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Subs by KalEl82
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I can see everything quite clearly.
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It has a stark beauty all its own.
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# See me when I float like a dove
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# Skies above...
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Magnificent desolation.
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- Beautiful view.
- Isn't that something?
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# Take me away #
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If I were to ask you,
"Where do you come from?"
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What would you say?
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What story would you tell?
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You might say,
"Well, I come from my home town..."
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Or "my city," or "my country."
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If you have
a particularly wide perspective,
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you might say,
"I come from Planet Earth."
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But what is the largest structure
that we could legitimately call home?
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Well, I would argue, it's that.
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That faint arc of light
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that stretches across the sky
from horizon to horizon
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is an outer spiral arm
of our galaxy, the Milky Way,
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our home island of 400 billion stars.
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The Milky Way takes its name
from the dense band of stars
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that sweeps across the sky
on the clearest of nights.
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From our vantage point, here on Earth,
we see the galaxy from within.
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But if we could
travel outside the galaxy,
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we would see the entire structure.
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The Milky Way revealed as an island
of light surrounded by darkness.
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Hundreds of billions of stars
in a single disk,
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that's existed
since the universe was young.
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Only now are we
able to explore its history.
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How it was born,
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how, through a series
of remarkable events,
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it grew to become
the galaxy we inhabit today,
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and how, eventually, it will end.
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We've discovered our own part
in this story, too,
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living, as we do,
inside the Milky Way,
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just over halfway
along one of its magnificent arms,
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around a small but familiar star.
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The Milky Way is an island in a sense.
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Every star you can see in the night sky
is a part of our galaxy.
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Our nearest neighbouring large galaxy
is over two million light years away.
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So it certainly feels as if
we are isolated and alone,
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adrift in an ocean of dark.
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And that is true to a point.
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There is no conceivable technology
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that will ever allow us
to leave our island physically.
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But science allows us to leave
the Milky Way in our imaginations,
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to view our galaxy
from impossible perspectives
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in both space and time,
and to tell its story.
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One mission,
more than any other,
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has deepened our understanding
of the galaxy,
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a spacecraft bearing the name
of an ancient Greek goddess...
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Everything functioning beautifully.
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...Gaia...
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Coming up
on separation of the boosters.
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...ancestral mother
of all life on Earth.
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The four boosters,
the four points of light, falling away.
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Gaia's mission,
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to map the locations
of billions of stars in the Milky Way,
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nearly all of them for the first time.
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Gaia spins on its axis,
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its sensors scanning the galaxy
in all directions.
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Every star is mapped
an average of 70 times,
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allowing Gaia to calculate
the speed and direction of each one,
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pinpointing their locations with
accuracies up to 1 ,000th of 1 %,
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over 1 .5 million stars every hour.
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Almost two billion in total so far.
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To create a map
like nothing ever seen before.
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The Gaia data is by far
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the most detailed star map
ever produced,
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a revolution in our understanding
of the Milky Way.
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This is the data,
and it looks like a,
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you know, an artist's impression
of a galaxy,
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something from science fiction.
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But this is a high-precision 3D map
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of our home, of our island of stars,
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and we can even fly through it.
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Such is the precision
of the mapping of the position.
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All these points of light are stars,
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some of them as far as 30,000
light years out from the solar system.
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The map allows us to journey through
the galaxy at impossible speeds,
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bringing distant stars within reach.
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But this is also
a journey through time.
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The extraordinary thing about this map
is that it's alive in a sense.
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I mean, Gaia didn't just measure
the positions of these stars,
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it measured their velocities.
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And that means we can tell
where those stars are going,
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what the galaxy is
going to be like in the future.
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But also, we can tell
where they came from.
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So, what the galaxy
was like in the past.
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By reversing the direction
of every star,
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we can rewind their histories,
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travelling backwards in time
through billions of years.
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Gaia has initiated a new science,
a science of galactic archaeology,
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where we can ask questions
about the origins of our galaxy itself.
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The first galaxies emerged
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just a few hundred million years
after the Big Bang.
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The universe was criss-crossed
by a vast structure
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known as the cosmic web.
