All language subtitles for How the Universe Works s08e08 Monsters of the Milky Way.eng

26,000 light-years from Earth,

shrouded in cosmic dust and gas

is a mysterious region
of space...

the center of the Milky Way.

The center of the Milky Way
galaxy is one of the strangest,

most exotic and violent places
in our galaxy.

Gas streaming everywhere,

radiation blasting out,

stars moving willy-nilly.

And at the very heart
is the mysterious black hole,

4 million times
the mass of the sun.

Now we're exploring
the center of the Milky Way

like never before,

uncovering powerful forces
that affect us all.

Everything that happens at the
center of the Milky Way galaxy

really is connected
to what's going on

in the rest of the Milky Way.

Understanding
the center of our galaxy

unlock secrets of our past,
present and future.

captions paid for by
discovery communications

March 2019.

We focus the XMM-Newton
space telescope

on a region of space
around Sagittarius A-star,

the supermassive black hole
at the heart of our galaxy.

We spot two huge columns of gas
glowing in X-ray light.

The columns seem to be coming
from Sagittarius A-star.

We see giant fountains of gas

extending outward
from the central region

as though it's like a wind
or a giant expulsion event.

The fountains of gas
extend 500 light-years above

and below the supermassive
black hole.

That's over a million times
the distance

from the sun to Neptune.

It looks like this material

is actually leaving
the vicinity of the black hole,

like it's burping out
these giant, hot X-ray chimneys.

So why is Sagittarius
A-star burping out hot gas?

Typically, around a black hole,
you have an accretion disk

funneling material
into the black hole,

but all of it doesn't end up
in the black hole.

There is a little bit of gas

falling onto it right now,
even as I'm speaking, right?

As gas falls toward
the supermassive black hole,

it becomes super heated.

It liberates an enormous amount
of energy

and that energy
has to go somewhere.

As gas spirals towards
the black hole,

some of the material accelerates
to near the speed of light.

It blasts out from
the accretion disk...

...creating chimneys
of superheated gas

that seem to connect

to two of the largest
structures in the galaxy...

the Milky Way's Fermi bubbles.

A few years ago,
we noticed that, in fact,

there are these giant bubbles
coming out of the very heart

of the Milky Way galaxy.

In each direction,

there's a bubble
25,000 light-years long.

But the gas-filled bubbles

dwarf the chimneys
of superheated gas.

Scientists wonder
if another more powerful force

blew the bubbles.

So what could have created
all of this superheated gas

that actually blew these
tremendously large bubbles?

Supermassive black
holes in other galaxies

might offer clues.

Black holes at
the centers of galaxies

go through different phases.

So they can be either active
or they can be calm.

Sometimes black holes
at the centers of galaxies

go through an active phase.

And when that happens,

the black hole is actively
feeding on material around it,

which means it's growing

and it also gives off
huge jets of radiation.

Calm supermassive black holes

release a trickle of hot gas.

But when lots of material
falls on them,

they can shoot out jets up
to millions of light-years long.

At the current time,

Sagittarius A-star is
what we call quiescent.

It's quiet.

There is some material
swirling around it,

but really not very much.

But we don't think
that's always been the case.

The centers of galaxies
are busy places.

There are stars there.
There's gas there.

There's dust there,

and sometimes these things
fall into that black hole.

6 million years ago,

Sagittarius A-star
may have had a feeding frenzy...

...eating too much

and blasting out
the remains in huge jets.

Those jets plow
through the galaxy

initially at near the speed
of light.

And as they do so,
they can wreak havoc

or sculpt the evolution
of the galaxy

that
they're propagating through.

Sagittarius A-star's jets

blasted gas out of the galaxy,

creating the scars we see
as the Fermi bubbles.

Now, whatever caused those jets
seems to have turned off.

It's not happening anymore

and we're seeing sort
of the leftovers of them.

But this is clearly a sign

that sometime in the past
few million years,

the black hole
in the center of our galaxy,

Sagittarius A-star, was actively
feeding on material around it.

Material was falling into it
and blasting out this stuff.

The jets left
destruction in their wake.

They may have also affected
the growth of our entire galaxy.

These structures at the center
of our galaxy are important

because they can either
shut off star formation

or they can trigger
star formation.

