole.

Perhaps the universe
has found a way

to skip the star phase

and go directly
to the black hole.

Clouds of gas may have
built massive black holes

in a process
called direct collapse.

As they collapsed,
they never even formed a star.

They just collapsed straight
into a giant black hole.

Through this direct
collapse theory,

you can form really
big black holes.

Imagine what it's like seeing
one of these giant clouds

of gas collapsing down
into a black hole.

You might think
you start with, okay,

cloud of gas slowly collapsing,

and, boop, it's a black hole.

That wouldn't be the case.
It would be more like

giant cloud of gas
starts collapsing,

then... aah! Black hole.

It's believed
that direct collapse

could have created black holes

up to a million times
the mass of the Sun,

much bigger
than from the collapse

of a single star.

These early black holes

are sort of like the galaxies
that never were.

They were gonna make galaxies.

But instead, they collapsed
into very massive black holes.

For direct collapse
to form a black hole,

the conditions need
to be precise.

The clouds must be
very symmetrical,

forming a smooth ball.

If you have a ball of gas
that isn't quite a ball,

that's not quite homogeneous,

as it collapses, it'll fragment.

And it'll fragment into objects
that won't form black holes.

So you want it to be hot enough

that it stays
one big, giant thing.

But it does need to cool
a little bit, right,

so that you get it
to collapse in on itself.

You have to get
uniform collapse over time

of a very large amount
of hydrogen gas, presumably,

which is the original matter
in the universe,

collapsing spherically
symmetrically,

without fragmenting,

over a period of less
than 500 million years.

Direct collapse
may have created black holes

a million times
the mass of the Sun.

But it can't completely explain

the 12 billion solar-mass
supermassive black holes

we see in the early universe.

Maybe gigantic supermassive
black holes were created

by strange, unseen forces.

Maybe they were created

by the mysterious dark universe.

Astronomers looking
deep into the early universe

have discovered gigantic
supermassive black holes.

This is a pretty deep mystery.

There are these
supermassive black holes

that exist
in the very early universe.

And by all accounts,
they should not exist.

According to
the normal laws of physics,

it shouldn't have been possible

for them to grow so big
so quickly.

For astrophysicists,
understanding how black holes

have grown to be so large is
one of our biggest mysteries.

We need some other way

of growing these
supermassive black holes.

There needs to be
some other mechanism

that allows them
to get that massive so early.

But what is that?

Everything we can see
in the night sky

makes up just 4.8%

of all the matter in the cosmos.

The rest is the dark universe,

including dark matter.

We can't see it, feel it,
or detect it directly.

But we know dark matter
is there.

Its gravity is tugging
on everything around it.

And we're beginning
to understand

it plays a fundamental role

in the formation
of the universe.

Most of the stuff
that collects together

gravitationally is dark matter.

So perhaps black holes form

somehow with the inclusion
of dark matter.

One way of looking at it
is there's six times

as much dark matter
as normal matter.

So there's six times
as much food out there

for the black holes to eat

if they're able to tap
into this dark stuff.

Maybe these supermassive
black holes are growing

by eating dark matter.

There are
some tantalizing clues.

The largest supermassive
black holes

don't live in the galaxies
with the most regular matter.

They live in the galaxies
with the most dark matter.

The one thing we know
about dark matter right now

is that it has gravity.

And a black hole runs
on gravity.

It attracts anything with mass.

So there's no reason to assume

that black holes would
only eat regular matter.

And now we know that there's far
more dark matter out there.

Maybe dark matter
helps the black holes eat.

Maybe in some ways,
dark matter is a feeder

for these supermassive
black holes.

Perhaps what really grows
a supermassive black hole

is all of the regular matter
being directed into the center

by the dark matter around it.

Maybe the dark
matter's powerful gravity

sucks in regular matter

and funnels it
into the black hole.

In a sense, the dark
matter is greasing the wheels.

