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The Milky Way, our home,

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formed not long after
the Big Bang.

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One of trillions in the
universe.

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This is our galaxy.

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♪♪

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Billions of planets orbiting
billions of stars.

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We are only just beginning
to understand its true place

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in the universe.

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It was only a hundred years ago,

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people thought our Milky Way
was the entire universe.

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♪♪

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If we really want to understand
where we come from

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and how the galaxy was formed,

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we can't just look in our cosmic
sort of backyard.

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We need to look much further
afield.

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And when we do,

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we discover a universe
in turmoil.

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Our history is made up
of multiple collisions

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and interactions
with our neighbors.

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♪♪

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Our Milky Way is not static.

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It is dynamic and it has

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such a rich, dynamic history.

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And our place in it

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far from secure.

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♪♪

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A collision can change
the structure of a galaxy,

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reorders the stars,
so you end up with something

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that looks different,
that behaves differently.

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Now we can see our galaxy's
future and its inevitable end.

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The Andromeda galaxy
is actually heading towards us

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at about 250,000 miles per hour.

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It will be a really nice sight,
actually.

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You know, just watch it coming.

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I mean, there's nothing
you can do about it

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except sit back
and enjoy the view.

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♪♪

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It's all coming together
to tell us about how we got here

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and what our place
in the universe really is.

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♪♪

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"The Milky Way,"
right now, on "NOVA."

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♪♪

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♪ See me when I float
like a dove ♪

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♪ The skies above are lined
with trees ♪

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♪ I'm on my knees,
begging please ♪

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♪ Come and take me away ♪

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♪♪

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Above us in the night sky,

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visible all around the world,

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the Milky Way wraps its arms
across the sky,

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a band of stars like no other.

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When the Milky Way
is up overhead,

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the skies are so brilliantly
bright that I swear

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the, the band of the Milky Way,
the disk of our own galaxy,

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quite literally casts a shadow.

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♪♪

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Our Milky Way is this really
incredibly beautiful place.

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It's this wonderful collection

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of beautiful stars, gas,
and dust that all kind of

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swirls together,

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almost like an abstract
painting.

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We've been trying

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to understand the band of stars
that stretches across

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the night sky since the time
of the ancient Greeks.

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♪♪

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Humans have been looking up

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at the night sky
since the dawn of time

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because we want to know
what's out there.

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Because the story of our galaxy

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is the story of every one of us.

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How does it all fit together?

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What are we part of?

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Can we understand it?

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♪♪

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The Milky Way galaxy
takes its name from the dense

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band of stars that we see
from Earth,

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when in fact, it's a structure
that entirely surrounds us.

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Every star in the sky is part of
it, including our sun.

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♪♪

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When looking into the night sky,

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you would see this band of stars
stretched across it,

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which actually corresponds
to the disk of the Milky Way.

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So we actually live
inside the Milky Way.

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♪♪

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Our galaxy is a spiral galaxy.

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And we can build up
this picture,

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which we have been doing for

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hundreds of years so far,
since the first astronomers,

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like Galileo, to, to kind of
build up this beautiful picture

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of our Milky Way.

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Right in the center,
you have a bulge.

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Then you have a pancake-like
structure.

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That's the disk,
and that's where we are.

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And then further out,
you have a faint halo of stars

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that goes quite far beyond
the disk.

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It's this beautiful
spiral structure

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of hundreds of billions of stars
all orbiting around

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a supermassive black hole right
at the center of the galaxy.

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♪♪

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The Milky Way's complex
structure has taken

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billions of years to evolve.

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And yet, it's one of the most
familiar forms in nature.

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So let's start
at the very center,

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and in the center, there is
a very old bulge,

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contains most of the old stars.

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And this is the remnants
of the first stars that formed

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in our part of the universe.

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Right at the very heart of it,

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there is a
supermassive black hole.

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That is the core of the
Milky Way as we know it.

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And then around that's
the bulge,

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and then there's this
big bar structure,

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mostly old stars,

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and that's what drives
the spiral arms.

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And so we can then say,
"Where are we in all of this?"

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We know pretty well where
the sun is.

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And hey presto, one sun,
and it'll be about there,

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roughly halfway from the center
to the outer

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spiral arm structures, and this
is where the sun lives today.

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The Milky Way's elegant spirals

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are the signature
of its dynamic history.

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The challenge is how to observe
it and tease out that history

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from our position on the inside.

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One of the problems of trying
to study the Milky Way

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from our position here on Earth
is that it's really hard

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to get a sense of what
the galaxy looks like overall.

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So, if we really want to
understand where we come from

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and how the galaxy was formed,
we can't just look in

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our cosmic sort of backyard.

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We need to look much further
afield.

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Clues to how the Milky Way

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formed and evolved emerged

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in the 1990s, with the launch of

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the most ambitious
space telescope at the time.