As those jets
propagate through the galaxy,

they pile up gas

and that gas can be then
triggered into star formation.

But these jets can also impart
so much heat or energy feedback

into the environment that
they prevent star formation.

So black holes in many ways
conduct an orchestra,

instructing or dictating
when stars can and cannot form.

In the center of the Milky Way,

star-formation rates seem low.

The jets could be responsible.

But in 2017,
the Alma telescope discovered

that change is coming.

So Alma's actually been
able to peer in

to the heart of our galaxy

and see that near
all this destruction,

there might actually be a new
generation of stars forming.

Today, our calm
supermassive black hole

could be helping
star formation in the core.

But the Fermi bubbles could be
evidence of a time

when Sagittarius A-star
shut down star formation.

Could the supermassive
black hole

roar back to life in the future?

Sagittarius A-star
could roar back to life

by just dumping some gas
onto it.

And there's a lot of gas
at the center of our galaxy

and it could wander into
the proximity

of Sagittarius A-star

and ultimately fall
onto the event horizon

and that would light it up.

If Sagittarius A-star
eats enough gas...

...it could shut down
star formation in the galaxy

for millions of years.

It could also give off X-rays
and gamma rays

that may hit the Earth.

Thankfully, our central
supermassive black hole

is pretty quiet
and massive feeding events,

massive energy events
are very, very rare.

We don't necessarily
have much to worry about.

Sagittarius A-star
has reshaped our galaxy.

If we want to survive
in the universe,

we need to know more about
this monster black hole.

The Event Horizon Telescope
is on a mission to do just that.

Question is can it succeed?

The center of the Milky Way

is home to a supermassive
black hole,

Sagittarius A-star.

At least we think it is.

We've never seen
the supermassive

black hole directly.

But we have seen stars
racing around the core.

The speeds of the stars

zipping around the center
of our Milky Way galaxy

indicate that there's
something very massive

and very compact there,

indeed, 4 million times
as massive as our Sun

in a volume smaller
than that of our solar system.

It's got to be
a black hole basically.

By measuring the orbits of stars

in our galaxy center...

...we estimate
that Sagittarius A-star

is over a hundred times
wider than our sun.

But despite its size,
the black hole is hidden.

One of the immediate challenges

of actually observing
black holes

is the fact that they don't emit
light and so you can't see them.

Right? So we've never
actually seen a black hole.

We've only seen the stuff
around a black hole

or we have seen the effects
that that black hole

imparts on its ambient
surroundings.

That's where the Event
Horizon Telescope came in.

Its goal was to photograph
Sagittarius A-star,

not the black hole itself,
but its shadow.

Around it is this a gas that is
moving around the black hole

that's super heated
to millions of degrees.

And what the Event
Horizon Telescope

is trying to see is
the shadow of a black hole.

Light from the hot gas
around Sagittarius A-star

frames the giant shadow.

It could be up to
93 million miles across.

Problem is Sagittarius
A-star is so far away

that the supermassive black hole
is still incredibly hard to see.

Sagittarius A-star is big,

but it's
26,000 light-years away.

A single light-year
is 6 trillion miles.

So this is a long, long walk.

And even though it's big,

that distance shrinks
its apparent size

to just
a tiny little dot on the sky.

To see the tiny dot,

we need a telescope
the size of the Earth.

How do you possibly do that?

You can't build
that telescope, right?

Well, there's a trick.

You actually get
a few different telescopes

and you spread them out
over the surface of the Earth.

And when we had
all of these sites together,

we wind up being able
to take an image of something

that is really,
really impossibly small.

To gather enough light
to see a target this small,

the team take
long-exposure images

of Sagittarius A-star's
shadow...

...but there's a problem.

The accretion disk moves
too much for us

to capture a clear image.

When you're taking
a long exposure of a person,

right,
you need them to be really,

really still, right?

Because if they're moving around
a lot,

they're going to blur
the image out.

And that kind of thing
is happening

when we observe
Sagittarius A-star

because it is unwilling
to sit still for us.

It is booming and banging
and flashing

on the timescale
of literally hours.

As glowing material
orbits the black hole

at 30% the speed of light,

Sagittarius A-star's
shadow blurs.

Future developments may allow us

to see Sagittarius A-star
clearly.

