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NARRATOR: Twenty-six thousand
light years from Earth...
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..shrouded in cosmic dust and gas...
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..is a mysterious region of space.
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The centre of the Milky Way.
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The centre of the Milky Way galaxy
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is one of the strangest, most
exotic,
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and violent places in our galaxy.
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Gas streaming everywhere,
radiation blasting out.
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Stars moving willy-nilly.
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And at the very heart
is the mysterious black hole,
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four million times
the mass of the sun.
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Now we're exploring the centre
of the Milky Way like never before.
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Uncovering powerful forces
that affect us all.
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Everything that happens at the
centre of the Milky Way galaxy
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really is connected to what's going
on in the rest of the Milky Way.
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Understanding the centre
of our galaxy,
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unlock secrets of our past,
present and future.
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March 2019,
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we focus the XMM-Newton
space telescope
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on a region of space
around Sagittarius A-star...
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..the supermassive black hole
at the heart of our galaxy.
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We spot two huge columns of gas
blowing an X-ray light.
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The column seemed to be coming
from Sagittarius A-star.
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We see giant fountains
of gas extending outward
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from the central region,
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as though it's like a wind
or a giant expulsion event.
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The fountains of gas
extend 500 light years
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above and below
the supermassive black hole.
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That's over a million times
the distance from the sun to Neptune.
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It looks like this material
is actually leaving
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the vicinity of the black hole,
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like it's burping out these giant,
hot X-ray chimneys.
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So why is Sagittarius A-star
burping out hot gas?
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Typically, around a black hole,
you have an accretion disc
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funnelling material
into the black hole,
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but all of it doesn't end up
in the black hole.
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There is a little bit of gas
falling onto it right now,
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even as I'm speaking, right?
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As gas falls toward
the supermassive black hole,
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it becomes superheated, it liberates
an enormous amount of energy,
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and that energy has to go somewhere.
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As gas spirals
towards the black hole,
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some of the material accelerates
to near the speed of light.
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It blasts out
from the accretion disc...
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..creating chimneys
of superheated gas
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that seem to connect to two of
the largest structures in the galaxy,
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the Milky Way's Fermi Bubbles.
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A few years ago, we noticed
that there are these giant bubbles
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coming out of the very heart
of the Milky Way galaxy.
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In each direction, there's a bubble
25,000 light years long.
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But the gas-filled bubbles
dwarf the chimneys.
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Scientists wonder if another
more powerful force blew the bubbles.
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So what could have created
all of the superheated gas
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that actually blew
these tremendously large bubbles?
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Black holes in other galaxies
might offer clues.
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Black holes at the centres of
galaxies
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go through different phases,
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so they can be either active
or they can be calm.
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Sometimes black holes
at the centres of galaxies
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go through an active phase.
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And when that happens,
the black hole is actively feeding
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on material around it,
which means it's growing,
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and it also gives off
huge jets of radiation.
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Calm supermassive black holes
release a trickle of hot gas.
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But when lots of material
falls on them,
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they can shoot out jets
up to millions of light years long.
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At the current time, Sagittarius
A-star is what we call quiescent.
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It's quiet. There's some material
swirling around it,
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but really, not very much.
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But we don't think
that's always been the case.
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The centres of galaxies
are busy places.
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There are stars there, there's gas
there, there's dust there,
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and sometimes, these things
fall into that black hole.
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Six million years ago,
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Sagittarius A-star
may have had a feeding frenzy.
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Eating too much, and blasting out
the remains in huge jets.
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Those jets plough through the
galaxy,
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initially at near the speed of
light.
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And as they do so,
they can wreak havoc
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or sculpt the evolution of the
galaxy
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that they're propagating through.
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Sagittarius A-star's jets
blasted gas out of the galaxy...
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..creating the scars we see
as the Fermi Bubbles.
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Now, whatever caused those jets
seems to have turned off,
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it's not happening any more,
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and we're seeing sort of
the leftovers of them.
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This is clearly a sign that sometime
in the past few million years,
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the black hole in the centre
of our galaxy, Sagittarius A-star,
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was actively feeding
on material around it.
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Material was falling into it
and blasting out this stuff.
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The jets left destruction
in their wake.
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They may also have affected
the growth of our entire galaxy.
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These structures at the centre
of our galaxy are important
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because they can either
shut off star formation
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or they can trigger star formation.
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As those jets propagate through
the galaxy, they pile up gas,
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and that gas can be then triggered
into star formation.
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But these jets can also impart
so much heat, or energy feedback,
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into the environment,
that they prevent star formation.
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So black holes, in many ways,
conduct an orchestra,
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instructing or dictating
when stars can and cannot form.
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In the centre of the Milky Way,
star formation rates seem low.
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The jets could be responsible.
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But in 2017, the ALMA telescope
discovered that change is coming.
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So ALMA's actually been able to
peer into the heart of our galaxy
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and see that near
all this destruction,
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there might actually be
a new generation of stars forming.
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Today, our calm,
supermassive black hole
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could be helping star formation
in the core.
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But the Fermi Bubbles
could be evidence of the time
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when Sagittarius A-star
shut down star formation.
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Could the supermassive black hole
roar back to life in the future?
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Sag A-star could roar back to life
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by just dumping some gas onto it.
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And there's a lot of gas
at the centre of our galaxy,
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and it could wander into
the proximity of Sagittarius A-star
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and ultimately fall
onto the event horizon,
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and that would light it up.
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If Sagittarius A-star
eats enough gas...
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..it could shut down
star formation in the galaxy
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for millions of years.
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It could also give off
X-rays and gamma rays
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that may hit the Earth.
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Thankfully, our central
supermassive black hole
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is pretty quiet,
and massive feeding events,
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massive energy events,
are very, very rare.
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We don't necessarily
have much to worry about.
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Sagittarius A-star
has reshaped our galaxy.
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If we want to survive
in the universe,
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we need to know more
about this monster black hole.
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The Event Horizon telescope
is on a mission to do just that.
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Question is, will it succeed?
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The centre of the Milky Way is home
to a supermassive black hole,
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Sagittarius A-star.