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Five, four, three, two, one.

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And lift-off of the
space shuttle Discovery

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with the Hubble Space Telescope.

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♪♪

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Our window on the universe.

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♪♪

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Standing by for SRB separation.

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Both solid rocket boosters
have separated.

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♪♪

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♪♪

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The Hubble Space Telescope
was one of the greatest feats

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in space missions
of human history.

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This 2.4-meter piece of glass,

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we've turned it on our universe

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and it has enabled
untold advances.

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♪♪

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Images from Hubble

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transformed astronomy...
Transformed science.

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♪♪

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Hubble isn't just focused on
the Milky Way.

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It also looks beyond,
much deeper into space.

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The data from Hubble
is unsurpassed.

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It gives us the sharpest views
of galaxies

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and the distant universe.

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Hubble's a little bit like
a time machine.

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It's able to pick up light
from galaxies

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that come from very far away.

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And because they've come from
very far away,

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we're looking at them
a completely different time,

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far back in time.

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♪♪

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To look far back in time,
Hubble trains its gaze

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on one tiny blank patch of sky

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for over 11 days.

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What appeared was pretty
incredible.

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We were able to see galaxies
in this ultra-deep field

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that is farther away
than we've ever, ever looked.

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♪♪

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So it's really given us an idea

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of how many galaxies
there are out there

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and the variety of galaxies
out there.

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♪♪

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It's a very hard number
to estimate,

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but it is absolutely
in the trillions.

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Their morphology
can be incredibly complex:

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big train wreck mergers

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or absolutely, stunningly,
beautifully round

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grand design spirals

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and everything in between.

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♪♪

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There are starburst galaxies

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that are generating new stars
at prodigious rates

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and there are small galaxies,

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which are my favorite.

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We call them dwarf galaxies.

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And they may be thousands
of times less massive

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than the Milky Way,

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but they're actually the most
common galaxy in the universe.

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♪♪

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Hubble tells us there are
trillions of galaxies

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in the universe.

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And by focusing on the ones
that are the farthest away,

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it looks deep back in time,

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giving us a picture
of what galaxies look like

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in their infancy.

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♪♪

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And they started forming in an
era of immense cosmic activity.

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Not long after the universe
began.

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Before the Milky Way forms,

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space is filled
with a vast structure

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known as the cosmic web.

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Hydrogen and helium gas collect
along the web's filaments.

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00:12:07,358 --> 00:12:13,721
But the web itself is made from
something more mysterious.

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It's called dark matter.

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00:12:18,655 --> 00:12:20,729
Dark matter is something
that has gravity

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but produces no light.

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It surrounds us.

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In fact, it dominates the mass
in our own galaxy.

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00:12:27,583 --> 00:12:29,340
And yet we don't know
what it is.

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We, we can't touch it,
we can't feel it.

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00:12:33,056 --> 00:12:35,368
Galaxies really need dark matter
because it's kind of like

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the glue that binds them all
together.

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You can almost say it's like
the seed of galaxy formation.

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It creates these huge structures
into which ordinary matter falls

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and then that matter
all gets compressed

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and can turn into stars.

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00:12:47,713 --> 00:12:50,135
And that really is, then,
what seeds galaxy formation

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00:12:50,177 --> 00:12:51,487
as a whole.

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The first stars are born
where the filaments cross

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00:12:59,489 --> 00:13:04,727
and dark matter is at
its densest,

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00:13:04,769 --> 00:13:07,585
drawing large amounts
of gas together

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00:13:07,618 --> 00:13:10,554
until it collapses under
its own gravity.

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Causing stars to ignite.

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00:13:22,275 --> 00:13:27,327
♪♪

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00:13:38,723 --> 00:13:44,605
New stars in their billions
are bound together by gravity,

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orbiting a common center.

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These are the first galaxies.

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00:13:54,788 --> 00:13:59,579
Among them, the Milky Way,
in its embryonic form.

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00:13:59,620 --> 00:14:04,378
♪♪

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00:14:04,420 --> 00:14:08,480
A whirling disk of gas and stars

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00:14:08,517 --> 00:14:12,063
surrounded by an invisible halo
of dark matter.

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00:14:17,702 --> 00:14:22,405
♪♪

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00:14:26,214 --> 00:14:28,222
Across the universe,

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00:14:28,262 --> 00:14:31,231
hundreds of billions of galaxies
are forming.

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00:14:34,375 --> 00:14:38,118
Some... a few dozen...
Are born very close

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00:14:38,151 --> 00:14:40,934
to our own Milky Way.

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00:14:40,967 --> 00:14:47,809
♪♪

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00:14:47,847 --> 00:14:54,177
Over time, gravity draws
these galaxies ever closer

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00:14:54,216 --> 00:14:58,014
to form what we know as
the local group.

