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NARRATOR: Twenty-six thousand light years from Earth...

..shrouded in cosmic dust and gas...

..is a mysterious region of space.

The centre of the Milky Way.

The centre 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,

four million times the mass of the sun.

Now we're exploring the centre of the Milky Way like never before.

Uncovering powerful forces that affect us all.

Everything that happens at the centre of the Milky Way galaxy

really is connected to what's going on in the rest of the Milky Way.

Understanding the centre of our galaxy,

unlock secrets of our past, present and future.

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 blowing an X-ray light.

The column seemed 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 disc

funnelling 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 superheated, 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 disc...

..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 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.

Scientists wonder if another more powerful force blew the bubbles.

So what could have created all of the superheated gas

that actually blew these tremendously large bubbles?

Black holes in other galaxies might offer clues.

Black holes at the centres of galaxies

go through different phases,

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

Sometimes black holes at the centres 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's some material swirling around it,

but really, not very much.

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

The centres 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.

Six 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 plough 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 any more,

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

This is clearly a sign that sometime in the past few million years,

the black hole in the centre 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 also have affected the growth of our entire galaxy.

These structures at the centre 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 centre 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 into 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 the time

when Sagittarius A-star shut down star formation.

Could the supermassive black hole roar back to life in the future?

Sag A-star could roar back to life

by just dumping some gas onto it.

And there's a lot of gas at the centre 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, will it succeed?

The centre 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 centre

of our Milky Way galaxy indicate

that there's something very massive and very compact there.

Indeed, four million times as massive as our Sun,

in a volume smaller than that of our solar system.

It's gotta be a black hole basically.

By measuring the orbits of stars in our galaxy's centre...

..we estimate that Sagittarius A-star is over 100 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.

So we've never actually seen a black hole.

We've only seen the stuff around a black hole.

We have seen the effects of that black hole in parts

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 gas that is moving around the black hole

that is super-heated to millions of degrees.

And what the Event Horizon telescope is trying to see

is the shadow of the 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

the supermassive black hole is still incredibly hard to see.

Sagittarius A-star is big,

but it's 26,000 light years away.

You know, a single light year is six 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 get a few different telescopes

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

And when we add 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 the small target,

the team takes long exposure images of Sagittarius A-star's shadow.

But there's a problem.

The accretion disc moves too much

for us to capture a clear image of Sagittarius A-star's shadow.

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

you need them to be really, really still.

Because if they're moving a lot, they're gonna 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 time scale 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 a 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 1,000 times more massive than Sagittarius A-star,

and has a far larger accretion disc.

When photographing a black hole, size matters,

because big accretion discs project more stable light,

so images of them don't blur as much.

In April 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.

(APPLAUSE)

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 from 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 six and half 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,

"Uh, 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 supermassive 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 disc

releases huge, powerful flares,

and they could be pointed right at us.

In 2018, astronomers were studying a special star.

The star is called S-2...

..and it passes close to Sagittarius A-star every 16 years.

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 centre 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 LA 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's 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 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 disc

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 supermassive black holes have these super-heated,

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 super-heated gas and plasma.

Solar flares release as much energy

as ten million volcanic explosions.

Flares from Sagittarius A-star's accretion disc...

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

It's kinda 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 disc.

This gas that's in this accretion disc 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,

or looking at the accretion disc 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 gotta realise, 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's centre,

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 centre of the Milky Way.

Thousands more may be hiding from sight.

The supermassive black hole, Sagittarius A-star,

dominates the centre of the Milky Way...

..affecting star formation...

..and carving out vast gas bubbles in space.

But Sagittarius A-star may 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 2018, astronomers, led by Columbia University,

revealed the results of a hunting mission

in the centre 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.

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.

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's very far away, say, at the galactic centre.

The glowing discs of material and X-ray binary systems

are almost a million times smaller

than the accretion disc 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, three-lightyear wide patch of space

at the galactic centre.

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 are very likely maybe swarms of black holes

racing around Sagittarius A-star.

But X-ray binaries that are powerful enough for us to detect

are incredibly rare.

We estimate that for the dozen X-ray binaries discovered,

there could be up to one thousand more.

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

in this three-lightyear region of space

They may be small, but they're dangerous.

Stellar mass black holes

are by far deadlier than supermassive black holes.

Stellar mass black holes are more likely to tear things apart

when they get too close.

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

but are several times the mass of the sun,

and that makes their gravitational pull increase rapidly

as you get close to one.

Even 100 miles from the event horizon,

the gravitational pull can be thousands of times stronger

just a single yard closer to the black hole.

Strong enough to tear you or your spaceship apart.

Supermassive black holes like Sagittarius A-star are different.

It's over four million times the mass of the sun,

and its event horizon is so large

that the gravitational forces increase

much more slowly as you get near.

The event horizon extends for so far

that you can be sucked in without knowing it,

and you're lost, and you're not ripped apart,

as you pass through the event horizon.

You can be trapped forever, but not realise it yet.

Why are these black holes swarming in the galaxy's centre?

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 centres 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 centre 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 centre 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 centre of the Milky Way.

A surprising observation indicates

that there is a lot of antimatter in the centre of our galaxy.

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 came from...

..but we discovered it was gamma radiation

coming from somewhere in the centre 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 wanna have some in your kitchen.

This stuff is very, very explosive, if you wanna 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 centre of the Milky Way.

We see antimatter throughout the galaxy,

but strangely, the galactic centre

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 ten 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 supermassive 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 did not 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 disc 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.

The 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 the larger universe.

It's not dangerous. It's very far away from us,

and it's fascinating...

..that all of this antimatter's being produced in our galaxy.

So just sit back and enjoy the fireworks.

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 have had

supermassive black holes in their centres.

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 even have happened

to a smaller galaxy's supermassive black hole.

It is entirely possible there are supermassive black holes

wandering around out there, not in the centre.

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 gonna 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 arrives in the centre.

On the way in, it could cause havoc for solar systems like our own.

If a solar system is disturbed

by a wandering black hole...

..then the nice, ordered architecture,

or the orbits of its planets, are completely disrupted.

In our solar system, the orbits of the planets are perfectly ordered.

If a black hole came even a little too close,

its gravity could throw the planets into chaos.

Some planets might be flung out of our solar system,

others might be plunged down into the nuclear furnace of the sun.

Some planets could face the ultimate destruction...

..being eaten by the rogue black hole.

If we can't see them,

could one be about to wander through our solar system?

So, hypothetically, a wandering black hole could,

yes, impact our solar system,

but I can't overstate how profoundly unlikely that is to happen.

Galaxies are mostly empty space.

We are just this speck in this cosmic vortex that is our galaxy,

and so our solar system is a really, really, really small target.

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 just shaking it really hard.

And if this happens in our galaxy, the amount of energy emitted,

that would be... That would be bad.

When the black holes collide, they release more energy

than all of the stars in the universe combined.

Should we be panicked about this? And the answer is, no.

The Earth has been orbiting the sun for four and a half 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 history.

But for now, the centre of our galaxy

is relatively quiet,

but it's still a terrible place to be.

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

It's nowhere you wanna be. It's a bad neighbourhood.

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

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 discs, 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 centre 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 centre of it all.

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

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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