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At least, we think it is.
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We've never seen
the supermassive black hole directly,
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but we have seen stars
racing around the core.
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The speeds of the stars
zipping around the centre
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of our Milky Way galaxy indicate
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that there's something very massive
and very compact there.
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Indeed, four million times
as massive as our Sun,
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in a volume smaller
than that of our solar system.
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It's gotta be a black hole
basically.
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By measuring the orbits of stars
in our galaxy's centre...
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..we estimate that Sagittarius A-star
is over 100 times wider
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than our sun.
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But despite its size,
the black hole is hidden.
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One of the immediate challenges
of actually observing black holes
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is the fact that they don't emit
light, and so you can't see them.
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So we've never actually
seen a black hole.
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We've only seen the stuff
around a black hole.
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We have seen the effects
of that black hole in parts
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on its ambient surroundings.
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That's where
the Event Horizon Telescope came in.
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Its goal was to photograph
Sagittarius A-star.
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Not the black hole itself,
but its shadow.
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Around it is this gas that is
moving around the black hole
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that is super-heated
to millions of degrees.
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And what the Event Horizon telescope
is trying to see
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is the shadow of the black hole.
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Light from the hot gas
around Sagittarius A-star
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frames the giant shadow.
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It could be
up to 93 million miles across.
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Problem is, Sagittarius A-star
is so far away
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the supermassive black hole
is still incredibly hard to see.
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Sagittarius A-star is big,
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but it's 26,000 light years away.
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You know, a single light year
is six trillion miles.
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So this is a long, long walk.
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And even though it's big,
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that distance shrinks
its apparent size
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to just a tiny little dot on the
sky.
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To see the tiny dot, we need
a telescope the size of the Earth.
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How do you possibly do that?
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You can't build that telescope,
right?
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Well, there's a trick. You get
a few different telescopes
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and you spread them out
over the surface of the Earth.
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And when we add
all of these sites together,
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we wind up being able
to take an image of something
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that is really, really
impossibly small.
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To gather enough light
to see the small target,
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the team takes long exposure images
of Sagittarius A-star's shadow.
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But there's a problem.
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The accretion disc moves too much
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for us to capture a clear image
of Sagittarius A-star's shadow.
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When you're taking
a long exposure of a person,
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you need them
to be really, really still.
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Because if they're moving a lot,
they're gonna blur the image out.
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And that kind of thing is happening
when we observe Sagittarius A-star,
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because it is unwilling
to sit still for us.
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It is booming and banging and
flashing
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on the time scale of literally
hours.
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As glowing material
orbits the black hole
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at 30% the speed of light,
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Sagittarius A-star's shadow blurs.
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Future developments may allow us
to see Sagittarius A-star clearly.
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For now, we can't capture
an accurate image
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of our galaxy's
supermassive black hole.
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But the hunt to see a supermassive
black hole wasn't over.
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The Event Horizon telescope
turned to another galaxy,
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54 million light years away, M87.
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M87 is an absolute beast of a
galaxy.
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It's a so-called
'brightest cluster galaxy.'
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These are among the largest galaxies
in the universe.
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And M87 is home to another
supermassive black hole,
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the giant M87 star.
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M87 star is so massive
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that the gravitational region
that's interesting
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is actually easier to image
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than the black hole
in our own galaxy.
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M87 star is over 1,000 times
more massive than Sagittarius A-star,
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and has a far larger accretion disc.
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When photographing a black hole,
size matters,
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because big accretion discs
project more stable light,
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00:15:31,840 --> 00:15:34,400
so images of them don't blur as much.
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In April 2019, the Event Horizon
team unveiled their image.
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We have seen
what we thought was unseeable.
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We have seen
and taken a picture of a black hole.
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00:15:52,320 --> 00:15:54,320
(APPLAUSE)
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I've been working on this project
for almost six years now,
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and so this is something
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we've been looking forward
to for a really long time.
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Capturing this image
took decades of work
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by hundreds of scientists
from all over the world.
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I was really stunned.
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00:16:21,520 --> 00:16:23,800
Suddenly when you say,
'That's the real thing',
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that's amazing.
It really affected me.
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This is something six and half
billion times the mass of the sun,
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55 million light years away,
and we're looking at it?
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00:16:37,280 --> 00:16:40,360
So, when you look at the image,
it's totally fine,
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00:16:40,440 --> 00:16:42,280
you're totally forgiven for
thinking,
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00:16:42,360 --> 00:16:44,000
"Uh, it looks a little blurry."
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00:16:44,080 --> 00:16:49,360
But I cannot reiterate enough
how profound this image actually is.
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00:16:50,800 --> 00:16:53,680
We are seeing
just a hair's width away
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00:16:53,760 --> 00:16:57,200
from a discontinuity in the fabric
of space time itself.
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00:16:57,280 --> 00:17:01,120
Actually seeing so close
to an actual event horizon,
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00:17:01,200 --> 00:17:04,720
a discontinuity in the fabric of
space-time, never seemed possible.
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00:17:07,240 --> 00:17:09,600
This image of the heart
of a distant galaxy...
239
00:17:10,800 --> 00:17:14,320
..helps us understand supermassive
black holes like never before.
240
00:17:17,280 --> 00:17:20,240
When we observe supermassive
black holes in other galaxies,
241
00:17:20,320 --> 00:17:22,320
including the one in M87,
242
00:17:22,400 --> 00:17:25,080
we're able to learn more
about the big picture
243
00:17:25,160 --> 00:17:28,200
of how these massive black holes
form and evolve over time.
244
00:17:28,280 --> 00:17:31,320
And that, in turn, helps us
understand how our Milky Way galaxy
245
00:17:31,400 --> 00:17:33,760
and its supermassive black hole
has formed.
246
00:17:36,080 --> 00:17:38,480
By studying, not just making
images of black holes,
247
00:17:38,560 --> 00:17:40,560
but making videos of black holes,
248
00:17:40,640 --> 00:17:43,520
and seeing as that gas
is spinning around it,
249
00:17:43,600 --> 00:17:45,600
we can try to map around a black
hole
250
00:17:45,680 --> 00:17:47,920
more precisely,
and learn about its dynamics.