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Great filaments of dark matter
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along which gravity attracted
ever denser concentrations of gas
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separated by immense tracts
of empty space.
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The first stars were born
where the filaments crossed,
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where the gas was dense enough
to collapse under its own gravity,
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and for the stars to ignite.
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New stars formed in their billions,
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bound together
by their mutual gravitational pull.
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These were the first galaxies.
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Amongst them, the Milky Way,
in its embryonic form,
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far smaller
and more irregular in structure
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than the mature spiral galaxy
we inhabit today.
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The exact details
of the Milky Way's birth
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remained a subjective research.
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But thanks to modern day observations,
the story of how our galaxy grew
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from those early beginnings
is coming into much sharper relief.
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The Gaia data allows us to see
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how the Milky Way
evolved throughout its history.
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And one of the clues
that it's had an interesting history
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can be seen in this animation.
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You see that most of the stars
orbit in very regular orbits
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around the centre of the Milky Way.
That's exactly what you'd expect.
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But you can see here
that some of the stars
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have very different orbits, indeed.
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They seem to be flying
all over the place.
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And that tells us
that something dramatic happened
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at some point as our galaxy
made its way through the universe.
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Across the universe, hundreds of
billions of galaxies were forming.
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Some, just a few dozen, were born
close enough to the Milky Way
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that their mutual gravitational pull
drew them together,
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forming what we now know
as the local group of galaxies,
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our home archipelago.
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Six billion years
before the earth formed,
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some of the Milky Way stars
already had their own planets,
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early worlds that were about to witness
the transformation of the galaxy.
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The wonderful thing about astronomy
is that you can look up into the sky,
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and even if you can't see worlds,
you can imagine them,
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and you can imagine their stories.
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Like, over there,
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close to the bright star, Vega,
is Kepler-444.
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The faint ancient star
and planets orbiting around it
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that's witnessed
pretty much the entire history
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of the Milky Way galaxy.
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And then, maybe
swinging around in the sky,
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just close to the Plough constellation
that everybody can recognise,
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and follow it down.
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There's a really faint star there,
you can't see it with the naked eye.
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It's so nondescript
it doesn't even have a name.
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It's got a number.
It's got HD 73394.
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But that star is an alien star.
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It was born in another galaxy,
and it entered the Milky Way
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in a galactic collision
with a smaller galaxy,
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and Kepler-444 over there
witnessed it all,
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and witnessed the Milky Way
being thrown into chaos.
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Kepler-444 was orbited
by five planets...
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and something new
had appeared in their skies.
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A smaller galaxy
was approaching the Milky Way,
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with stars that burn bright blue,
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Gaia-Enceladus,
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a member of the local group,
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roughly a quarter
of the size of our own galaxy.
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Over hundreds of millions of years,
the galaxies collided...
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The stars of Gaia-Enceladus penetrating
deep into the Milky Way's heart.
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But our galaxy held its ground,
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capturing billions of incoming stars.
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An entire galaxy, swallowed whole.
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These alien stars
remain in our galaxy to this day.
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The Gaia data tell us that
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collisions are the driving force
of galactic evolution.
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Some galaxies cease to exist
as independent islands of stars,
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while others grow and prosper.
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The survival of the fittest, writ large.
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"When galaxies collide."
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You know, that phrase puts images of
Hollywood disaster movies into the mind,
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of stars getting ripped apart.
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But that's not what happens at all.
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I mean, you imagine
that our sun were,
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say, the size of a small pebble
or a grain of sand.
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The nearest neighbouring star
in this region of the galaxy
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will be somewhere over by those hills.
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The distances between stars is immense.
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The stars don't collide.
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So, when galaxies interact,
the stars get scattered.
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The shape of the galaxy changes,
but nothing gets destroyed.
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And, in fact, sometimes
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galactic collisions
can be engines of creation.
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Gaia-Enceladus,
the alien galaxy,
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had brought with it
fresh supplies of interstellar gas,
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the raw material of star formation.
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For a time, this gas heightened the rate
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at which the Milky Way
could produce new stars,
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helping it to grow.
