WEBVTT

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Neil Armstrong:
I can see everything quite clear! Y.

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It has a stark beauty all its own.

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B uzz a ld ri n: Magnificent desolation.

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- Beautiful view.
- Armstro n g: Isn't that something?

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Brian Cox: 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 ”m y city, ” or ”m y 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

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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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Cox: 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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Man: Everything functioning beautifully.

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Cox: Gaia...

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Man: Coming up
on separation of the boosters.

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Cox: Ancestral mother
of all life on earth.

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Man: The four boosters,
the four points of light, falling away.

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Cox: Ga ia'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 7, 000th of 7%,

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over 7.5 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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Cox: The gaia data is by far

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the most detailed star map
e ver 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 e very 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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Cox: New stars formed in their billions,

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bound together
by their mutual gra vitational 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 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 gra vita tional 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 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 ga ia 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 itsjourney 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 itsjourney

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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

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to help us travel through
time... 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'sjust 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

00:29:48.401 --> 00:29:50.401
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.

00:31:07.601 --> 00:31:12.041
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.

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As sagittarius dwarf
passed through the milky way,

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it brought fresh gas and fresh energy.

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The impact sent ripples
across the milky way,

00:32:39.161 --> 00:32:43.801
triggering another spectacular era
of star formation.

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And in the outer regions
of the galaxy...

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Our own star was born.

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The sun was soon joined by the earth...

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And together, they set out
on their journey through the galaxy.

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We were born in the milky way,

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but we may have been
conceived in a collision.

00:34:05.641 --> 00:34:09.881
Now, we can't say for certain that
the collision with sagittarius dwarf

00:34:09.961 --> 00:34:11.921
caused the formation of our sun.

00:34:12.001 --> 00:34:13.761
The data is not precise enough,

00:34:13.841 --> 00:34:16.441
and our understanding is
not deep enough for that.

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But what we can say is
that the birth of the sun

00:34:19.001 --> 00:34:22.041
coincided with
enhanced rates of star formation

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in the milky way,
caused by that collision.

00:34:25.441 --> 00:34:28.161
But that's not
quite the end of the story,

00:34:28.241 --> 00:34:32.161
because, in a very real sense,
the collision is still underway.

00:34:42.801 --> 00:34:45.241
The remains of sagittarius dwarf

00:34:45.321 --> 00:34:48.681
are still orbiting
on the fringes of the milky way.

00:35:02.801 --> 00:35:05.281
Over the last five billion years,

00:35:05.361 --> 00:35:09.081
the galaxy has crossed our path
two more times,

00:35:25.001 --> 00:35:29.721
Each interaction triggering
a new generation of star birth.

00:35:53.281 --> 00:35:58.641
A fresh sprinkling of light
inside our galaxy's spiral arms,

00:36:15.881 --> 00:36:21.121
The finishing touches
on a masterpiece of galactic creation.

00:36:33.961 --> 00:36:38.761
The poet, John donne, famously wrote,
"no man is an island entire of itself,

00:36:38.841 --> 00:36:42.481
"every man is a piece
of the continent, a part of the main,"

00:36:42.561 --> 00:36:44.561
by which, he meant that
no human being

00:36:44.641 --> 00:36:47.121
can isolate themselves
from the rest of humanity

00:36:47.201 --> 00:36:51.441
because our origins and our fates
are so deeply intertwined,

00:36:51.521 --> 00:36:54.601
and therefore, we must
care deeply for each other.

00:36:54.681 --> 00:36:59.521
And the same is true for galaxies.
No galaxy is an island entire of itself.

00:36:59.601 --> 00:37:03.681
And the history of the milky way
stretches back 13 billion years or more.

00:37:03.761 --> 00:37:06.521
That's pretty much
the entire history of the universe,

00:37:06.601 --> 00:37:10.241
and its story is a story
of collisions and interactions

00:37:10.321 --> 00:37:14.561
between galaxies, of rivers,
and flows and streams of stars

00:37:14.641 --> 00:37:19.761
stirring up the void and triggering
the formation of worlds like ours.

00:37:19.841 --> 00:37:24.081
I mean, you, me,
everyone can trace our origins

00:37:24.161 --> 00:37:27.681
back to a collision between galaxies.