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00:15:02,088 --> 00:15:06,945
Our local group is a set of
galaxies that lies in a volume

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00:15:06,984 --> 00:15:09,735
of the universe that we believe

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00:15:09,768 --> 00:15:12,868
is gravitationally bound
together.

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00:15:12,905 --> 00:15:17,281
Meaning that these galaxies are
close enough that at some point

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00:15:17,321 --> 00:15:20,192
they might all combine together
or collide together

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00:15:20,233 --> 00:15:24,904
to form one big, large galaxy.

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00:15:24,937 --> 00:15:26,913
The galaxies within
the local group can all feel

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00:15:26,953 --> 00:15:28,777
one another's gravity,

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so they're all sort of slowly
moving together with time.

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♪♪

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00:15:40,394 --> 00:15:44,552
♪♪

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00:15:44,586 --> 00:15:49,290
Just three billion years
after the Milky Way began,

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00:15:49,322 --> 00:15:55,750
it rises in the night sky
of its first planets,

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00:15:55,787 --> 00:16:00,644
but with only half the stars
and a more irregular structure

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00:16:00,684 --> 00:16:05,257
than the mature galaxy
we see today.

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00:16:05,291 --> 00:16:09,995
So how did our galaxy
get its spirals?

260
00:16:12,012 --> 00:16:18,123
To answer the question,
a new spacecraft is built.

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00:16:18,157 --> 00:16:21,420
Gaia will look directly
at the Milky Way itself.

262
00:16:24,461 --> 00:16:30,659
Its designers are determined
to overcome an age-old problem:

263
00:16:30,701 --> 00:16:34,411
how to measure the true distance
between stars.

264
00:16:34,445 --> 00:16:37,894
Being able to determine
the distance to objects

265
00:16:37,933 --> 00:16:41,033
is one of the most
fundamental things

266
00:16:41,069 --> 00:16:44,104
you need to do to understand
the structure of our universe.

267
00:16:44,141 --> 00:16:48,365
To measure the distances
accurately,

268
00:16:48,398 --> 00:16:51,498
Gaia's engineers must devise
an orbit for the craft

269
00:16:51,534 --> 00:16:56,172
big enough that it can measure
the same star from two points

270
00:16:56,207 --> 00:17:00,965
very far apart,
called a parallax measurement.

271
00:17:01,007 --> 00:17:03,790
Gaia will need to travel

272
00:17:03,823 --> 00:17:06,639
almost a million miles
from Earth.

273
00:17:06,672 --> 00:17:09,739
Attention pour la décompte
finale.

274
00:17:09,775 --> 00:17:12,526
Dix, neuf, huit,

275
00:17:12,559 --> 00:17:14,862
sept, six,

276
00:17:15,088 --> 00:17:18,220
cinq, quatre, trois,

277
00:17:18,256 --> 00:17:20,329
deux, un, top... décollage.

278
00:17:30,961 --> 00:17:33,148
I've been involved in Gaia since
the very beginning of it.

279
00:17:38,833 --> 00:17:41,681
It was a beautiful launch,
really spectacular.

280
00:17:44,593 --> 00:17:47,082
The spacecraft shares the name

281
00:17:47,121 --> 00:17:51,214
of the ancient Greek
Earth goddess, Gaia.

282
00:17:51,249 --> 00:17:54,666
It took four minutes.

283
00:17:54,706 --> 00:17:56,846
You could see the flame
of the rocket

284
00:17:56,882 --> 00:18:00,680
and you could see the individual
stages popping off.

285
00:18:00,722 --> 00:18:03,473
♪♪

286
00:18:03,507 --> 00:18:05,244
Then they got into this critical
state where they had to

287
00:18:05,267 --> 00:18:06,642
open up the sun shields.

288
00:18:06,675 --> 00:18:08,585
It was critical
that this opened up

289
00:18:08,626 --> 00:18:11,976
and protect the payload
from the sun.

290
00:18:12,019 --> 00:18:13,907
And that was the do-or-die
moment.

291
00:18:13,939 --> 00:18:18,610
♪♪

292
00:18:25,652 --> 00:18:29,036
Gaia's mission is to map
the true positions

293
00:18:29,076 --> 00:18:33,071
of a billion stars
in our Milky Way...

294
00:18:33,108 --> 00:18:38,860
Nearly all of them
for the first time.

295
00:18:38,900 --> 00:18:41,935
Before Gaia, we just looked at

296
00:18:41,973 --> 00:18:43,730
the images of our galaxy.

297
00:18:43,765 --> 00:18:46,319
We were missing half of
the information.

298
00:18:48,437 --> 00:18:49,813
Gaia is the first-ever

299
00:18:49,845 --> 00:18:51,472
precision distance
measuring machine

300
00:18:51,509 --> 00:18:53,518
that mankind has ever had.