It's sort of tilting
the table up

so that that food can
slide right in.

But now scientists
think the dark matter

may create gigantic
black holes directly

by igniting dark stars.

Some believe that dark matter

sparked early universe
super stars.

When they die, they leave behind
supermassive black holes.

Dark stars sound like

they come from
the fertile imagination

of some Sci-Fi writer.

But Dr. Katie Freese believes
they may explain

how early supermassive
black holes grew so fast.

Dark stars are amazing.

So, when we first had this idea,

we got excited really quickly,

because this is
a new type of star

that has never been seen before.

Dark stars may have
been some of the first stars

to form in the universe.

They sparked into life

when the universe was just
200 million years old.

But how could dark stars form
really massive black holes?

A newborn black hole can't weigh
more than its parent star.

So in order to give birth to
a really massive black hole,

the parent star has to be
supermassive, as well.

These early objects
are really strange.

They're very cool.

And they're really, really big.

The size of these things
is 10 times the distance

between the Sun and the earth.

But how is that possible?

Regular stars have
an upper size limit.

A star is a battle between
gravity pushing inwards

and nuclear fusion pushing out.

When the star grows too big,

its gravity
becomes overwhelming.

The delicate balance

between gravity
and fusion is broken.

Gravity wins out,
and the star collapses.

But dark stars
may have a work-around

that lets them
become supermassive.

So, they are made
of ordinary matter.

They're made
of hydrogen and helium.

But they're powered
by dark matter.

We don't know what
dark matter is made from.

But we do have theories
on how it might power a star.

One of the best ideas
we have for dark matter

is that it's made of weakly
interacting massive particles,

or wimps for short.

So, these wimps are
their own antimatter.

And that means, whenever
they encounter each other,

they annihilate and turn
into something else.

That means a lot of heat
is released, a lot of energy.

And it's that energy
that could power stars.

The energy
from the wimps' annihilations

keeps the star from
collapsing like a normal star.

So it's possible
that, in some stars,

their internal reactions

are actually being powered
by dark matter.

If that's the case, then you
could imagine situations

where, when that burns out,

you produce very massive
black holes.

So it could be that dark matter,

the physics of dark matter,

plays really important roles
in creating black holes

and their prevalence
in the universe.

The energy from the dark matter

allows the dark stars
to grow huge.

When they first form,
they're small.

They're about the mass
of the Sun.

But because they're so cool,

they keep accumulating matter

and growing, growing, growing.

And some of them will get
to be a million times

as massive as the Sun
and a billion times as bright.

But these giants
don't live for long.

Eventually,
the dark matter particles

wipe each other out completely.

And there is no more fuel

to keep the massive amount
of ordinary matter

from collapsing.

And then that's it.

There's nothing to sustain
this big, puffy object.

If it's big enough, you collapse
directly to a black hole.

A monster
supermassive black hole.

It's really fun
to think about the possibility

that the physics of dark matter

is actually helping
to power stars.

If so, it would bring, you know,

a whole new window
into our understanding

of stars and their evolution.

At the moment, dark stars
are just theoretical.

But when the powerful
James Webb telescope

comes online in 2018,

we may get our first glimpse.

We're gonna do an observing
run and look f

and so we're very excited.

If you would find an entirely
new type of star,

that would be huge.

While Katie Freese
looks for dark stars,

another team is investigating
another radical idea

that offers new insight

into how supermassive
black holes grow so huge.

They detect the faint echoes

of a violent event
from across the universe,

the remnants of
an extraordinary collision,

a supremely energetic event

that reveals
black holes are cannibals.

Our universe
is filled with enormous

supermassive black holes
that defy explanation.

Supermassive black holes
are one of the things

in the universe that,
when you run the physics,

when you run the math
of how did they evolve,

they really shouldn't be there.

It's still a profound mystery.

The universe hasn't
been around long enough

for regular black holes

to eat enough matter
to get supermassive.

So how did they get so big?