00:37:27.761 --> 00:37:32.961
You may be small, but you are
a consequence of grand events.

00:38:09.081 --> 00:38:11.761
And those grand events
haven't stopped.

00:38:11.841 --> 00:38:14.881
It just feels like it
because we don't perceive events

00:38:14.961 --> 00:38:18.841
that play out over billions of years,
involving billions of stars.

00:38:19.281 --> 00:38:22.641
But the unique thing
about this time in history

00:38:22.721 --> 00:38:24.641
is that we can speak
with some confidence,

00:38:24.721 --> 00:38:29.961
not only about our galaxy's past,
but also about our galaxy's future.

00:38:30.361 --> 00:38:34.401
And just as inexorably as
those great islands of stars

00:38:34.481 --> 00:38:39.001
drift through the universe,
change will come again.

00:39:05.601 --> 00:39:11.481
We move into the future with a new
understanding of our place in the galaxy.

00:39:24.881 --> 00:39:31.041
We are inhabitants of a small planet
orbiting around an ordinary star,

00:39:31.121 --> 00:39:34.761
where something extraordinary
has happened.

00:39:47.161 --> 00:39:52.361
But although the galaxy made us,
it wasn't made for us.

00:39:54.841 --> 00:39:58.321
We are accidental b y-products
of its history

00:40:00.241 --> 00:40:04.721
and we will be passive witnesses
to its ongoing evolution.

00:40:10.601 --> 00:40:13.081
The milky way is the great survivor,

00:40:13.161 --> 00:40:17.561
and the echoes of its turbulent history
are literally written across the sky.

00:40:17.881 --> 00:40:22.241
Over there in the southwest,
the remnants of sagittarius dwarf,

00:40:22.321 --> 00:40:25.201
the debris from that collision
still wandering around

00:40:25.281 --> 00:40:27.961
somewhere on the fringes
of the milky way.

00:40:28.521 --> 00:40:31.561
And in that direction,
as Sirius rises in the east

00:40:31.641 --> 00:40:33.641
in the constellation of canis major,

00:40:33.721 --> 00:40:36.081
there are the remains
of another dwarf galaxy

00:40:36.161 --> 00:40:38.601
that we think
collided with us long ago.

00:40:40.601 --> 00:40:43.681
So, the milky way
pretty much devours anything

00:40:43.761 --> 00:40:46.161
that comes into this region of space

00:40:46.241 --> 00:40:51.681
because it's the largest galaxy
in the neighbourhood, except for one.

00:40:59.601 --> 00:41:01.201
The local group is home

00:41:01.281 --> 00:41:05.561
to another galaxy
that rivals our own in size.

00:41:09.001 --> 00:41:13.761
A galaxy that's been
hiding in plain sight.

00:41:17.361 --> 00:41:21.001
Right up there, just between
the consolations of cassiopeia

00:41:21.081 --> 00:41:22.761
and the square of Pegasus,

00:41:22.841 --> 00:41:26.201
is a faint, misty patch of light
in the sky

00:41:26.281 --> 00:41:28.481
about twice the diameter of a full moon.

00:41:28.561 --> 00:41:30.521
So, you can certainly
see it with binoculars.

00:41:30.601 --> 00:41:32.041
And even in the city,

00:41:32.121 --> 00:41:34.601
I can take a photograph of it
with a camera like this.

00:41:37.721 --> 00:41:39.401
And there it is.

00:41:40.281 --> 00:41:43.721
That object is the Andromeda galaxy,

00:41:44.841 --> 00:41:49.521
and you see that it's a spiral shape.
You can see it even in this photograph.

00:41:50.921 --> 00:41:54.241
In many ways, Andromeda is our twin.

00:42:06.521 --> 00:42:11.001
And it's a twin that we've been
able to explore in incredible detail.

00:42:13.081 --> 00:42:19.681
Man: Three, two, one,
and lift-off of space shuttle Atlantis,

00:42:19.761 --> 00:42:23.001
on a final visit to enhance
the vision of hubble

00:42:24.881 --> 00:42:27.841
into the deepest grandeur
of our universe.

00:42:30.561 --> 00:42:32.961
Woman:
Standing by for srb separation.

00:42:45.681 --> 00:42:50.001
Cox: The hubble space telescope
is in its fourth decade of operation.