251
00:17:49,560 --> 00:17:53,600
An image of Sagittarius A-star
remains out of reach,
252
00:17:53,680 --> 00:17:57,760
but in 2018, it shows
a deadly side to its character.
253
00:18:00,160 --> 00:18:02,960
The supermassive black hole's
accretion disc
254
00:18:03,040 --> 00:18:06,360
releases huge, powerful flares,
255
00:18:06,440 --> 00:18:09,560
and they could be
pointed right at us.
256
00:18:13,120 --> 00:18:17,240
In 2018, astronomers were
studying a special star.
257
00:18:19,160 --> 00:18:21,240
The star is called S-2...
258
00:18:22,360 --> 00:18:26,280
..and it passes close
to Sagittarius A-star every 16 years.
259
00:18:28,120 --> 00:18:30,120
By studying this star's fly-by,
260
00:18:30,200 --> 00:18:33,120
we hope to learn more
about Sagittarius A-star.
261
00:18:35,160 --> 00:18:38,160
We think that S2
may be the very closest star
262
00:18:38,240 --> 00:18:41,480
to the supermassive black hole
in the centre of our galaxy.
263
00:18:41,560 --> 00:18:43,560
At closest approach to Sag A-star,
264
00:18:43,640 --> 00:18:47,800
S2 comes within 17 light hours
or so of the surface.
265
00:18:50,800 --> 00:18:53,640
The supermassive black hole's
powerful gravity
266
00:18:53,720 --> 00:18:57,480
accelerates the star
to 17 million miles an hour.
267
00:18:59,000 --> 00:19:02,760
That's fast enough to travel
from New York to LA in half a second.
268
00:19:04,720 --> 00:19:08,880
But it's not the star's speed
that excites scientists.
269
00:19:10,160 --> 00:19:13,160
This is a great star,
because it's on an elliptical orbit
270
00:19:13,240 --> 00:19:15,560
that takes it fairly far
from the black hole,
271
00:19:15,640 --> 00:19:20,160
but every few years, it passes right
above the supermassive black hole.
272
00:19:22,400 --> 00:19:25,920
As we tracked S2's swing
around Sagittarius A-star,
273
00:19:26,000 --> 00:19:29,720
we detected powerful bursts
of infrared light
274
00:19:29,800 --> 00:19:33,480
coming from the direction
of the supermassive black hole.
275
00:19:36,520 --> 00:19:38,360
There's a blob of gas that's
orbiting
276
00:19:38,440 --> 00:19:40,240
very close to the black hole,
277
00:19:40,320 --> 00:19:43,200
and it was flaring as it went
around.
278
00:19:43,280 --> 00:19:46,760
There were three separate flares of
light that they were able to detect.
279
00:19:48,680 --> 00:19:52,280
The flares didn't come directly
from the supermassive black hole.
280
00:19:54,480 --> 00:19:57,040
They came from material around it.
281
00:20:00,000 --> 00:20:02,720
The flares that were discovered
are thought to originate
282
00:20:02,800 --> 00:20:05,880
from magnetic storms in this very,
very hot turbulent gas
283
00:20:05,960 --> 00:20:08,000
around the black hole.
284
00:20:10,160 --> 00:20:12,320
The extreme heat
in the accretion disc
285
00:20:12,400 --> 00:20:15,760
strips electrons from atoms of gas.
286
00:20:15,840 --> 00:20:20,120
The stripped electrons
and hot gas form a plasma,
287
00:20:20,200 --> 00:20:22,920
which creates
powerful magnetic fields
288
00:20:23,000 --> 00:20:25,120
when accelerated to high speeds.
289
00:20:26,640 --> 00:20:30,360
Because some supermassive black
holes have these super-heated,
290
00:20:30,440 --> 00:20:33,640
rapidly spinning vortices of gas
swirling around them,
291
00:20:33,720 --> 00:20:38,160
you get these very, very powerful,
very tightly wound magnetic fields.
292
00:20:40,360 --> 00:20:42,720
And there's energy stored
in that magnetic field.
293
00:20:42,800 --> 00:20:45,360
It's like a bunch of piano wires
all tangled up,
294
00:20:45,440 --> 00:20:48,280
and if these things interact
with each other, they can snap.
295
00:20:48,360 --> 00:20:50,600
And when they snap,
that energy is released.
296
00:20:55,200 --> 00:20:57,360
You'll get this enormous
release of energy
297
00:20:57,440 --> 00:21:00,360
as these coils of magnetic fields
effectively snap,
298
00:21:00,440 --> 00:21:02,800
and when they do so...
299
00:21:04,040 --> 00:21:06,040
..just like on the surface
of our sun,
300
00:21:06,120 --> 00:21:08,120
they release an enormous
flare of gas.
301
00:21:12,040 --> 00:21:15,920
These powerful flares
can be millions of miles wide...
302
00:21:17,640 --> 00:21:20,960
..and come packed
with super-heated gas and plasma.
303
00:21:24,600 --> 00:21:27,560
Solar flares release as much energy
304
00:21:27,640 --> 00:21:30,440
as ten million volcanic explosions.
305
00:21:35,520 --> 00:21:38,240
Flares from Sagittarius A-star's
accretion disc...
306
00:21:40,520 --> 00:21:44,720
..are like millions of solar flares
all going off at once.
307
00:21:48,160 --> 00:21:51,520
It's kinda like comparing
a nuclear weapon to a firecracker.
308
00:21:54,160 --> 00:21:59,160
Sagittarius A-star's flares
release intense blasts of radiation,
309
00:21:59,240 --> 00:22:01,720
but by watching the flares
from Earth,
310
00:22:01,800 --> 00:22:03,800
we can learn about the orientation
311
00:22:03,880 --> 00:22:06,840
of the supermassive black hole's
accretion disc.