The most logical answer

is that large black holes
are born large,

around 1 to 2 billion
solar masses.

But that's still
over 10 times smaller

than the largest supermassive
black holes out there.

Given the time scales, it
doesn't seem to add up.

We need some other way

to make these
supermassive black holes.

And the question is,
what is that?

A clue came
from a large, isolated galaxy

200 million light-years away

in a quiet part of the universe.

Nestling alone was
a supermassive black hole

with a mass of 17 billion suns.

Normally, such monsters

are found in dense regions
of space

with lots of galaxies
and lots of stars.

This black holes doesn't match
its surroundings at all.

It's kind of like driving
to the middle of a desert

and coming across
the empire state building.

Now, the empire state building
belongs in the middle of a city.

And a black hole this big

belongs in a rich cluster
of galaxies.

This is the first time
astronomers have found

such a giant object

lurking in such a relatively
empty area of the universe.

So you got to ask the question,

if there's nothing else around,
how exactly do you grow

a 17-billion-solar-mass
black hole?

One possible answer
is the stuff of nightmares.

Maybe the story
of this black hole

is actually a little more
scary than we thought.

Maybe it's all alone

because it ate
all of its neighbors.

Maybe it was eating
more than galaxies.

Maybe it was eating
its own kind.

The thing about black holes
is they're omnivores.

They'll eat anything.

Anything that gets close them,
they'll gobble up.

One way black holes
can grow so large

is by eating other black holes.

So in a sense,
they may be cannibals.

Cannibal black holes
were just theoretical.

We'd never actually
seen them eat each other.

Then scientists detected
the faint echoes

of actual ripples
in space-time.

When engineers turned on

the laser interferometer
gravitational-wave observatory,

or LIGO for short,

they immediately picked up

the faint signal
of gravitational waves.

Gravitational waves are created

by huge explosions in space.

To make them, you need an almost
unimaginably energetic event,

something really, really big...

...something like
merging black holes.

A black hole merger
is the most violent,

the most energetic thing

that happens
in the universe, period.

Picture the scene,
1.3 billion years ago.

Two black holes circle each
other in a dance of death.

The larger black hole
pulls the smaller one inwards

until they're locked together
in a spiral.

Very, very slowly,
that orbit is decaying.

They're getting closer
and closer and closer.

And then they will merge
into one giant black hole,

truly one of the most dramatic
events in the universe.

Finally, they collide

in one of the largest bangs
since the big bang.

I would have loved
to have been able

to safely view the collision

of these two
black holes up close.

Imagine these two black holes

as they spiral
in toward each other,

going faster and faster
and faster and faster.

And then, suddenly, where
there appears to be nothing

or just distortions in space
in front of you,

suddenly, there is this
enormous burst of energy.

And everything
just rains around you.

By measuring the frequency

we can calculate the size
of the objects causing them.

When those two black holes,

weighing 29 solar masses

and 36 solar masses, collided,

they created a black hole
around twice the size.

In some ways,
it's very elegant and simple.

You take two black holes.
You spiral them in together.

And you end up
with one big black hole.

The event showed that black
holes can double their mass

through cannibalism... Almost.

The final black hole was less
than the sum of its parts.

There were 3 solar masses
missing.

That may not sound like a lot.

So let's put it in context.

Our sun is burning

about 100 billion
hydrogen bombs every second.

And over its
10-billion-year lifetime,

it will convert less
than maybe 1% of the mass

of the Sun to energy.

In 2/10 of a second,

3 times the mass
of the Sun in matter

got converted to energy
in that collision.

It was 36 septillion yottawatts.

What does that mean?
A lot of freaking energy.

That's more energy
in that 2/10 of a second

than is emitted by all the stars

in the visible universe
in the same time.

In its first run,
LIGO detected two collisions.

This suggests
that cannibal black holes

are relatively common

and that each feast
builds a larger black hole.