00:43:00.961 --> 00:43:02.761
Its ongoing mission has given us

00:43:02.841 --> 00:43:06.521
some of the most detailed images
of the universe ever seen.

00:43:19.761 --> 00:43:24.401
Over the years, hubble has frequently
turned its attention to Andromeda,

00:43:32.561 --> 00:43:35.681
2.5 million light years from earth.

00:43:39.481 --> 00:43:43.721
It's mapped a spiral structure
similar to that of the milky way

00:43:47.881 --> 00:43:52.041
with such fine precision that
we've been able to calculate

00:43:52.121 --> 00:43:55.561
not only the motion
of a ndromeda's stars,

00:43:55.641 --> 00:43:59.161
but also the motion
of the galaxy itself.

00:44:05.361 --> 00:44:10.361
And we now know that the entire galaxy
is heading towards us

00:44:10.761 --> 00:44:14.761
at over 400, 000 kilometres per hour.

00:44:27.161 --> 00:44:29.641
Now, you may think,
"well, what's one more collision?"

00:44:29.721 --> 00:44:32.081
I mean, the milky way
has survived all these collisions

00:44:32.161 --> 00:44:34.761
for pretty much
the entire history of the universe.

00:44:35.241 --> 00:44:40.201
Well, this one will be different
because Andromeda is bigger than us.

00:44:51.081 --> 00:44:55.521
The milky way, as we know it today,
will not be immortal

00:45:00.561 --> 00:45:03.641
And the earth will witness its demise.

00:45:14.121 --> 00:45:16.921
Two galaxies in a single sky,

00:45:17.001 --> 00:45:21.881
gradually but inexorably
merging into one.

00:46:08.801 --> 00:46:14.041
Cox: In the impact, there will be
a last colossal burst of star formation.

00:46:21.881 --> 00:46:25.161
But this will be very different
to previous collisions.

00:46:31.961 --> 00:46:36.361
This time our galaxy
will meet its match.

00:47:03.681 --> 00:47:07.761
The great galaxies will distort
each of the spiral arms.

00:47:09.961 --> 00:47:12.241
Stars will be scattered

00:47:14.481 --> 00:47:19.081
until no traces
of the original structures remain.

00:48:01.401 --> 00:48:05.121
The milky way's fate is sealed.

00:48:07.721 --> 00:48:10.921
Andromeda will be the first
of a series of mergers

00:48:11.001 --> 00:48:14.841
as the remaining galaxies
in our local group converge,

00:48:14.921 --> 00:48:17.801
drawn together by gra vity.

00:48:27.561 --> 00:48:31.841
But hubble has allowed us
to see even further into the future.

00:48:33.841 --> 00:48:37.201
It's looked out
far beyond the local group,

00:48:37.281 --> 00:48:40.481
towards the edge
of the observable universe,

00:48:40.561 --> 00:48:46.161
and seen that every distant galaxy
is receding from us.

00:48:56.521 --> 00:48:58.441
In a final twist,

00:48:58.521 --> 00:49:03.121
these retreating galaxies
tell us something profound

00:49:03.201 --> 00:49:06.601
about the nature of the universe itself.

00:49:10.441 --> 00:49:12.321
We live in an expanding universe.

00:49:12.401 --> 00:49:16.401
In fact, we live in a universe
that's accelerating in its expansion.

00:49:16.481 --> 00:49:20.001
So, all the galaxies are
rushing away from each other,

00:49:20.081 --> 00:49:24.081
and in the far future, they'll be
rushing away from each other so fast

00:49:24.161 --> 00:49:28.161
that even if we sent
a beam of light out to the galaxies,

00:49:28.241 --> 00:49:29.561
it would never catch them.

00:49:47.201 --> 00:49:51.481
Billions of years from now,
the remnants of the milky way will form

00:49:51.561 --> 00:49:55.481
part of a single,
gigantic collection of stars...

00:50:02.241 --> 00:50:05.721
The merged remains
of the local group...

00:50:09.801 --> 00:50:15.761
Alone, as every other galaxy
recedes into the distance.

00:50:24.681 --> 00:50:30.601
Eventually, all the galaxies
will fade from view,

00:50:30.681 --> 00:50:37.681
and our galaxy will stand at last
in perfect isolation...