301
00:18:53,557 --> 00:18:58,260
♪♪

302
00:18:58,294 --> 00:19:00,979
So how is it possible for Gaia
to map the Milky Way

303
00:19:01,014 --> 00:19:03,765
so accurately from within?

304
00:19:03,798 --> 00:19:06,963
♪♪

305
00:19:06,998 --> 00:19:10,741
First, it travels to its
distant vantage point

306
00:19:10,775 --> 00:19:16,079
called L2,
a gravitational sweet spot.

307
00:19:16,119 --> 00:19:19,284
It can hold here
with minimal fuel use

308
00:19:19,318 --> 00:19:25,910
as it follows the Earth in its
extensive orbit around the sun.

309
00:19:25,943 --> 00:19:27,936
Astronomy has always been
at the forefront of technology,

310
00:19:27,959 --> 00:19:29,728
but the kind of technology
we work with right now

311
00:19:29,751 --> 00:19:30,930
is absolutely amazing.

312
00:19:30,968 --> 00:19:34,962
♪♪

313
00:19:35,000 --> 00:19:39,441
With just a whisper of nitrogen
to help Gaia's telescopes

314
00:19:39,480 --> 00:19:43,059
sweep smoothly
through 360 degrees

315
00:19:43,096 --> 00:19:46,065
four times a day,

316
00:19:46,104 --> 00:19:49,683
it makes over one-and-a-half
million observations an hour.

317
00:19:49,721 --> 00:19:53,977
♪♪

318
00:19:54,009 --> 00:19:55,865
After four months,

319
00:19:55,897 --> 00:19:58,681
it has looked at the whole sky
at least once.

320
00:19:58,713 --> 00:20:03,286
♪♪

321
00:20:03,322 --> 00:20:06,105
Gaia gathers data
on the brightest stars

322
00:20:06,138 --> 00:20:10,928
across the whole sky...

323
00:20:10,970 --> 00:20:14,004
Stars within the disk
of the galaxy,

324
00:20:14,042 --> 00:20:17,010
from the center to the halo
and beyond.

325
00:20:17,050 --> 00:20:20,946
♪♪

326
00:20:20,986 --> 00:20:24,915
After it has traveled millions
of miles in its orbit,

327
00:20:24,954 --> 00:20:29,047
it observes the same stars
from a different vantage point.

328
00:20:29,083 --> 00:20:34,070
♪♪

329
00:20:34,107 --> 00:20:39,225
After nearly two years
of almost non-stop sky-scanning,

330
00:20:39,259 --> 00:20:42,609
scientists can triangulate
the true position

331
00:20:42,651 --> 00:20:45,948
of over a billion stars

332
00:20:45,979 --> 00:20:49,908
for the most accurate map
of the galaxy ever created.

333
00:20:50,140 --> 00:20:53,971
♪♪

334
00:20:54,012 --> 00:20:56,217
The Gaia map.

335
00:20:59,965 --> 00:21:01,875
The Gaia data has allowed us

336
00:21:01,917 --> 00:21:05,715
to see our own galaxy
like never before.

337
00:21:05,757 --> 00:21:07,645
I think that Gaia opened up

338
00:21:07,677 --> 00:21:11,573
a really new axis of information
to us

339
00:21:11,613 --> 00:21:14,298
that we just have never imagined
it would do.

340
00:21:14,333 --> 00:21:17,149
These are like having
completely, you know,

341
00:21:17,182 --> 00:21:19,125
revolutionary cartographers

342
00:21:19,166 --> 00:21:23,029
make an entirely new map
of our home galaxy.

343
00:21:28,190 --> 00:21:34,007
Finally, astronomers have
their Holy Grail:

344
00:21:34,046 --> 00:21:38,270
the Milky Way,
mapped in three dimensions.

345
00:21:41,279 --> 00:21:45,470
This is our first-ever honest
3D picture of the Milky Way.

346
00:21:45,503 --> 00:21:48,373
It's not a simulation
from a computer

347
00:21:48,416 --> 00:21:52,214
and it is not an, an attempt at
guessing the structure

348
00:21:52,256 --> 00:21:54,745
from approximate data.

349
00:21:54,784 --> 00:21:55,977
Every one of those stars
is individually measured

350
00:21:56,000 --> 00:21:57,659
to high precisions.

351
00:21:57,696 --> 00:22:00,730
So this means that we can
move ourselves around

352
00:22:00,768 --> 00:22:02,761
through this and see, well, what
does this bit of the Milky Way

353
00:22:02,784 --> 00:22:04,672
actually look like?

354
00:22:04,704 --> 00:22:06,442
And you decide you want to look
at it from far away,

355
00:22:06,465 --> 00:22:08,091
and you can do that.