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But long before our star was born,
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the Gaia-Enceladus collision era
drew to a close.
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What triggered the formation of the sun
has long remained a puzzle.
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But the Gaia telescope has discovered
new clues to its origin,
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in the events that followed
billions of years later,
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as our island of stars
continued to evolve.
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On the distant shores of the Milky Way,
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Gaia has investigated
a structure of epic proportions...
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A stream of stars
winding their way around the galaxy.
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This stream of stars is enormous.
It's almost unimaginable in scale.
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If you look up into the night sky,
those stars that you can see are,
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at most, a few thousand
light years away.
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You think about that,
the light began its journey to your eye
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from the most distant stars
when the pharaohs ruled Egypt.
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And then, if you look out
to the Milky Way,
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to the shores of our galaxy,
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you see light from a few tens
of thousands of light years away.
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I mean, that light began its journey
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when there were Neanderthals
here in Europe.
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But this stream of stars
wraps around the galaxy.
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It's hundreds of thousands
of light years in extent.
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A structure that large
demands an explanation.
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The stream is wreckage, it's footprints,
if you like, of a very violent event.
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Gaia has confirmed the origins
of this immense structure...
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through the telescope's unique ability
to help us travel through time...
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Backwards.
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The data tell a story
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of a new age of star birth,
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of the transformation of the Milky Way
triggered by another galactic collision.
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It was another galaxy
from our local group,
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Sagittarius dwarf,
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perhaps 20 times smaller
than the Milky Way,
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was torn apart in the impact.
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Sagittarius dwarf brought fresh supplies
of the vital ingredient for star birth.
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That is the sound of
the most common element in the universe.
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This radio telescope is pointing
towards the Milky Way,
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as she's just risen above the horizon
over there behind the clouds,
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and what you're listening to
is hydrogen gas.
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The radio telescope is detecting
the faint signal of hydrogen
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from across the galaxy.
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Hydrogen is found
throughout the Milky Way,
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sometimes in the form
of towering clouds light years high.
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These regions are star factories
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where the dense clouds of hydrogen gas
collapse under gravity,
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to forge new stars.
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Hydrogen atoms radiate radio waves
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at a very particular wavelength,
21 centimetres.
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And as I speak, that radiation has been
captured by that radio telescope.
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Imagine, there are atoms over there.
And by "over there,"
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I mean, what, thousands,
tens of thousands of light years away.
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And at some point,
way, way back in the past,
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out came the radiation,
and we can listen to it.
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So, we're listening
to the lifeblood of our galaxy.
245
00:32:03,961 --> 00:32:07,680
As Sagittarius dwarf
passed through the Milky Way,
246
00:32:08,241 --> 00:32:12,760
it brought fresh gas and fresh energy.
247
00:32:31,281 --> 00:32:35,040
The impact sent ripples
across the Milky Way,
248
00:32:39,161 --> 00:32:43,800
triggering another spectacular era
of star formation.
249
00:32:54,481 --> 00:32:58,360
And in the outer regions
of the galaxy...
250
00:33:03,921 --> 00:33:07,080
our own star was born.
251
00:33:18,441 --> 00:33:21,360
The sun was soon joined by the earth...
252
00:33:28,521 --> 00:33:33,640
and together, they set out
on their journey through the galaxy.
253
00:33:49,481 --> 00:33:52,000
We were born in the Milky Way,
254
00:33:56,441 --> 00:34:00,120
but we may have been
conceived in a collision.
255
00:34:05,641 --> 00:34:09,880
Now, we can't say for certain that
the collision with Sagittarius dwarf
256
00:34:09,961 --> 00:34:11,920
caused the formation of our sun.
257
00:34:12,001 --> 00:34:13,760
The data is not precise enough,
258
00:34:13,841 --> 00:34:16,440
and our understanding is
not deep enough for that.
259
00:34:16,521 --> 00:34:18,920
But what we can say is
that the birth of the sun
260
00:34:19,001 --> 00:34:22,040
coincided with
enhanced rates of star formation
261
00:34:22,121 --> 00:34:24,920
in the Milky Way,
caused by that collision.