312
00:22:09,200 --> 00:22:12,680
This gas that's in this accretion
disc around the black hole
313
00:22:12,760 --> 00:22:16,320
is like a friendly helper, shining
a flashlight back toward Earth,
314
00:22:16,400 --> 00:22:18,880
and we can watch the orbit
of these flashlights
315
00:22:18,960 --> 00:22:21,280
and help understand
the orientation of gas
316
00:22:21,360 --> 00:22:23,360
that swirls around the black hole.
317
00:22:27,040 --> 00:22:29,080
We think we're getting
a bird's-eye view of it,
318
00:22:29,160 --> 00:22:31,160
and looking down the barrel,
319
00:22:31,240 --> 00:22:33,840
or looking at the accretion disc
basically face on.
320
00:22:33,920 --> 00:22:35,760
That means that any material
321
00:22:35,840 --> 00:22:38,080
that gets blasted away from the
black hole
322
00:22:38,160 --> 00:22:40,360
could be aimed right at us.
323
00:22:43,800 --> 00:22:47,160
Should we be worried
about the flares reaching Earth?
324
00:22:48,880 --> 00:22:50,720
It sounds worrisome,
325
00:22:50,800 --> 00:22:53,440
this blob of gas emitting
these huge flares of light,
326
00:22:53,520 --> 00:22:57,040
but you gotta realise,
this is 26,000 light years away.
327
00:22:57,120 --> 00:23:00,200
That is a long way. It took
an extremely sensitive detector
328
00:23:00,280 --> 00:23:03,400
on one of the largest telescopes
on Earth
329
00:23:03,480 --> 00:23:05,840
to be able to see this at all.
330
00:23:05,920 --> 00:23:08,320
Earth is safe, for now,
331
00:23:08,400 --> 00:23:11,240
but the more we learn
about the galaxy's centre,
332
00:23:11,320 --> 00:23:13,320
the more terrifying it becomes.
333
00:23:14,960 --> 00:23:18,760
We know of Sagittarius A-star,
the central supermassive black hole,
334
00:23:18,840 --> 00:23:23,200
but now we're beginning to suspect
that it might not be alone.
335
00:23:25,200 --> 00:23:28,440
A dangerous swarm of black holes
could be racing
336
00:23:28,520 --> 00:23:31,160
around the centre of the Milky Way.
337
00:23:31,240 --> 00:23:34,360
Thousands more
may be hiding from sight.
338
00:23:39,720 --> 00:23:43,280
The supermassive black hole,
Sagittarius A-star,
339
00:23:43,360 --> 00:23:45,720
dominates the centre
of the Milky Way...
340
00:23:48,000 --> 00:23:50,320
..affecting star formation...
341
00:23:52,200 --> 00:23:55,560
..and carving out vast
gas bubbles in space.
342
00:23:57,720 --> 00:24:02,320
But Sagittarius A-star may not be
the only black hole in town,
343
00:24:02,400 --> 00:24:05,040
or even the most dangerous.
344
00:24:07,480 --> 00:24:10,320
We've known for a long time that
there's a supermassive black hole
345
00:24:10,400 --> 00:24:12,400
in the very heart of our galaxy,
346
00:24:12,480 --> 00:24:15,360
but there may be an angry swarm
of smaller black holes
347
00:24:15,440 --> 00:24:18,200
buzzing all around it.
348
00:24:18,280 --> 00:24:22,680
In April 2018, astronomers,
led by Columbia University,
349
00:24:22,760 --> 00:24:25,560
revealed the results
of a hunting mission
350
00:24:25,640 --> 00:24:27,840
in the centre of the galaxy.
351
00:24:27,920 --> 00:24:31,600
They'd used 12 years
of Chandra Observatory data
352
00:24:31,680 --> 00:24:34,480
to seek out stellar mass black holes.
353
00:24:35,920 --> 00:24:38,600
Black holes that are made
from the death of stars,
354
00:24:38,680 --> 00:24:42,360
from supernova explosions, are
called stellar mass black holes.
355
00:24:47,960 --> 00:24:52,360
These are made from stars that were
many times the mass of the sun.
356
00:24:55,760 --> 00:24:58,360
Finding stellar mass
black holes is tough.
357
00:25:00,280 --> 00:25:03,200
Light can't escape
a black hole's gravity,
358
00:25:03,280 --> 00:25:05,280
so we can't see them directly.
359
00:25:08,720 --> 00:25:12,480
And stellar mass black holes
are only tens of miles wide,
360
00:25:12,560 --> 00:25:15,000
making them
almost impossible to detect.
361
00:25:18,240 --> 00:25:22,400
So astronomers look for a special
type of stellar mass black hole.
362
00:25:26,360 --> 00:25:30,040
One of the ways that we look
for stellar mass black holes
363
00:25:30,120 --> 00:25:33,000
is that they often are vampires,
364
00:25:33,080 --> 00:25:35,960
eating a companion star.
365
00:25:39,720 --> 00:25:43,800
These vampires
are part of a binary pair,
366
00:25:43,880 --> 00:25:48,280
a stellar mass black hole in orbit
with a living star,
367
00:25:48,360 --> 00:25:51,480
the black hole
feasting on its partner.
368
00:25:53,280 --> 00:25:56,880
That black hole is like a very,
very deadly parasite for that star.
369
00:25:56,960 --> 00:25:59,720
It is ripping mass
off the surface of that star,
370
00:25:59,800 --> 00:26:03,200
and that matter is raining down
toward the black hole itself.
371
00:26:06,960 --> 00:26:09,040
And that material lights up,
372
00:26:09,120 --> 00:26:11,680
so this allows us
to hunt for black holes,
373
00:26:11,760 --> 00:26:14,600
not through taking pictures
of black holes directly,
374
00:26:14,680 --> 00:26:18,480
but through seeing the material
falling to its doom.
375
00:26:19,720 --> 00:26:23,640
Problem is, gas and dust
spread throughout the galaxy
376
00:26:23,720 --> 00:26:28,200
stops visible light
from the binary pair reaching Earth.
377
00:26:28,280 --> 00:26:31,520
But the binary pair
release another type of light
378
00:26:31,600 --> 00:26:36,720
that passes through the gas
and dust more easily, X-rays.