356
00:22:08,129 --> 00:22:09,385
Or you can zoom in close
and say, "I want to know

357
00:22:09,408 --> 00:22:10,871
"how that star cluster works.

358
00:22:10,912 --> 00:22:12,855
I'll go and sit inside it."

359
00:22:12,897 --> 00:22:13,898
Gaia can tell the difference
between a star

360
00:22:13,921 --> 00:22:15,777
that's at the front of
that cluster

361
00:22:15,809 --> 00:22:17,949
and a star that's at the back
of that cluster,

362
00:22:17,985 --> 00:22:21,565
even though the cluster itself
is 5,000 light years away.

363
00:22:21,601 --> 00:22:23,213
Gaia is not only measuring where
things are

364
00:22:23,265 --> 00:22:25,819
to delightful precision,

365
00:22:25,858 --> 00:22:27,484
but equally,
you can see things moving.

366
00:22:27,522 --> 00:22:33,568
And it's actually the moving
that's the critical bit.

367
00:22:33,601 --> 00:22:37,825
In addition to mapping stars
in three-dimensional space,

368
00:22:37,858 --> 00:22:40,958
Gaia captured another dimension,

369
00:22:40,994 --> 00:22:45,502
the result of its repeated trips
around the sun:

370
00:22:45,538 --> 00:22:48,889
time.

371
00:22:48,931 --> 00:22:50,688
This data could help us
understand

372
00:22:50,723 --> 00:22:55,427
how our galaxy evolved.

373
00:22:55,459 --> 00:22:59,203
Gaia doesn't just tell us where
the stars are in the sky,

374
00:22:59,235 --> 00:23:03,873
but also how fast they're moving
across the sky and towards us,

375
00:23:03,908 --> 00:23:05,884
and that's an essential bit

376
00:23:05,924 --> 00:23:10,016
of information to understand how
things change over time.

377
00:23:10,052 --> 00:23:14,493
Once scientists know
how a star is moving,

378
00:23:14,532 --> 00:23:17,053
they can use Newtonian mechanics

379
00:23:17,093 --> 00:23:21,349
to calculate where it is going.

380
00:23:21,381 --> 00:23:25,157
And using the same calculations,
they can reverse the motion

381
00:23:25,189 --> 00:23:29,893
of the star to uncover
where it has been.

382
00:23:29,925 --> 00:23:34,367
This new data is revolutionizing
a field of science

383
00:23:34,405 --> 00:23:38,400
known as galactic archaeology.

384
00:23:38,437 --> 00:23:41,602
Galactic archaeology
is the process of identifying

385
00:23:41,638 --> 00:23:44,574
the history and the motion
of stars

386
00:23:44,614 --> 00:23:48,193
so you can figure out
where stars come from,

387
00:23:48,231 --> 00:23:53,502
how old they are, and how their
motions change over time.

388
00:23:53,542 --> 00:23:55,682
♪♪

389
00:23:55,718 --> 00:23:57,967
What's been really incredible
about Gaia is, if we couple it

390
00:23:57,990 --> 00:24:01,669
with spectra that we're
observing back on Earth,

391
00:24:01,703 --> 00:24:03,679
we're able to date the stars
and really use them

392
00:24:03,719 --> 00:24:05,444
as the fossils that they're
supposed to be.

393
00:24:05,479 --> 00:24:09,222
So this means we can work out
what the fossils tell us about

394
00:24:09,255 --> 00:24:11,249
the evolutionary events that
happened in the Milky Way's past

395
00:24:11,272 --> 00:24:12,735
and then date them.

396
00:24:12,776 --> 00:24:15,297
So put them in
chronological order.

397
00:24:15,336 --> 00:24:17,791
So we combine everything
together in order to get

398
00:24:17,832 --> 00:24:21,663
a really clear understanding
of how the Milky Way came to be.

399
00:24:21,704 --> 00:24:26,691
♪♪

400
00:24:26,729 --> 00:24:30,440
This new data from Gaia has
helped scientists spot a pattern

401
00:24:30,473 --> 00:24:32,547
between the Milky Way

402
00:24:32,585 --> 00:24:38,980
and our neighborhood cluster
of galaxies, the local group.

403
00:24:39,017 --> 00:24:41,234
The important thing to know
about our galactic neighbors

404
00:24:41,257 --> 00:24:42,802
in the local group
is that nothing's actually

405
00:24:42,825 --> 00:24:44,135
sitting still.

406
00:24:44,169 --> 00:24:46,624
Gravity means that we're all
moving towards or away

407
00:24:46,665 --> 00:24:47,625
from each other and we're

408
00:24:47,657 --> 00:24:50,473
sort of playing a dance
out there.

409
00:24:50,506 --> 00:24:53,638
♪♪

410
00:24:53,674 --> 00:24:55,235
Gravity is

411
00:24:55,274 --> 00:24:57,217
the great cosmic attractor.