For now, we can't capture
an accurate image

of our galaxy's
supermassive black hole.

But the hunt to see
a supermassive black hole

wasn't over.

The Event Horizon Telescope
turned to another galaxy

54 million
light-years away... M87.

M87 is an absolute beast
of a galaxy.

It's the so-called
brightest cluster galaxy.

These are among the largest
galaxies in the universe.

And M87 is home

to another supermassive
black hole...

the giant M87 star.

M87 star is so massive
that the gravitational region

that's interesting
is actually easier to image

than the black hole
in our own galaxy.

M87 star is over
a thousand times more massive

than Sagittarius A-star

and has a far
larger accretion disk.

When photographing a black hole,
size matters,

because big accretion disks
project more stable light,

so images of them
don't blur as much.

In April of 2019,

the event horizon team
unveiled their image.

We have seen what we thought
was unseeable.

We have seen and taken a picture
of a black hole.

I've been working
on this project

for almost six years now,
and so, this is something

we've been looking forward to
for a really long time.

Capturing this image
took decades of work

by hundreds of scientists
all over the world.

I was really stunned.

Suddenly, when you say that's
the real thing, that's amazing.

It really affected me.

This is something
6 1/2 billion times

the mass of the Sun,

55 million light-years away
and we're looking at it.

So when you look at
the image, it's totally fine.

You're totally forgiven
for thinking,

"ah, it looks a little blurry."

But I cannot reiterate enough

how profound
this image actually is.

We are seeing just a hair's
width away from a discontinuity

in the fabric
of space-time itself.

Actually seeing so close
to an actual event horizon,

a discontinuity in the fabric
of space-time,

never seemed possible.

This image of the
heart of a distant galaxy

helps us understand supermassive
black holes like never before.

When we observe
supermassive black holes

in other galaxies,
including the one in m87,

we're able to learn more about
the big picture

of how these massive black holes
form and evolve over time.

And that in turn,
helps us understand

how our Milky Way galaxy

and its super massive
black hole has formed.

By studying, not just
making images of black holes,

but making videos
of black holes,

and seeing as that gas
is spinning around it,

we can try to map around
a black hole more precisely

and learn about its dynamics.

An image of Sagittarius A-star

remains out of reach,

but in 2018, it shows
a deadly side to its character.

The supermassive black
hole's accretion disk

releases huge, powerful flares,

and they could be pointed
right at us.

In 2018, astronomers
were studying a special star

orbiting our galaxy's
supermassive black hole.

The star passes close
to Sagittarius A-star...

every 16 years.

It's called S2, and by studying
this star's fly-by,

we hope to learn more
about Sagittarius A-star.

We think that S2 may
be the very closest star

to the supermassive black hole
in the center of our galaxy.

At closest approach
to sag A-star,

S2 comes within 17 light hours
or so of the surface.

The supermassive black
hole's powerful gravity

accelerates the star
to 17 million miles an hour.

That's fast enough to travel
from New York to L.A.

In half a second,

but it's not the star's speed
that excites scientists.

This is a great star, because
it's on an elliptical orbit

that takes it fairly far
from the black hole,

but every few years,

it passes right above
the supermassive black hole.

As we tracked S2's
swing around Sagittarius A-star,

we detected powerful
bursts of infrared light

coming from the direction
of the supermassive black hole.

There's a blob of gas

that is orbiting very close
to the black hole,

and it was flaring
as it went around.

There were three separate
flares of light

that they were able to detect.

The flares didn't come directly

from
the supermassive black hole,

they came from
the material around it.

The flares that were discovered
are thought to originate

from magnetic storms

in this very, very hot turbulent
gas around the black hole.

The extreme heat
in the accretion disk

strips electrons
from atoms of gas.

The stripped electrons
and hot gas form a plasma,

which creates powerful
magnetic fields

when accelerated to high speeds.

Because some super
massive black holes

have these superheated,

rapidly spinning vortices
of gas swirling around them,

you get these very,
very powerful,

very tightly wound
magnetic fields.

And there's energy stored
in that magnetic field.

It's like a bunch of piano wires
all tangled up.

And if these things interact
with each other, they can snap,

and when they snap,
that energy is released.