But so far,
the largest black hole

these mergers have produced

is 62 solar masses,

not close to the largest
supermassives we've found.

It's hard to imagine,
in 13.8 billion years,

that there'd be enough
collisions of 30-solar-mass

black holes to build up to form a
billion-solar-mass black hole.

That's 100 million collisions.

So maybe small
black holes eating each other

isn't the solution.

Maybe supermassive black holes

are eating each other.

If so, could
the supermassive black hole

at the heart of our own galaxy
be on the menu?

We've found
supermassive black holes

so large, they defy explanation.

They're too big to have grown

by simply eating
the matter around them.

They can't form the same way
that regular black holes do.

There must be something else
that happens that lets them grow

to such enormous mass.

Too large to have
grown from dark stars

and too big to have grown
from regular black holes

simply eating each other.

Merging black holes
almost certainly play a role

in our understanding
of supermassive black holes.

We think that supermassive black
holes themselves also merge

and have merged regularly over
the course of the universe.

Now, whether this
merging activity itself

is enough to make them that big,

the jury is still out on that.

Now a newly discovered
type of galaxy

may provide an answer.

It's called w2246-0526.

And we can't see it.

But we can detect
the heat it gives off.

This galaxy is an example

of a rare class
of objects called hot dogs.

One of the funnier terms f

is a hot dog galaxy.

And no, this is not
some delicious sausage snack.

In fact, it means "hot,
dust-obscured galaxy."

It's called obscured
because it's shrouded

in so much dust and gas,
the only light that escapes

is infrared in the form of heat.

All this heat must be
coming from somewhere.

So in the core,
there is a cauldron,

a seething
supermassive black hole,

the likes of which
we can't even imagine.

Of all the supermassive
black holes we know of,

the ones that are obscured
in these hot dog galaxies

may be the ones
that are the most ravenous,

consuming many millions of times

the mass of the Sun.

Scientists theorize
that hot dogs

could be the offspring

of cannibal giant black holes.

When the monstrous
black holes merge,

they drag gas and dust
with them.

This brings more food
to the table,

allowing the new black hole
to gorge itself.

When you have these
two galaxies merging,

they have all-new food.

It's a brand-new dinner plate,

a brand-new buffet
of food to eat.

The combination
of cannibalism and fresh food

allows the black holes
to grow super large.

Perhaps this is how
the supermassive black hole

at the center of our galaxy

grew when it was young.

But what's the future

of our supermassive
Sagittarius "a" -star?

As far as
supermassive black holes go,

Sagittarius "a" -star

is actually still kind of
in the minor leagues.

It's small.
But it's not done yet.

It's still eating.
It's still growing.

And in around 4 billion years,

it's going to become
25 times larger,

because it's going to be
eaten by its neighbor.

The giant Andromeda galaxy
is heading our way.

And it's going to engulf
our milky way.

When galaxies merge,

their central supermassive
black holes merge.

Andromeda's huge
supermassive black hole

will drag Sagittarius "a" -star
into orbit...

...gradually drawing it closer
and closer

until it devours it.

The new supermassive
black hole will weigh

around 100 million solar masses.

But the disruption
to the new galaxy

will provide the new
supermassive black hole

with plenty to eat

and the opportunity to grow
a whole lot bigger.

At present,
there are many theories

of how supermassive
black holes get so big.

Most likely, it's a
combination of them all.

But however it happens,
we can be pretty sure

it's one of the most spectacular
things in the universe.

The jury's still out on exactly

how supermassive black holes
become so massive.

Making all
the black holes we see

probably requires
a pretty diverse cookbook.

So any physicist who's looking
for a really simple,

single answer
for how they get made,

they're probably
gonna be disappointed.

It's probably a pretty complex
thing that's going on.

It could be through eating.

It could be through
eating and merging.

And usually, the answer
is somewhere in the middle.

So they will merge
with other black holes.

And they'll also have
a few snacks between mergers.