262
00:34:25,441 --> 00:34:28,160
But that's not
quite the end of the story,
263
00:34:28,241 --> 00:34:32,160
because, in a very real sense,
the collision is still underway.
264
00:34:42,801 --> 00:34:45,240
The remains of Sagittarius dwarf
265
00:34:45,321 --> 00:34:48,680
are still orbiting
on the fringes of the Milky Way.
266
00:35:02,801 --> 00:35:05,280
Over the last five billion years,
267
00:35:05,361 --> 00:35:09,080
the galaxy has crossed our path
two more times,
268
00:35:25,001 --> 00:35:29,720
each interaction triggering
a new generation of star birth.
269
00:35:53,281 --> 00:35:58,640
A fresh sprinkling of light
inside our galaxy's spiral arms,
270
00:36:15,881 --> 00:36:21,120
the finishing touches
on a masterpiece of galactic creation.
271
00:36:33,961 --> 00:36:38,760
The poet, John Donne, famously wrote,
"No man is an island entire of itself,
272
00:36:38,841 --> 00:36:42,480
"every man is a piece
of the continent, a part of the main,"
273
00:36:42,561 --> 00:36:44,560
by which, he meant that
no human being
274
00:36:44,641 --> 00:36:47,120
can isolate themselves
from the rest of humanity
275
00:36:47,201 --> 00:36:51,440
because our origins and our fates
are so deeply intertwined,
276
00:36:51,521 --> 00:36:54,600
and therefore, we must
care deeply for each other.
277
00:36:54,681 --> 00:36:59,520
And the same is true for galaxies.
No galaxy is an island entire of itself.
278
00:36:59,601 --> 00:37:03,680
And the history of the Milky Way
stretches back 13 billion years or more.
279
00:37:03,761 --> 00:37:06,520
That's pretty much
the entire history of the universe,
280
00:37:06,601 --> 00:37:10,240
and its story is a story
of collisions and interactions
281
00:37:10,321 --> 00:37:14,560
between galaxies, of rivers,
and flows and streams of stars
282
00:37:14,641 --> 00:37:19,760
stirring up the void and triggering
the formation of worlds like ours.
283
00:37:19,841 --> 00:37:24,080
I mean, you, me,
everyone can trace our origins
284
00:37:24,161 --> 00:37:27,680
back to a collision between galaxies.
285
00:37:27,761 --> 00:37:32,960
You may be small, but you are
a consequence of grand events.
286
00:38:09,081 --> 00:38:11,760
And those grand events
haven't stopped.
287
00:38:11,841 --> 00:38:14,880
It just feels like it
because we don't perceive events
288
00:38:14,961 --> 00:38:18,840
that play out over billions of years,
involving billions of stars.
289
00:38:19,281 --> 00:38:22,640
But the unique thing
about this time in history
290
00:38:22,721 --> 00:38:24,640
is that we can speak
with some confidence,
291
00:38:24,721 --> 00:38:29,960
not only about our galaxy's past,
but also about our galaxy's future.
292
00:38:30,361 --> 00:38:34,400
And just as inexorably as
those great islands of stars
293
00:38:34,481 --> 00:38:39,000
drift through the universe,
change will come again.
294
00:39:05,601 --> 00:39:07,400
We move into the future
295
00:39:07,481 --> 00:39:11,480
with a new understanding
of our place in the galaxy.
296
00:39:24,881 --> 00:39:31,040
We are inhabitants of a small planet
orbiting around an ordinary star,
297
00:39:31,121 --> 00:39:34,760
where something extraordinary
has happened.
298
00:39:47,161 --> 00:39:52,360
But although the galaxy made us,
it wasn't made for us.
299
00:39:54,841 --> 00:39:58,320
We are accidental by-products
of its history
300
00:40:00,241 --> 00:40:04,720
and we will be passive witnesses
to its ongoing evolution.
301
00:40:10,601 --> 00:40:13,080
The Milky Way is the great survivor,
302
00:40:13,161 --> 00:40:17,560
and the echoes of its turbulent history
are literally written across the sky.