412
00:24:57,258 --> 00:25:00,837
This dance of the galaxy
and its neighbors

413
00:25:00,874 --> 00:25:03,625
have been going on
for billions of years.

414
00:25:03,659 --> 00:25:05,864
♪♪

415
00:25:05,898 --> 00:25:08,616
Gaia is only just now revealing
the steps

416
00:25:08,651 --> 00:25:12,329
to this intricate
intergalactic dance.

417
00:25:12,363 --> 00:25:16,739
♪♪

418
00:25:16,779 --> 00:25:20,195
When the Gaia satellite started
producing its data,

419
00:25:20,236 --> 00:25:22,539
and astronomers started
analyzing this data,

420
00:25:22,571 --> 00:25:25,060
there was something rather
curious.

421
00:25:25,099 --> 00:25:27,206
A large sample of stars
were found that seemed to be

422
00:25:27,244 --> 00:25:29,612
rotating in the opposite
direction to the majority

423
00:25:29,644 --> 00:25:31,783
of stars in the Milky Way disk.

424
00:25:31,820 --> 00:25:33,828
And that's really unusual.

425
00:25:33,868 --> 00:25:36,553
And it was really surprising.

426
00:25:36,588 --> 00:25:38,531
So that means that not all
of the stars

427
00:25:38,573 --> 00:25:40,297
that make up our galaxy,
the Milky Way,

428
00:25:40,333 --> 00:25:42,406
were actually born here.

429
00:25:42,445 --> 00:25:45,293
They probably came from
a different galaxy altogether.

430
00:25:45,325 --> 00:25:47,286
So they're almost these alien
stars that have been brought in.

431
00:25:47,309 --> 00:25:51,336
Gaia's data led scientists

432
00:25:51,373 --> 00:25:55,814
to make an astonishing
discovery.

433
00:25:55,854 --> 00:25:57,895
So the most mind-blowing thing

434
00:25:57,934 --> 00:26:01,284
is that those stars
are the remnants

435
00:26:01,325 --> 00:26:03,693
of a humongous collision,

436
00:26:03,726 --> 00:26:06,116
and they actually come from
another galaxy.

437
00:26:06,158 --> 00:26:10,283
♪♪

438
00:26:17,902 --> 00:26:24,549
If we could travel back in time
ten billion years

439
00:26:24,591 --> 00:26:30,637
and land on one of the earliest
planets within our Milky Way,

440
00:26:30,672 --> 00:26:34,983
we'd see something spectacular
in the night sky.

441
00:26:35,024 --> 00:26:41,998
♪♪

442
00:26:42,032 --> 00:26:47,663
Billions of stars coming into
view heading towards us.

443
00:26:47,697 --> 00:26:52,619
♪♪

444
00:26:52,657 --> 00:26:55,506
The Milky Way is about
to collide

445
00:26:55,537 --> 00:26:58,353
with another galaxy from
our local group.

446
00:26:58,385 --> 00:27:01,866
♪♪

447
00:27:01,906 --> 00:27:04,689
Called Gaia-Enceladus.

448
00:27:07,889 --> 00:27:11,982
♪♪

449
00:27:15,634 --> 00:27:21,833
♪♪

450
00:27:24,146 --> 00:27:28,075
A quarter of the size
of our galaxy,

451
00:27:28,115 --> 00:27:33,746
Gaia-Enceladus is drawn
into the Milky Way,

452
00:27:33,779 --> 00:27:36,301
bringing disorder to its
flat disk.

453
00:27:39,763 --> 00:27:41,258
When you look at
a galaxy merger,

454
00:27:41,300 --> 00:27:43,308
it looks like an incredibly
violent process,

455
00:27:43,347 --> 00:27:45,487
but it's actually something
that's incredibly elegant,

456
00:27:45,523 --> 00:27:48,339
and that is because galaxies
are, ultimately,

457
00:27:48,372 --> 00:27:50,195
mostly empty space.

458
00:27:50,228 --> 00:27:53,709
And so when galaxies collide
or crash together,

459
00:27:53,749 --> 00:27:55,954
they pass through one another
like ghosts.

460
00:27:55,988 --> 00:27:59,732
The chance for a star-star
collision in a galaxy merger

461
00:27:59,765 --> 00:28:03,344
is actually exquisitely low.

462
00:28:03,381 --> 00:28:05,684
It's really quite
a beautiful process,

463
00:28:05,717 --> 00:28:09,264
because the way in which
the mutual gravity

464
00:28:09,301 --> 00:28:11,986
of these two galaxies actually
interact with one another

465
00:28:12,021 --> 00:28:15,022
causes one to start
sort of spiraling around.