379
00:26:36,800 --> 00:26:39,000
The system itself is emitting
X-rays,
380
00:26:39,080 --> 00:26:41,240
so they're called 'X-ray binaries.'
381
00:26:41,320 --> 00:26:44,000
So, these are useful
because the X-ray emission
382
00:26:44,080 --> 00:26:47,680
can be very powerful, and can be
potentially seen from the Earth,
383
00:26:47,760 --> 00:26:51,520
even though the binary's very far
away, say, at the galactic centre.
384
00:26:54,560 --> 00:26:58,000
The glowing discs of material
and X-ray binary systems
385
00:26:58,080 --> 00:27:00,600
are almost a million times smaller
386
00:27:00,680 --> 00:27:03,680
than the accretion disc
surrounding Sagittarius A-star,
387
00:27:03,760 --> 00:27:08,960
too small for us to see the material
swirling around them in detail.
388
00:27:11,600 --> 00:27:15,560
So we see the X-ray binaries
as pinpricks of X-ray light.
389
00:27:18,840 --> 00:27:22,520
Astronomers detect
12 of these X-ray binaries
390
00:27:22,600 --> 00:27:25,680
in a small, three-lightyear wide
patch of space
391
00:27:25,760 --> 00:27:27,760
at the galactic centre.
392
00:27:27,840 --> 00:27:30,640
And that means that there could be
a much larger collection
393
00:27:30,720 --> 00:27:33,200
of these relatively tiny
stellar mass black holes
394
00:27:33,280 --> 00:27:35,280
in the heart of our galaxy.
395
00:27:37,240 --> 00:27:39,600
If black holes form
the way we think they do,
396
00:27:39,680 --> 00:27:42,560
there are very likely maybe
swarms of black holes
397
00:27:42,640 --> 00:27:44,640
racing around Sagittarius A-star.
398
00:27:47,320 --> 00:27:50,880
But X-ray binaries that
are powerful enough for us to detect
399
00:27:50,960 --> 00:27:53,560
are incredibly rare.
400
00:27:55,440 --> 00:27:59,800
We estimate that for the dozen
X-ray binaries discovered,
401
00:27:59,880 --> 00:28:02,360
there could be up to one thousand
more.
402
00:28:07,040 --> 00:28:11,120
In total, there could be 20,000
stellar mass black holes
403
00:28:11,200 --> 00:28:14,240
in this three-lightyear
region of space
404
00:28:23,680 --> 00:28:27,040
They may be small,
but they're dangerous.
405
00:28:27,120 --> 00:28:29,120
Stellar mass black holes
406
00:28:29,200 --> 00:28:31,920
are by far deadlier
than supermassive black holes.
407
00:28:32,000 --> 00:28:36,240
Stellar mass black holes are
more likely to tear things apart
408
00:28:36,320 --> 00:28:39,120
when they get too close.
409
00:28:39,200 --> 00:28:43,520
These stellar mass black holes
are only tens of miles wide,
410
00:28:43,600 --> 00:28:46,680
but are several times
the mass of the sun,
411
00:28:46,760 --> 00:28:50,240
and that makes their
gravitational pull increase rapidly
412
00:28:50,320 --> 00:28:52,600
as you get close to one.
413
00:28:52,680 --> 00:28:55,520
Even 100 miles from the event
horizon,
414
00:28:55,600 --> 00:28:59,080
the gravitational pull can be
thousands of times stronger
415
00:28:59,160 --> 00:29:02,080
just a single yard closer
to the black hole.
416
00:29:04,280 --> 00:29:08,120
Strong enough to tear you
or your spaceship apart.
417
00:29:17,160 --> 00:29:22,000
Supermassive black holes like
Sagittarius A-star are different.
418
00:29:22,080 --> 00:29:25,560
It's over four million times
the mass of the sun,
419
00:29:25,640 --> 00:29:28,280
and its event horizon is so large
420
00:29:28,360 --> 00:29:30,720
that the gravitational
forces increase
421
00:29:30,800 --> 00:29:33,240
much more slowly as you get near.
422
00:29:35,400 --> 00:29:37,720
The event horizon extends for so far
423
00:29:37,800 --> 00:29:40,160
that you can be sucked in
without knowing it,
424
00:29:40,240 --> 00:29:42,720
and you're lost,
and you're not ripped apart,
425
00:29:42,800 --> 00:29:44,800
as you pass through
the event horizon.
426
00:29:46,000 --> 00:29:48,880
You can be trapped forever,
but not realise it yet.
427
00:29:51,640 --> 00:29:55,640
Why are these black holes swarming
in the galaxy's centre?
428
00:29:55,720 --> 00:29:59,200
It appears they've migrated
from the rest of the Milky Way.
429
00:30:01,080 --> 00:30:03,160
Through a process
called dynamical friction,
430
00:30:03,240 --> 00:30:05,720
black holes can actually sink
to the centres of galaxies
431
00:30:05,800 --> 00:30:08,320
very, very rapidly,
like dropping a stone into a pond.
432
00:30:09,720 --> 00:30:13,000
What that means is that an errant,
wandering black hole
433
00:30:13,080 --> 00:30:16,240
might eventually find its way
toward the centre of our own galaxy,
434
00:30:16,320 --> 00:30:18,320
where Sagittarius A-star resides.
435
00:30:20,160 --> 00:30:23,440
As stellar mass black holes
orbit the galaxy,
436
00:30:23,520 --> 00:30:28,280
they interact gravitationally with
stars and clouds of gas and dust.
437
00:30:31,160 --> 00:30:33,360
These interactions
push the black holes
438
00:30:33,440 --> 00:30:35,720
towards the centre of the galaxy...
439
00:30:37,360 --> 00:30:39,360
..where the black holes swarm.
440
00:30:44,000 --> 00:30:47,680
A swarm of stellar mass
black holes sounds deadly,
441
00:30:47,760 --> 00:30:52,600
but it may not be the most lethal
thing in the centre of the Milky Way.
442
00:30:55,120 --> 00:30:57,640
A surprising observation indicates
443
00:30:57,720 --> 00:31:01,440
that there is a lot of antimatter
in the centre of our galaxy.