You'll get this
enormous release of energy

as these coils of magnetic
fields effectively snap.

And when they do so, just like
on the surface of our sun,

they release
an enormous flare of gas.

These powerful flares
can be millions of miles wide

and come packed with superheated
gas and plasma.

Solar flares release
as much energy

as 10 million volcanic
explosions.

Flares from Sagittarius
A-star's accretion disk

are like millions of solar
flares all going off at once.

It's kind of like comparing a
nuclear weapon to a firecracker.

Sagittarius A-star's flares

release intense blasts
of radiation,

but by watching the flares
from Earth,

we can learn about
the orientation

of the supermassive
black hole's accretion disk.

This gas that's in
this accretion disk

around the black hole
is like a friendly helper

shining a flashlight
back toward Earth.

And we can watch the orbit
of these flashlights

and help understand
the orientation of gas

that swirls
around the black hole.

We think we're getting
a bird's-eye view of it.

And looking down the barrel,

we're looking at the accretion
disk basically face-on.

That means that any material
that gets blasted away

from the black hole
could be aimed right at us.

Should we be worried
about the flares reaching Earth?

It sounds worrisome,
this blob of gas

emitting these
huge flares of light,

but you've got to realize,
this is 26,000 light-years away.

That is a long way.

It took an extremely sensitive
detector

on one of the largest
telescopes on Earth

to be able to see this at all.

Earth is safe for now,

but the more we learn
about the galaxy center,

the more terrifying it becomes.

We know of Sagittarius A-star,

the central supermassive
black hole,

but now we're beginning
to suspect

that it might not be alone.

A dangerous swarm of black holes

could be racing around
the center of the Milky Way.

Thousands more may
be hiding from sight.

The supermassive black
hole, Sagittarius A-star,

dominates the center
of the Milky Way...

...affecting star formation...

...and carving out vast
gas bubbles in space.

But Sagittarius A-star might not
be the only black hole in town,

or even the most dangerous.

We've known for a long time

that there's a supermassive
black hole

in the very heart of our galaxy,

but there may be an angry swarm
of smaller black holes,

buzzing all around it.

In April of 2018,

astronomers led
by Columbia university

revealed the results
of a hunting mission

in the center of the galaxy.

They'd used 12 years
of Chandra observatory data

to seek out
stellar mass black holes.

Black holes that are made
from the death of stars,

from supernova explosions,

are called stellar
mass black holes.

And these are made from stars

that were many times
the mass of the sun.

Finding stellar mass
black holes is tough.

Light can't escape
a black hole's gravity,

so we can't see them directly.

And stellar mass black holes
are only tens of miles wide,

making them almost impossible
to detect.

So astronomers look
for a special type

of stellar mass black hole.

One of the ways that we look
for stellar mass black holes,

is that they often are vampires
eating a companion star.

These vampires are
part of a binary pair,

a stellar mass black hole
in orbit with a living star,

the black hole feasting
on its partner.

That black hole

is like a very, very deadly
parasite for that star.

It is ripping mass
off the surface of that star,

and that matter is raining down
toward the black hole itself.

And that material lights up,

so this allows us
to hunt for black holes,

not through taking pictures
of black holes directly,

but through seeing the material
falling to its doom.

The problem is,

gas and dust spread
throughout the galaxy

stops visible light from
the binary pair reaching Earth.

But the binary pair release
another type of light

that passes through the gas
and dust more easily...

X-rays.

The system itself
is emitting X-rays,

so they're called
X-ray binaries.

So these are useful,
because the X-ray emission

can be very powerful

and can be potentially seen
from the Earth,

even though the binary
is very far away,

say, at the galactic center.

The glowing disks of
material in X-ray binary systems

are almost a million times
smaller than the accretion disk

surrounding Sagittarius A-star,
too small

for us to see the material
swirling around them in detail.

So, we see the X-ray binaries
as pinpricks of X-ray light.

Astronomers detect
12 of these X-ray binaries

in a small 3-light-year-wide
patch of space

at the galactic center.

And that means that there could
be a much larger collection

of these relatively tiny stellar
mass black holes

in the heart of our galaxy.

If black holes form the way
we think they do,

there very likely
may be swarms of black holes

racing around
Sagittarius A-star.