303
00:40:17,881 --> 00:40:22,240
Over there in the southwest,
the remnants of Sagittarius dwarf,
304
00:40:22,321 --> 00:40:25,200
the debris from that collision
still wandering around
305
00:40:25,281 --> 00:40:27,960
somewhere on the fringes
of the Milky Way.
306
00:40:28,521 --> 00:40:31,560
And in that direction,
as Sirius rises in the east
307
00:40:31,641 --> 00:40:33,640
in the constellation of Canis Major,
308
00:40:33,721 --> 00:40:36,080
there are the remains
of another dwarf galaxy
309
00:40:36,161 --> 00:40:38,600
that we think
collided with us long ago.
310
00:40:40,601 --> 00:40:43,680
So, the Milky Way
pretty much devours anything
311
00:40:43,761 --> 00:40:46,160
that comes into this region of space
312
00:40:46,241 --> 00:40:51,680
because it's the largest galaxy
in the neighbourhood, except for one.
313
00:40:59,601 --> 00:41:01,200
The local group is home
314
00:41:01,281 --> 00:41:05,560
to another galaxy
that rivals our own in size.
315
00:41:09,001 --> 00:41:13,760
A galaxy that's been
hiding in plain sight.
316
00:41:17,361 --> 00:41:21,000
Right up there, just between
the consolations of Cassiopeia
317
00:41:21,081 --> 00:41:22,760
and the Square of Pegasus,
318
00:41:22,841 --> 00:41:26,200
is a faint, misty patch of light
in the sky
319
00:41:26,281 --> 00:41:28,480
about twice the diameter of a full moon.
320
00:41:28,561 --> 00:41:30,520
So, you can certainly
see it with binoculars.
321
00:41:30,601 --> 00:41:32,040
And even in the city,
322
00:41:32,121 --> 00:41:34,600
I can take a photograph of it
with a camera like this.
323
00:41:37,721 --> 00:41:39,400
And there it is.
324
00:41:40,281 --> 00:41:43,720
That object is the Andromeda galaxy,
325
00:41:44,841 --> 00:41:49,520
and you see that it's a spiral shape.
You can see it even in this photograph.
326
00:41:50,921 --> 00:41:54,240
In many ways, Andromeda is our twin.
327
00:42:06,521 --> 00:42:11,000
And it's a twin that we've been
able to explore in incredible detail.
328
00:42:13,081 --> 00:42:19,680
Three, two, one,
and lift-off of Space Shuttle Atlantis,
329
00:42:19,761 --> 00:42:23,000
on a final visit to enhance
the vision of Hubble
330
00:42:24,881 --> 00:42:27,840
into the deepest grandeur
of our universe.
331
00:42:30,561 --> 00:42:32,960
Standing by for SRB separation.
332
00:42:45,681 --> 00:42:50,000
The Hubble Space Telescope
is in its fourth decade of operation.
333
00:43:00,961 --> 00:43:02,760
Its ongoing mission has given us
334
00:43:02,841 --> 00:43:06,520
some of the most detailed images
of the universe ever seen.
335
00:43:19,761 --> 00:43:24,400
Over the years, Hubble has frequently
turned its attention to Andromeda,
336
00:43:32,561 --> 00:43:35,680
2.5 million light years from Earth.
337
00:43:39,481 --> 00:43:43,720
It's mapped a spiral structure
similar to that of the Milky Way
338
00:43:47,881 --> 00:43:52,040
with such fine precision that
we've been able to calculate
339
00:43:52,121 --> 00:43:55,560
not only the motion
of Andromeda's stars,
340
00:43:55,641 --> 00:43:59,160
but also the motion
of the galaxy itself.
341
00:44:05,361 --> 00:44:10,360
And we now know that the entire galaxy
is heading towards us
342
00:44:10,761 --> 00:44:14,760
at over 400,000 kilometres per hour.
343
00:44:27,161 --> 00:44:29,640
Now, you may think,
"Well, what's one more collision?"