444
00:31:04,040 --> 00:31:07,960
In 2017, astronomers tried to solve
445
00:31:08,040 --> 00:31:10,600
a decades old cosmic mystery.
446
00:31:13,640 --> 00:31:17,880
Unexplained high energy radiation
streaming through our galaxy.
447
00:31:22,200 --> 00:31:25,000
At first, we didn't know
where it came from...
448
00:31:27,200 --> 00:31:30,160
..but we discovered
it was gamma radiation
449
00:31:30,240 --> 00:31:33,400
coming from somewhere
in the centre of the Milky Way.
450
00:31:36,120 --> 00:31:38,840
The question is,
what's making these gamma rays?
451
00:31:38,920 --> 00:31:42,160
That's hard to do. It's not like
you can rub your hands together
452
00:31:42,240 --> 00:31:44,640
and generate gamma rays.
453
00:31:44,720 --> 00:31:47,560
When we took a closer look
at the gamma rays,
454
00:31:47,640 --> 00:31:49,720
we discovered the signature
455
00:31:49,800 --> 00:31:53,240
of the most explosive substance
in the universe,
456
00:31:53,320 --> 00:31:55,320
antimatter.
457
00:31:57,920 --> 00:32:02,560
Antimatter is like normal matter,
but with opposite charge.
458
00:32:02,640 --> 00:32:05,480
That's it. It's matter's evil twin.
459
00:32:07,880 --> 00:32:11,920
When evil twin meets good twin,
it is not a happy reunion.
460
00:32:15,880 --> 00:32:19,960
Antimatter is scary. It's not like
you wanna have some in your kitchen.
461
00:32:20,040 --> 00:32:23,720
This stuff is very, very explosive,
if you wanna think of it that way.
462
00:32:23,800 --> 00:32:27,920
If it touches normal matter,
it releases a huge amount of energy.
463
00:32:29,640 --> 00:32:33,960
When matter and antimatter combine,
they annihilate each other
464
00:32:34,040 --> 00:32:36,520
and transform
into high energy radiation,
465
00:32:36,600 --> 00:32:38,640
just like the gamma rays
466
00:32:38,720 --> 00:32:41,360
seen streaming out of the centre
of the Milky Way.
467
00:32:43,840 --> 00:32:46,320
We see antimatter
throughout the galaxy,
468
00:32:46,400 --> 00:32:48,720
but strangely, the galactic centre
469
00:32:48,800 --> 00:32:53,000
seemed to have 40% more antimatter
than anywhere else.
470
00:32:55,400 --> 00:32:57,400
Right now,
in the heart of our galaxy,
471
00:32:57,480 --> 00:33:00,320
we actually observe
fountains of antimatter
472
00:33:00,400 --> 00:33:04,800
that are producing ten trillion tons
of antimatter every second.
473
00:33:06,360 --> 00:33:09,320
One of the big questions that we've
wondered about for a very long time
474
00:33:09,400 --> 00:33:11,920
is what's the origin of this stuff?
475
00:33:14,360 --> 00:33:16,880
Initially, there were
several suspects.
476
00:33:18,040 --> 00:33:20,040
One possible source of antimatter
477
00:33:20,120 --> 00:33:23,400
is the central supermassive
black hole, Sagittarius A-star.
478
00:33:23,480 --> 00:33:26,480
Matter can be swirling around this,
and it can have such high energy
479
00:33:26,560 --> 00:33:29,120
that it can create antimatter.
480
00:33:31,960 --> 00:33:34,880
But the antimatter isn't coming
from a single point.
481
00:33:34,960 --> 00:33:38,160
It's spread across thousands
of light years of space.
482
00:33:40,280 --> 00:33:44,560
So Sagittarius A-star can't be
the source of the gamma ray stream.
483
00:33:48,120 --> 00:33:50,840
Another suspect was dark matter.
484
00:33:53,600 --> 00:33:57,080
One of the biggest mysteries in the
universe right now is dark matter.
485
00:33:57,160 --> 00:33:59,920
We know that the majority
of mass in the universe
486
00:34:00,000 --> 00:34:03,360
is not in the same form that we are,
it's not made of atoms.
487
00:34:03,440 --> 00:34:05,960
But whatever sort of particle it is
or may be,
488
00:34:06,040 --> 00:34:09,000
if these things collide,
they can produce antimatter,
489
00:34:09,080 --> 00:34:11,080
and that will produce the gamma
rays.
490
00:34:11,160 --> 00:34:13,760
So it's possible that as we look
into the heart of the galaxy
491
00:34:13,840 --> 00:34:15,840
and see these extra gamma rays,
492
00:34:15,920 --> 00:34:18,000
that's the signal
that dark matter is there.
493
00:34:21,480 --> 00:34:24,480
But the gamma ray stream we detected
494
00:34:24,560 --> 00:34:27,640
is too weak to have been created
by dark matter.
495
00:34:30,840 --> 00:34:33,600
Then we had a breakthrough.
496
00:34:33,680 --> 00:34:38,400
We discovered that a special metal
called Titanium-44
497
00:34:38,480 --> 00:34:41,800
could be responsible
for the gamma ray stream.
498
00:34:46,000 --> 00:34:49,120
Titanium-44 is a highly
radioactive element.
499
00:34:49,200 --> 00:34:52,440
That means that it wants to decay
into other types of nuclei.
500
00:34:55,880 --> 00:35:00,520
When Titanium-44 decays,
it gives off antimatter.
501
00:35:00,600 --> 00:35:03,640
But to produce the antimatter
seen in the galaxy's core,
502
00:35:03,720 --> 00:35:06,600
you would need a lot of Titanium-44.
503
00:35:07,960 --> 00:35:12,160
It could be created
in rare energetic events,
504
00:35:12,240 --> 00:35:15,600
in the collision of two dead stars,
505
00:35:15,680 --> 00:35:18,480
white dwarfs.
506
00:35:18,560 --> 00:35:20,080
A white dwarf star
507
00:35:20,160 --> 00:35:22,320
is a star that did
not have enough mass when it died
508
00:35:22,400 --> 00:35:24,400
to actually become a supernova.