But X-ray binaries
that are powerful enough for us

to detect are incredibly rare.

So we estimate that for the
dozen X-ray binaries discovered,

there could be up
to a thousand more.

In total, there could be
20,000 stellar mass black holes

in this 3-light-year
region of space.

Why are these black holes
swarming in the galaxy center?

It appears they've migrated
from the rest of the Milky Way.

Through a process
called dynamical friction,

black holes can actually sink
to the centers of galaxies

very, very rapidly, like
dropping a stone into a pond.

What that means
is that an errant,

wandering black hole
might eventually

find its way toward the center
of our own galaxy,

where Sagittarius A-star
resides.

As stellar mass black
holes orbit the galaxy,

they interact gravitationally

with stars and clouds
of gas and dust.

These interactions
push the black holes

towards the center
of the galaxy,

where the black holes swarm.

A swarm of stellar mass
black holes sounds deadly,

but it may not be
the most lethal thing

in the center of the Milky Way.

A surprising observation
indicates that there is a lot

of antimatter
in the center of our galaxy.

And when antimatter
meets matter,

the results are explosive.

In 2017,

astronomers tried to solve
a decades-old cosmic mystery...

...unexplained
high-energy radiation

streaming through our galaxy.

At first, we didn't know
where it was from.

But we discovered
it was gamma radiation

coming from somewhere
in the center of the Milky Way.

The question is,
what's making these gamma rays?

That's hard to do.

It's not like
you can rub your hands together

and generate gamma rays.

When we took a closer
look at the gamma rays,

we discovered the signature
of the most explosive substance

in the universe... antimatter.

Antimatter is like normal matter

but with opposite charge.

That's it.
It's matter's evil twin.

When evil twin meets good twin,

it is not a happy reunion.

Antimatter is scary.

It's not like you want
to have some in your kitchen.

This stuff is very,
very explosive,

if you want to think
of it that way.

If it touches normal matter,

it releases a huge
amount of energy.

When matter
and antimatter combine,

they annihilate each other
and transform

into high-energy radiation,
just like the gamma rays

seen streaming out
of the center of the Milky Way.

We see antimatter
throughout the galaxy,

but strangely,
the galactic center

seemed to have 40% more
antimatter than anywhere else.

Right now in the heart
of our galaxy,

we actually observe
fountains of antimatter

that are producing
10 trillion tons

of antimatter every second.

One of the big questions
that we've wondered about

for a very long time, is what's
the origin of this stuff?

Initially,
there were several suspects.

One possible source
of antimatter

is the central black hole,
Sagittarius A-star.

Matter can be swirling
around this

and it can have such high energy

that it can create antimatter.

But the antimatter

isn't coming
from a single point,

it's spread across thousands
of light-years of space.

So Sagittarius A-star

can't be the source
of the gamma-ray stream.

Another suspect was dark matter.

One of the biggest mysteries
in the universe

right now is dark matter.

We know that the majority
of mass in the universe

is not in the same form
that we are.

It's not made of atoms,

but whatever sort of particle
it is or may be,

if these things collide,
they can produce antimatter,

and that will produce
the gamma rays.

So it's possible that as we look
into the heart of the galaxy

and see these extra gamma rays,

that's the signal
that dark matter is there.

But the gamma ray
stream we detected is too weak

to have been created
by dark matter.

Then we had a breakthrough.

We discovered that a special
metal called titanium-44

could be responsible
for the gamma-ray stream.

Titanium-44 is a highly
radioactive element.

That means that
it wants to decay

into other types of nuclei.

When titanium-44 decays,

it gives off antimatter.

But to produce the antimatter
seen in the galaxy's core,

you would need
a lot of titanium-44.

It could be created
in rare energetic events,

in the collision of two dead
stars... white dwarfs.

A white dwarf star is a star
that didn't have enough mass

when it died
to actually become a supernova.

It just sort of cools off
as a dead little cinder.

But what if you have
two white dwarfs

that are orbiting
around each other,

and as they come closer
and closer and collide,

all of a sudden now,
you have enough mass

to actually kick
a supernova explosion off.

These particular kinds
of supernovae

are very good
at producing titanium-44.

So these kinds of supernovas
are very, very good

at making antimatter.

These supernovas
erupt in the core of the galaxy

once every 2,000 years.