344
00:44:29,721 --> 00:44:32,080
I mean, the Milky Way
has survived all these collisions
345
00:44:32,161 --> 00:44:34,760
for pretty much
the entire history of the universe.
346
00:44:35,241 --> 00:44:40,200
Well, this one will be different
because Andromeda is bigger than us.
347
00:44:51,081 --> 00:44:55,520
The Milky Way, as we know it today,
will not be immortal
348
00:45:00,561 --> 00:45:03,640
and the earth will witness its demise.
349
00:45:14,121 --> 00:45:16,920
Two galaxies in a single sky,
350
00:45:17,001 --> 00:45:21,880
gradually but inexorably
merging into one.
351
00:46:08,801 --> 00:46:14,040
In the impact, there will be
a last colossal burst of star formation.
352
00:46:21,881 --> 00:46:25,160
But this will be very different
to previous collisions.
353
00:46:31,961 --> 00:46:36,360
This time our galaxy
will meet its match.
354
00:47:03,681 --> 00:47:07,760
The great galaxies will distort
each of the spiral arms.
355
00:47:09,961 --> 00:47:12,240
Stars will be scattered
356
00:47:14,481 --> 00:47:19,080
until no traces
of the original structures remain.
357
00:48:01,401 --> 00:48:05,120
The Milky Way's fate is sealed.
358
00:48:07,721 --> 00:48:10,920
Andromeda will be the first
of a series of mergers
359
00:48:11,001 --> 00:48:14,840
as the remaining galaxies
in our local group converge,
360
00:48:14,921 --> 00:48:17,800
drawn together by gravity.
361
00:48:27,561 --> 00:48:31,840
But Hubble has allowed us
to see even further into the future.
362
00:48:33,841 --> 00:48:37,200
It's looked out
far beyond the local group,
363
00:48:37,281 --> 00:48:40,480
towards the edge
of the observable universe,
364
00:48:40,561 --> 00:48:46,160
and seen that every distant galaxy
is receding from us.
365
00:48:56,521 --> 00:48:58,440
In a final twist,
366
00:48:58,521 --> 00:49:03,120
these retreating galaxies
tell us something profound
367
00:49:03,201 --> 00:49:06,600
about the nature of the universe itself.
368
00:49:10,441 --> 00:49:12,320
We live in an expanding universe.
369
00:49:12,401 --> 00:49:16,400
In fact, we live in a universe
that's accelerating in its expansion.
370
00:49:16,481 --> 00:49:20,000
So, all the galaxies are
rushing away from each other,
371
00:49:20,081 --> 00:49:24,080
and in the far future, they'll be
rushing away from each other so fast
372
00:49:24,161 --> 00:49:28,160
that even if we sent
a beam of light out to the galaxies,
373
00:49:28,241 --> 00:49:29,560
it would never catch them.
374
00:49:47,201 --> 00:49:51,480
Billions of years from now,
the remnants of the Milky Way will form
375
00:49:51,561 --> 00:49:55,480
part of a single,
gigantic collection of stars...
376
00:50:02,241 --> 00:50:05,720
The merged remains
of the local group...
377
00:50:09,801 --> 00:50:15,760
Alone, as every other galaxy
recedes into the distance.
378
00:50:24,681 --> 00:50:30,600
Eventually, all the galaxies
will fade from view,
379
00:50:30,681 --> 00:50:37,680
and our galaxy will stand at last
in perfect isolation...
380
00:50:39,921 --> 00:50:44,040
An island unto itself.
381
00:50:52,121 --> 00:50:56,280
I think we live at a fortunate time
in the history of the universe
382
00:50:56,361 --> 00:50:59,760
because we can look into the sky
and see the galaxies.
383
00:50:59,961 --> 00:51:04,120
The astronomers of the far future might
imagine that they live in a universe
384
00:51:04,201 --> 00:51:08,680
populated by countless billions
of islands of billions of stars.
385
00:51:09,001 --> 00:51:11,040
But they won't be able to prove it.
386
00:51:11,361 --> 00:51:17,280
They won't be able to see the true scale
and majesty of the universe.