509
00:35:24,480 --> 00:35:26,840
It just sort of cools off
as a dead little cinder.
510
00:35:26,920 --> 00:35:28,560
But what if you have two white
dwarfs
511
00:35:28,640 --> 00:35:30,400
that are orbiting around each other?
512
00:35:30,480 --> 00:35:33,360
And as they come closer
and closer and collide,
513
00:35:33,440 --> 00:35:35,640
all of a sudden now
you have enough mass
514
00:35:35,720 --> 00:35:37,920
to actually kick
a supernova explosion off.
515
00:35:42,720 --> 00:35:44,720
These particular kinds of supernovae
516
00:35:44,800 --> 00:35:47,080
are very good
at producing Titanium-44.
517
00:35:49,520 --> 00:35:52,200
So these kinds of supernovas
are very, very good
518
00:35:52,280 --> 00:35:54,280
at making antimatter.
519
00:35:56,360 --> 00:35:59,720
These supernovas erupt
in the core of the galaxy
520
00:35:59,800 --> 00:36:02,120
once every 2,000 years.
521
00:36:04,640 --> 00:36:07,800
But outside of the core,
in the disc of the galaxy
522
00:36:07,880 --> 00:36:09,880
where our solar system orbits...
523
00:36:12,040 --> 00:36:15,160
..these supernovas happen
three times as often.
524
00:36:17,000 --> 00:36:20,080
So, the gamma ray observations
were wrong,
525
00:36:20,160 --> 00:36:23,280
there isn't more antimatter
in the heart of the galaxy.
526
00:36:24,600 --> 00:36:28,960
It's our region of the galaxy
that contains the most antimatter.
527
00:36:31,840 --> 00:36:34,720
The question is, are we in danger?
528
00:36:36,440 --> 00:36:38,080
If you take an ounce of matter
529
00:36:38,160 --> 00:36:40,240
and an ounce of antimatter
and collide them,
530
00:36:40,320 --> 00:36:43,000
you're generating
a megaton of energy,
531
00:36:43,080 --> 00:36:47,120
the equivalent of a million tons
of TNT exploding.
532
00:36:48,480 --> 00:36:51,880
So you don't need much antimatter
to generate a vast amount of energy.
533
00:36:53,240 --> 00:36:55,520
But the thing you have
to remember is we live
534
00:36:55,600 --> 00:36:59,120
in this wonderful dramatic
environment of the larger universe.
535
00:36:59,200 --> 00:37:01,640
It's not dangerous.
It's very far away from us,
536
00:37:01,720 --> 00:37:03,720
and it's fascinating...
537
00:37:05,920 --> 00:37:09,080
..that all of this antimatter's
being produced in our galaxy.
538
00:37:09,160 --> 00:37:11,560
So just sit back
and enjoy the fireworks.
539
00:37:19,000 --> 00:37:23,280
The Milky Way is around 100,000
light years across,
540
00:37:23,360 --> 00:37:27,360
and it's home
to at least 200 billion stars,
541
00:37:27,440 --> 00:37:30,920
but it hasn't always been this large.
542
00:37:31,000 --> 00:37:34,000
We know that our Milky Way galaxy
543
00:37:34,080 --> 00:37:36,400
grew to the size it is now,
which is huge,
544
00:37:36,480 --> 00:37:38,920
by eating other galaxies.
545
00:37:39,000 --> 00:37:41,800
And some of these galaxies
would have had
546
00:37:41,880 --> 00:37:44,360
supermassive black holes
in their centres.
547
00:37:47,760 --> 00:37:51,440
When the Milky Way's gravity
pulled in smaller galaxies,
548
00:37:51,520 --> 00:37:55,200
most of their material
merged with the Milky Way.
549
00:37:57,160 --> 00:37:59,640
But some material, like stars,
550
00:37:59,720 --> 00:38:04,200
could've been slung tens of thousands
of light years out of the Milky Way.
551
00:38:07,160 --> 00:38:09,320
This could even have happened
552
00:38:09,400 --> 00:38:12,360
to a smaller
galaxy's supermassive black hole.
553
00:38:12,440 --> 00:38:16,920
It is entirely possible
there are supermassive black holes
554
00:38:17,000 --> 00:38:20,000
wandering around out there,
not in the centre.
555
00:38:22,680 --> 00:38:24,800
So how could it be possible
that there's actually
556
00:38:24,880 --> 00:38:26,880
a supermassive black hole
close to us,
557
00:38:26,960 --> 00:38:29,040
wandering around,
but we never even see it?
558
00:38:29,120 --> 00:38:32,560
Well, remember, 'black hole'
means it's really, really black.
559
00:38:32,640 --> 00:38:35,280
It actually absorbs radiation
and any energy,
560
00:38:35,360 --> 00:38:37,960
so unless something
is falling into a black hole
561
00:38:38,040 --> 00:38:40,280
or orbiting around it,
you're not gonna see it.
562
00:38:41,600 --> 00:38:44,320
And so if this supermassive
black hole were hypothetically
563
00:38:44,400 --> 00:38:46,400
wandering the outskirts
of our galaxy,
564
00:38:46,480 --> 00:38:49,960
well, there's a lot less gas there
for that black hole to run into,
565
00:38:50,040 --> 00:38:53,520
and if there's no gas around that
black hole, we will not see it.
566
00:38:56,120 --> 00:38:58,520
The rogue supermassive black hole
567
00:38:58,600 --> 00:39:02,280
may not stay in the outskirts
of the galaxy forever,
568
00:39:02,360 --> 00:39:06,720
gravitational interactions slowly
pull it back into the Milky Way.
569
00:39:08,280 --> 00:39:10,520
Billions of years later,
570
00:39:10,600 --> 00:39:14,000
the supermassive black hole
arrives in the centre.
571
00:39:18,440 --> 00:39:23,000
On the way in, it could cause havoc
for solar systems like our own.
572
00:39:24,440 --> 00:39:27,400
If a solar system is disturbed
573
00:39:27,480 --> 00:39:30,200
by a wandering black hole...