00:50:39.921 --> 00:50:44.041
An island unto itself.

00:50:52.121 --> 00:50:56.281
I think we live at a fortunate time
in the history of the universe

00:50:56.361 --> 00:50:59.761
because we can look into the sky
and see the galaxies.

00:50:59.961 --> 00:51:04.121
The astronomers of the far future might
imagine that they live in a universe

00:51:04.201 --> 00:51:08.681
populated by countless billions
of islands of billions of stars.

00:51:09.001 --> 00:51:11.041
But they won't be able to prove it.

00:51:11.361 --> 00:51:17.281
They won't be able to see the true scale
and majesty of the universe.

00:51:43.401 --> 00:51:45.801
We've been trying to understand
the band of stars

00:51:45.881 --> 00:51:49.241
that stretches across the night sky
since the time of the ancient Greeks.

00:51:51.321 --> 00:51:54.801
Vasily belokurov: The story
of our galaxy, the milky way,

00:51:54.881 --> 00:51:59.041
how it started, how it was formed,
and how it's transformed

00:51:59.121 --> 00:52:01.441
is really the story of us.

00:52:02.081 --> 00:52:03.961
Das: Inside the milky way,
you always have

00:52:04.041 --> 00:52:07.521
a slightly skewed perspective
of the way the milky way looks.

00:52:07.601 --> 00:52:10.041
So, we're in it. And so,
what we would like to do

00:52:10.121 --> 00:52:12.681
is go above it and look down
and see what it's like.

00:52:12.761 --> 00:52:14.761
Now, you can't do that
unless you could travel

00:52:14.841 --> 00:52:16.561
at millions of times the speed of light.

00:52:16.641 --> 00:52:18.641
We can't.
So, the only way we can do it

00:52:18.721 --> 00:52:22.481
is by working out accurately
where all the stars are,

00:52:22.561 --> 00:52:25.081
how far away they are,
from us, in particular.

00:52:35.721 --> 00:52:38.881
Gilmore: Gaia is
a European space agency spacecraft,

00:52:38.961 --> 00:52:41.321
which is, in principle,
a very simple little thing.

00:52:41.401 --> 00:52:43.521
It's two telescopes
collecting the light,

00:52:43.601 --> 00:52:45.561
putting it down onto one giant camera,

00:52:45.641 --> 00:52:47.481
biggest camera
ever put in space, actually.

00:52:50.121 --> 00:52:52.761
Das: It can observe
the positions of stars so accurately

00:52:52.841 --> 00:52:56.721
that you could see the edge of
a euro coin on the moon from earth,

00:52:56.801 --> 00:52:59.001
and that is just mind-blowing.

00:53:14.841 --> 00:53:17.241
Gilmore: It was a beautiful launch.
Really spectacular.

00:53:23.321 --> 00:53:24.921
And then it got into
this critical state

00:53:25.001 --> 00:53:27.041
where they had to
open up the sun shields.

00:53:27.121 --> 00:53:31.881
It was critical that this opened up
and protect the payload from the sun.

00:53:32.481 --> 00:53:34.401
And that was the do-or-die moment.

00:53:43.041 --> 00:53:44.241
Man: There's the good news.

00:53:51.561 --> 00:53:53.201
Gilmore: Gaia works
by measuring parallax.

00:53:53.241 --> 00:53:56.081
This is exactly the same way
your eyes and brain work

00:53:56.161 --> 00:53:58.721
so you can tell
how far away something is

00:53:58.801 --> 00:54:01.961
because of the slight difference
in angle from this eye to that eye.

00:54:02.841 --> 00:54:04.161
And so, what we do with gaia

00:54:04.241 --> 00:54:07.361
is have a picture in the summer
and a picture in the winter,

00:54:07.441 --> 00:54:09.921
and in that stage, gaia has gone
halfway around the sun.

00:54:10.001 --> 00:54:14.521
And so, its two eyes are twice
the radius of the earth's orbit apart.

00:54:14.921 --> 00:54:19.081
And that's how we do parallax.
All it is is a big version of your head.

00:54:26.641 --> 00:54:29.961
Das: The last data released from gaia
was in December 2020,

00:54:30.041 --> 00:54:33.481
and what's been really exciting is that
we've been able to get the distances

00:54:33.561 --> 00:54:36.601
and the motions of the star
to a much better level of accuracy.