466
00:28:15,062 --> 00:28:17,018
Once it, you know, plunges in,
it spirals around it,

467
00:28:17,046 --> 00:28:20,495
and then comes back and returns,
so it's kind of like, you know,

468
00:28:20,533 --> 00:28:22,706
two objects in a sort of
celestial ballet

469
00:28:22,742 --> 00:28:24,750
around one another.

470
00:28:24,790 --> 00:28:26,815
A collision can change the
structure of a galaxy,

471
00:28:26,838 --> 00:28:30,036
reorders the stars in the
galaxy, gives them new orbits,

472
00:28:30,071 --> 00:28:32,210
moves the gas
into different places.

473
00:28:32,247 --> 00:28:34,207
And so you end up with something
that looks different,

474
00:28:34,230 --> 00:28:34,995
that behaves differently.

475
00:28:35,031 --> 00:28:39,342
♪♪

476
00:28:39,382 --> 00:28:43,158
The invisible driver
of all these interactions

477
00:28:43,191 --> 00:28:48,462
is the same stuff that formed
the galaxies in the first place:

478
00:28:48,503 --> 00:28:51,123
dark matter.

479
00:28:51,159 --> 00:28:53,265
♪♪

480
00:28:53,304 --> 00:28:56,022
Because it accounts for most of
the gravity in the galaxy,

481
00:28:56,055 --> 00:28:57,966
it is dark matter that
determines

482
00:28:58,008 --> 00:29:00,463
how violent the collision is,
how rapidly

483
00:29:00,504 --> 00:29:04,050
and with what intensity galaxies
come together when they collide.

484
00:29:04,089 --> 00:29:05,977
In many ways,

485
00:29:06,009 --> 00:29:09,588
it determines how galaxies
end up after a collision.

486
00:29:09,625 --> 00:29:12,375
♪♪

487
00:29:18,617 --> 00:29:23,125
Just a few billion years after
the Milky Way formed,

488
00:29:23,161 --> 00:29:25,432
already much more massive than
Gaia-Enceladus...

489
00:29:28,922 --> 00:29:32,185
the Milky Way's gravity
overwhelms its neighbor.

490
00:29:35,674 --> 00:29:38,260
Absorbing it entirely.

491
00:29:38,298 --> 00:29:41,495
♪♪

492
00:29:49,915 --> 00:29:53,943
The Milky Way is bigger by
a billion stars.

493
00:29:53,979 --> 00:30:00,505
♪♪

494
00:30:00,539 --> 00:30:01,849
For the first time ever,

495
00:30:01,884 --> 00:30:04,187
we have seen how our Milky Way
has grown bigger.

496
00:30:07,708 --> 00:30:10,098
What we've learnt
from this collision

497
00:30:10,140 --> 00:30:13,010
is really about how much richer
our galaxy grew,

498
00:30:13,053 --> 00:30:15,639
but it doesn't actually tell us
about us yet.

499
00:30:18,141 --> 00:30:21,753
To find out how
our solar system got here,

500
00:30:21,789 --> 00:30:23,929
scientists have been
tracing the history

501
00:30:23,965 --> 00:30:27,611
of another unusual group
of stars.

502
00:30:27,645 --> 00:30:31,541
♪♪

503
00:30:31,581 --> 00:30:35,161
They loop around our
galactic disk

504
00:30:35,197 --> 00:30:41,047
in a spectacular trail called
the Sagittarius Stream.

505
00:30:41,086 --> 00:30:43,079
So the Sagittarius Stream
is really interesting

506
00:30:43,102 --> 00:30:44,925
because it might actually help
us understand

507
00:30:44,958 --> 00:30:47,828
where we came from.

508
00:30:47,871 --> 00:30:50,490
It is what's known as a
tidal stream,

509
00:30:50,526 --> 00:30:51,738
which is a stream of stars

510
00:30:51,774 --> 00:30:54,263
that have been stretched
across the night sky

511
00:30:54,302 --> 00:30:57,205
due to the gravity of the
Milky Way.

512
00:30:57,247 --> 00:31:00,030
The Sagittarius Stream
is so big,

513
00:31:00,063 --> 00:31:03,064
it goes all the way up and even
all the way down,

514
00:31:03,103 --> 00:31:06,748
so we can just carry
the Milky Way from its handle.

515
00:31:06,783 --> 00:31:10,526
It's really, really large
stream.

516
00:31:10,560 --> 00:31:13,408
The trail of stars we see today

517
00:31:13,440 --> 00:31:17,151
is named after the galaxy that
they used to belong to,

518
00:31:17,183 --> 00:31:21,243
Sagittarius Dwarf.

519
00:31:21,280 --> 00:31:24,128
The Sagittarius galaxy

520
00:31:24,160 --> 00:31:27,227
was discovered by a student
and myself in the '90s.