387
00:51:43,401 --> 00:51:45,800
We've been trying to understand
the band of stars
388
00:51:45,881 --> 00:51:49,240
that stretches across the night sky
since the time of the ancient Greeks.
389
00:51:51,321 --> 00:51:54,800
The story
of our galaxy, the Milky Way,
390
00:51:54,881 --> 00:51:59,040
how it started, how it was formed,
and how it's transformed
391
00:51:59,121 --> 00:52:01,440
is really the story of us.
392
00:52:02,081 --> 00:52:03,960
Inside the Milky Way,
you always have
393
00:52:04,041 --> 00:52:07,520
a slightly skewed perspective
of the way the Milky Way looks.
394
00:52:07,601 --> 00:52:10,040
So, we're in it. And so,
what we would like to do
395
00:52:10,121 --> 00:52:12,680
is go above it and look down
and see what it's like.
396
00:52:12,761 --> 00:52:14,760
Now, you can't do that
unless you could travel
397
00:52:14,841 --> 00:52:16,560
at millions of times the speed of light.
398
00:52:16,641 --> 00:52:18,640
We can't.
So, the only way we can do it
399
00:52:18,721 --> 00:52:22,480
is by working out accurately
where all the stars are,
400
00:52:22,561 --> 00:52:25,080
how far away they are,
from us, in particular.
401
00:52:35,721 --> 00:52:38,880
Gaia is
a European Space Agency spacecraft,
402
00:52:38,961 --> 00:52:41,320
which is, in principle,
a very simple little thing.
403
00:52:41,401 --> 00:52:43,520
It's two telescopes
collecting the light,
404
00:52:43,601 --> 00:52:45,560
putting it down onto one giant camera,
405
00:52:45,641 --> 00:52:47,480
biggest camera
ever put in space, actually.
406
00:52:50,121 --> 00:52:52,760
It can observe
the positions of stars so accurately
407
00:52:52,841 --> 00:52:56,720
that you could see the edge of
a Euro coin on the moon from Earth,
408
00:52:56,801 --> 00:52:59,000
and that is just mind-blowing.
409
00:53:14,841 --> 00:53:17,240
It was a beautiful launch.
Really spectacular.
410
00:53:23,321 --> 00:53:24,920
And then it got into
this critical state
411
00:53:25,001 --> 00:53:27,040
where they had to
open up the sun shields.
412
00:53:27,121 --> 00:53:31,880
It was critical that this opened up
and protect the payload from the sun.
413
00:53:32,481 --> 00:53:34,400
And that was the do-or-die moment.
414
00:53:43,041 --> 00:53:44,240
There's the good news.
415
00:53:51,561 --> 00:53:53,201
Gaia works
by measuring parallax.
416
00:53:53,241 --> 00:53:56,080
This is exactly the same way
your eyes and brain work
417
00:53:56,161 --> 00:53:58,720
so you can tell
how far away something is
418
00:53:58,801 --> 00:54:01,960
because of the slight difference
in angle from this eye to that eye.
419
00:54:02,841 --> 00:54:04,160
And so, what we do with Gaia
420
00:54:04,241 --> 00:54:07,360
is have a picture in the summer
and a picture in the winter,
421
00:54:07,441 --> 00:54:09,920
and in that stage, Gaia has gone
halfway around the sun.
422
00:54:10,001 --> 00:54:14,520
And so, its two eyes are twice
the radius of the earth's orbit apart.
423
00:54:14,921 --> 00:54:19,080
And that's how we do parallax.
All it is is a big version of your head.
424
00:54:26,641 --> 00:54:29,960
The last data released from Gaia
was in December 2020,
425
00:54:30,041 --> 00:54:33,480
and what's been really exciting is that
we've been able to get the distances
426
00:54:33,561 --> 00:54:36,600
and the motions of the star
to a much better level of accuracy.
427
00:54:40,481 --> 00:54:43,040
Most of the stars
in the disk of the Milky Way
428
00:54:43,121 --> 00:54:44,520
all move in the same direction,
429
00:54:44,601 --> 00:54:47,680
rotating clockwise
around the centre of the galaxy.