00:54:40.481 --> 00:54:43.041
Michelle Collins: Most of the stars
in the disk of the milky way

00:54:43.121 --> 00:54:44.521
all move in the same direction,

00:54:44.601 --> 00:54:47.681
rotating clockwise
around the centre of the galaxy.

00:54:48.561 --> 00:54:52.001
And one of the most exciting things
that came out of the first data release

00:54:52.081 --> 00:54:55.721
was that a large sample of stars
were found that seemed to be rotating

00:54:55.801 --> 00:54:58.361
in the opposite direction
to the majority of stars

00:54:58.441 --> 00:55:01.921
in the milky way disk,
and that's really surprising.

00:55:05.161 --> 00:55:07.441
They probably came from
a different galaxy altogether.

00:55:07.521 --> 00:55:10.401
So, they're almost these alien stars
that have been brought in.

00:55:13.721 --> 00:55:15.561
Cox: Alien stars from galaxies

00:55:15.641 --> 00:55:19.761
that, long ago, shared
our own corner of the universe.

00:55:21.361 --> 00:55:23.961
Das: The important thing to know
about our galactic neighbours

00:55:24.041 --> 00:55:26.281
is that nothing's
actually sitting still.

00:55:26.521 --> 00:55:28.561
We're all moving towards
or away from each other.

00:55:28.641 --> 00:55:30.721
We're sort of playing a dance out there.

00:55:34.681 --> 00:55:36.921
Cox: And driving
the dance of the galaxies

00:55:37.001 --> 00:55:40.561
is the universe's
most elusive form of matter.

00:55:42.801 --> 00:55:46.201
Dark matter is something
that has gravity, but produces no light.

00:55:46.281 --> 00:55:51.041
It surrounds us. In fact, it dominates
the mass in our own galaxy.

00:55:51.121 --> 00:55:56.081
And yet, we don't know what it is.
We can't touch it. We can't feel it.

00:55:58.361 --> 00:56:00.961
Gilmore: We were able to start
measuring very accurately

00:56:01.081 --> 00:56:05.321
the way stars move from radial velocities,
that's just towards and away from us,

00:56:05.401 --> 00:56:08.561
and this allowed us to
measure accurately for the first time

00:56:08.641 --> 00:56:11.001
how the dark matter
was distributed near us.

00:56:13.721 --> 00:56:17.041
Cox: The team have pieced together
how dark matter orchestra Ted

00:56:17.121 --> 00:56:19.761
a series of galactic collisions...

00:56:22.241 --> 00:56:24.601
That spanned billions of years.

00:56:27.481 --> 00:56:30.401
Dark matter is really important
in galaxy collisions

00:56:30.481 --> 00:56:31.721
because it's so abundant.

00:56:31.801 --> 00:56:32.921
So, it's really driving

00:56:33.001 --> 00:56:35.681
the gravitational interaction
between the galaxies.

00:56:41.681 --> 00:56:45.121
It is dark matter that determines
how violent a collision is,

00:56:45.201 --> 00:56:47.721
how rapidly, and with what intensity

00:56:47.801 --> 00:56:50.041
galaxies come together
when they collide.

00:56:52.681 --> 00:56:58.001
In many ways, it determines
how galaxies end up after a collision.

00:57:02.961 --> 00:57:04.641
Gilmore: So, the thing
that gaia showed us

00:57:04.721 --> 00:57:08.881
is not that it's plausible that
this happened. It showed it did happen.

00:57:08.961 --> 00:57:10.441
It happened in just this way.

00:57:10.521 --> 00:57:13.641
So, it's not speculation any more.
It's quantitative science.

00:57:17.801 --> 00:57:19.601
Rosario: The galaxy is a dynamic thing.

00:57:19.681 --> 00:57:21.761
It's a living organism, if you want.

00:57:21.841 --> 00:57:25.561
It is breathing. It is changing.
It is transforming.

00:57:30.081 --> 00:57:31.961
Collins: It's all
coming together in the end

00:57:32.041 --> 00:57:33.961
to tell us about how we got here

00:57:34.041 --> 00:57:36.361
and what our place
in the universe really is.