574
00:39:31,840 --> 00:39:35,920
..then the nice,
ordered architecture,
575
00:39:36,000 --> 00:39:40,440
or the orbits of its planets,
are completely disrupted.
576
00:39:42,280 --> 00:39:47,280
In our solar system, the orbits
of the planets are perfectly ordered.
577
00:39:48,760 --> 00:39:51,920
If a black hole came
even a little too close,
578
00:39:52,000 --> 00:39:54,920
its gravity could throw the planets
into chaos.
579
00:39:57,800 --> 00:40:01,280
Some planets might be flung out
of our solar system,
580
00:40:01,360 --> 00:40:05,000
others might be plunged down
into the nuclear furnace of the sun.
581
00:40:12,600 --> 00:40:16,000
Some planets could face
the ultimate destruction...
582
00:40:18,400 --> 00:40:21,200
..being eaten
by the rogue black hole.
583
00:40:26,280 --> 00:40:29,000
If we can't see them,
584
00:40:29,080 --> 00:40:32,000
could one be about to wander
through our solar system?
585
00:40:34,640 --> 00:40:38,800
So, hypothetically,
a wandering black hole could,
586
00:40:38,880 --> 00:40:41,040
yes, impact our solar system,
587
00:40:41,120 --> 00:40:44,600
but I can't overstate how profoundly
unlikely that is to happen.
588
00:40:44,680 --> 00:40:46,680
Galaxies are mostly empty space.
589
00:40:46,760 --> 00:40:50,800
We are just this speck in this
cosmic vortex that is our galaxy,
590
00:40:50,880 --> 00:40:55,240
and so our solar system is a really,
really, really small target.
591
00:40:58,440 --> 00:41:01,320
When this rogue
supermassive black hole meets up
592
00:41:01,400 --> 00:41:04,920
with Sagittarius A-star,
the fuse is lit.
593
00:41:07,520 --> 00:41:10,200
The pair spiral towards each other...
594
00:41:13,160 --> 00:41:15,720
..spinning faster and faster,
595
00:41:15,800 --> 00:41:18,040
reaching up to
half the speed of light.
596
00:41:23,400 --> 00:41:26,840
Finally, the two black holes merge.
597
00:41:32,960 --> 00:41:36,360
You would have an enormously
energetic event on your hands.
598
00:41:36,440 --> 00:41:40,080
Those supermassive black holes
could, in principle, merge together,
599
00:41:40,160 --> 00:41:42,960
create a huge blast
of gravitational waves,
600
00:41:43,040 --> 00:41:46,600
accompanied by a profoundly
energetic flash of light,
601
00:41:46,680 --> 00:41:49,200
that could, in principle,
endanger all life on Earth.
602
00:41:55,760 --> 00:42:00,360
It's literally a stretching
and contracting of space itself.
603
00:42:00,440 --> 00:42:02,640
It's like grabbing
the framework of space
604
00:42:02,720 --> 00:42:04,720
and just shaking it really hard.
605
00:42:04,800 --> 00:42:08,600
And if this happens in our galaxy,
the amount of energy emitted,
606
00:42:08,680 --> 00:42:11,520
that would be... That would be bad.
607
00:42:14,240 --> 00:42:17,480
When the black holes collide,
they release more energy
608
00:42:17,560 --> 00:42:20,240
than all of the stars
in the universe combined.
609
00:42:28,320 --> 00:42:31,400
Should we be panicked about this?
And the answer is, no.
610
00:42:31,480 --> 00:42:34,400
The Earth has been orbiting the sun
for four and a half billion years
611
00:42:34,480 --> 00:42:36,480
without any incident, right?
612
00:42:36,560 --> 00:42:38,840
We're pretty safe from them.
613
00:42:38,920 --> 00:42:43,360
If we were around to see
the two black holes collide,
614
00:42:43,440 --> 00:42:47,200
we'd witness the most destructive
light show in history.
615
00:42:50,000 --> 00:42:53,160
But for now, the centre of our galaxy
616
00:42:53,240 --> 00:42:55,600
is relatively quiet,
617
00:42:55,680 --> 00:42:58,600
but it's still a terrible place
to be.
618
00:43:02,520 --> 00:43:05,640
The centre of our Milky Way
is not a friendly place.
619
00:43:05,720 --> 00:43:09,480
It's nowhere you wanna be.
It's a bad neighbourhood.
620
00:43:09,560 --> 00:43:13,600
You've got tons of stars,
tons of radiation,
621
00:43:13,680 --> 00:43:17,440
stars are being born
and dying and exploding.
622
00:43:17,520 --> 00:43:19,680
You've got the central
supermassive black hole,
623
00:43:19,760 --> 00:43:22,160
you've got a potential
swarm of black holes,
624
00:43:22,240 --> 00:43:24,560
you've got accretion discs,
you've got flares,
625
00:43:24,640 --> 00:43:27,280
you've got magnetic outbursts,
you've got jets.
626
00:43:28,720 --> 00:43:31,120
Let's just stay out here
in the suburbs, all right?
627
00:43:32,600 --> 00:43:34,600
The centre of our galaxy
628
00:43:34,680 --> 00:43:38,040
is one of the most nightmarish
places in the cosmos.
629
00:43:40,840 --> 00:43:44,400
It's also home to some
of the most incredible forces
630
00:43:44,480 --> 00:43:46,480
the universe has to offer.
631
00:43:49,280 --> 00:43:52,000
Whatever the future holds
for our galaxy...
632
00:43:53,960 --> 00:43:57,920
..the core of the Milky Way
will be at the centre of it all.
633
00:43:59,440 --> 00:44:03,040
Our home galaxy, the Milky Way,
is our safe harbour,
634
00:44:03,120 --> 00:44:05,720
our island in this vast cosmic
ocean.
635
00:44:05,800 --> 00:44:08,480
And so to understand
the heart of our galaxy
636
00:44:08,560 --> 00:44:12,840
is to understand our home
in this cosmic void.
637
00:44:12,920 --> 00:44:15,840
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