430
00:54:48,561 --> 00:54:52,000
And one of the most exciting things
that came out of the first data release
431
00:54:52,081 --> 00:54:55,720
was that a large sample of stars
were found that seemed to be rotating
432
00:54:55,801 --> 00:54:58,360
in the opposite direction
to the majority of stars
433
00:54:58,441 --> 00:55:01,920
in the Milky Way disk,
and that's really surprising.
434
00:55:05,161 --> 00:55:07,440
They probably came from
a different galaxy altogether.
435
00:55:07,521 --> 00:55:10,400
So, they're almost these alien stars
that have been brought in.
436
00:55:13,721 --> 00:55:15,560
Alien stars from galaxies
437
00:55:15,641 --> 00:55:19,760
that, long ago, shared
our own corner of the universe.
438
00:55:21,361 --> 00:55:23,960
The important thing to know
about our galactic neighbours
439
00:55:24,041 --> 00:55:26,280
is that nothing's
actually sitting still.
440
00:55:26,521 --> 00:55:28,560
We're all moving towards
or away from each other.
441
00:55:28,641 --> 00:55:30,720
We're sort of playing a dance out there.
442
00:55:34,681 --> 00:55:36,920
And driving
the dance of the galaxies
443
00:55:37,001 --> 00:55:40,560
is the universe's
most elusive form of matter.
444
00:55:42,801 --> 00:55:46,200
Dark matter is something
that has gravity, but produces no light.
445
00:55:46,281 --> 00:55:51,040
It surrounds us. In fact, it dominates
the mass in our own galaxy.
446
00:55:51,121 --> 00:55:56,080
And yet, we don't know what it is.
We can't touch it. We can't feel it.
447
00:55:58,361 --> 00:56:00,960
We were able to start
measuring very accurately
448
00:56:01,081 --> 00:56:03,360
the way stars move
from radial velocities,
449
00:56:03,441 --> 00:56:05,320
that's just towards and away from us,
450
00:56:05,401 --> 00:56:08,560
and this allowed us to
measure accurately for the first time
451
00:56:08,641 --> 00:56:11,000
how the dark matter
was distributed near us.
452
00:56:13,721 --> 00:56:17,040
The team have pieced together
how dark matter orchestrated
453
00:56:17,121 --> 00:56:19,760
a series of galactic collisions...
454
00:56:22,241 --> 00:56:24,600
that spanned billions of years.
455
00:56:27,481 --> 00:56:30,400
Dark matter is really important
in galaxy collisions
456
00:56:30,481 --> 00:56:31,720
because it's so abundant.
457
00:56:31,801 --> 00:56:32,920
So, it's really driving
458
00:56:33,001 --> 00:56:35,680
the gravitational interaction
between the galaxies.
459
00:56:41,681 --> 00:56:45,120
It is dark matter that determines
how violent a collision is,
460
00:56:45,201 --> 00:56:47,720
how rapidly, and with what intensity
461
00:56:47,801 --> 00:56:50,040
galaxies come together
when they collide.
462
00:56:52,681 --> 00:56:58,000
In many ways, it determines
how galaxies end up after a collision.
463
00:57:02,961 --> 00:57:04,640
So, the thing
that Gaia showed us
464
00:57:04,721 --> 00:57:08,880
is not that it's plausible that
this happened. It showed it did happen.
465
00:57:08,961 --> 00:57:10,440
It happened in just this way.
466
00:57:10,521 --> 00:57:13,640
So, it's not speculation any more.
It's quantitative science.
467
00:57:17,801 --> 00:57:19,600
The galaxy is a dynamic thing.
468
00:57:19,681 --> 00:57:21,760
It's a living organism, if you want.
469
00:57:21,841 --> 00:57:25,560
It is breathing. It is changing.
It is transforming.
470
00:57:30,081 --> 00:57:31,960
It's all
coming together in the end
471
00:57:32,041 --> 00:57:33,960
to tell us about how we got here
472
00:57:34,041 --> 00:57:36,360
and what our place
in the universe really is.
473
00:57:36,384 --> 00:57:39,384
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