But outside of the core
in the disk of the galaxy

where our solar system orbits...

...these supernovas happen
three times as often.

So the gamma ray observations
were wrong.

There isn't more antimatter
in the heart of the galaxy.

It's our region of the galaxy

that contains
the most antimatter.

Question is, are we in danger?

If you take an ounce of matter

and an ounce of antimatter
and collide them,

you're generating
a megaton of energy,

the equivalent of a million tons
of TNT exploding.

So you don't need
much antimatter to generate

a vast amount of energy.

But the thing you have
to remember is we live

in this wonderful,
dramatic environment

of a larger universe.

It's not dangerous.

It's very far away from us,
and it's fascinating.

But all of this antimatter
is being produced in our galaxy,

so just sit back
and enjoy the fireworks.

The center of the Milky Way

is violent and extreme,

but things could get
a whole lot worse.

Rogue supermassive black holes

could be
lurking near our galaxy,

and they have the power
to end life as we know it.

The Milky Way is around
100,000 light-years across,

and it's home to at least
200 billion stars,

but it hasn't
always been this large.

We know that our Milky Way
galaxy grew to the size

it is now, which is huge,
by eating other galaxies.

And some of these galaxies
would've had

supermassive black holes
in their centers.

When the Milky Way's gravity

pulled in smaller galaxies,

most of their material
merged with the Milky Way,

but some material like stars,

could've been slung tens
of thousands of light-years

out of the Milky Way.

This could've happened
to a smaller galaxy's

super massive black hole.

It is entirely possible

there are supermassive
black holes

wandering around out there,
not in the center.

So how could it be
possible that there's actually

a supermassive black hole
close to us wandering around,

but we never even see it?

Well, remember black hole
means it's really, really black.

It actually absorbs radiation
and any energy.

So unless something is falling
into a black hole

or orbiting around it,
you're not going to see it.

And so, if this
supermassive black hole

were hypothetically wandering
the outskirts of our galaxy,

well, there's a lot less
gas there

for that black hole to run into.

And if there's no gas around

that black hole,
we will not see it.

The rogue supermassive
black hole may not stay

in the outskirts
of the galaxy forever.

Gravitational interactions
slowly pull it back

into the Milky Way.

Billions of years later,

the supermassive black hole
could arrive in the center.

When this rogue supermassive
black hole meets up with

Sagittarius A-star,
the fuse is lit.

The pair spiral
towards each other...

...spinning faster and faster,

reaching up to half
the speed of light.

Finally,
the two black holes merge.

You would have

an enormously energetic event
on your hands.

Those supermassive black holes
could, in principle,

merge together, create a huge
blast of gravitational waves,

accompanied by a profoundly
energetic flash of light

that could, in principle,
endanger all life on Earth.

It's literally a stretching

and contracting of space itself.

It's like grabbing
the framework of space

and it's shaking it really hard.

And if this happens
in our galaxy,

the amount of energy emitted,
that would be bad.

When the black holes collide,

they release more energy

than all the stars
in the galaxy combined.

Should we be panicked
about this?

And the answer is no.

The Earth has been orbiting
the sun

for 4 1/2 billion years
without any incident, right?

We're pretty safe from them.

If we were around to
see the two black holes collide,

we'd witness the most
destructive light show

in the history of the galaxy.

But for now, the center of our
galaxy is relatively quiet,

but it's still
a terrible place to be.

The center of our Milky Way
is not a friendly place.

It's nowhere you want to be.
It's a bad neighborhood.

You've got tons of stars,
tons of radiation,

and stars are being born
and dying and exploding.

You've got the central
supermassive black hole.

You've got a potential swarm
of black holes.

You've got accretion disks.
You've got flares.

You've got magnetic outbursts.
You've got jets.

Let's just stay out here
in the suburbs, all right?

The center of our galaxy

is one of the most nightmarish
places in the cosmos.

It's also home to some
of the most incredible forces

the universe has to offer.

Whatever the future
holds for our galaxy...

...the core of the Milky Way
will be at the center of it all.

Our home galaxy, the Milky Way,
is our safe harbor,

our island in this vast,
cosmic ocean.

And so to understand
the heart of our galaxy,

is to understand our home
in this cosmic void.

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