521
00:31:27,265 --> 00:31:30,048
Most of the Sagittarius galaxy
is actually spread out

522
00:31:30,080 --> 00:31:32,056
into two streams, one in the
front and out the back,

523
00:31:32,096 --> 00:31:34,912
like giant comet tails wrapping
around the entire sky,

524
00:31:34,945 --> 00:31:37,564
going out for maybe
100,000 light years away.

525
00:31:37,601 --> 00:31:39,642
♪♪

526
00:31:39,681 --> 00:31:41,591
We could see these,

527
00:31:41,634 --> 00:31:43,744
but it wasn't possible to
understand how they got there.

528
00:31:46,178 --> 00:31:49,343
Now, with Gaia,
we have motions of these stars,

529
00:31:49,377 --> 00:31:51,932
so we can see what direction
they're moving in,

530
00:31:51,970 --> 00:31:53,965
which ones are going fast,
which ones are going slow.

531
00:31:55,874 --> 00:31:57,850
For the first time ever,

532
00:31:57,890 --> 00:32:00,412
it's been possible to say,
"Ah, this is what happened!"

533
00:32:03,330 --> 00:32:05,666
The Sagittarius Stream is
essentially the tidal debris

534
00:32:05,698 --> 00:32:09,311
that has been left over when
a dwarf galaxy,

535
00:32:09,347 --> 00:32:11,769
the Sagittarius Dwarf, actually
plunged into the Milky Way.

536
00:32:16,867 --> 00:32:19,716
By studying the stream of stars,

537
00:32:19,747 --> 00:32:22,596
scientists have uncovered
the story

538
00:32:22,627 --> 00:32:25,214
of a much more recent
galactic collision.

539
00:32:29,636 --> 00:32:32,321
This time,
with a much smaller galaxy.

540
00:32:36,484 --> 00:32:40,708
When the Sagittarius galaxy
orbited into the Milky Way,

541
00:32:40,741 --> 00:32:42,749
it came, foolishly,
rather far in.

542
00:32:42,789 --> 00:32:49,730
♪♪

543
00:32:49,765 --> 00:32:53,344
As it dives towards the
Milky Way,

544
00:32:53,382 --> 00:32:56,383
the dwarf galaxy begins
to have its stars pulled off.

545
00:33:00,934 --> 00:33:02,095
When it goes through the disk,

546
00:33:02,118 --> 00:33:04,705
it punches a hole in the disk,
and the stars get

547
00:33:04,742 --> 00:33:07,875
put in particular patterns.

548
00:33:07,910 --> 00:33:09,840
And it's got stretched into
these two great long streams.

549
00:33:09,863 --> 00:33:15,615
♪♪

550
00:33:15,655 --> 00:33:19,933
The much smaller galaxy
encroaches upon the Milky Way,

551
00:33:19,975 --> 00:33:22,463
just like Gaia-Enceladus did,

552
00:33:22,503 --> 00:33:25,351
but the timing is intriguing,

553
00:33:25,383 --> 00:33:29,727
because this collision happens
just before

554
00:33:29,768 --> 00:33:33,696
the birth of
our own solar system.

555
00:33:33,736 --> 00:33:36,835
One of the most important
consequences of galaxy mergers,

556
00:33:36,872 --> 00:33:38,641
like the destruction of the
Sagittarius Dwarf galaxy

557
00:33:38,664 --> 00:33:40,422
by the Milky Way,

558
00:33:40,456 --> 00:33:45,062
is a new, fresh injection
of gas into the galaxy, right?

559
00:33:45,096 --> 00:33:47,999
And it is gas,
particularly cold gas,

560
00:33:48,041 --> 00:33:50,944
that is the fuel from which
all stars are born.

561
00:33:50,985 --> 00:33:52,896
For star formation to occur,

562
00:33:52,937 --> 00:33:54,726
basically,
the colder, the better.

563
00:33:54,761 --> 00:33:56,802
♪♪

564
00:34:05,482 --> 00:34:08,996
The most important gas
that the collisions bring

565
00:34:09,033 --> 00:34:11,423
is made of one of the oldest

566
00:34:11,466 --> 00:34:14,401
and most ubiquitous elements
in the universe.

567
00:34:25,739 --> 00:34:30,530
So what I'm listening to here
is the lifeblood of our galaxy,

568
00:34:30,571 --> 00:34:33,125
hydrogen.

569
00:34:33,163 --> 00:34:35,074
We can detect it with
our radio telescopes,

570
00:34:35,115 --> 00:34:37,385
like in this case,
pointing right at the Milky Way.

571
00:34:37,420 --> 00:34:40,683
Hydrogen is the most common
element in the universe,

572
00:34:40,715 --> 00:34:44,033
and it's in our own galaxy.

573
00:34:44,076 --> 00:34:46,052
We don't see gas with our eyes,

574
00:34:46,092 --> 00:34:48,427
and therefore we are
not used to the idea

575
00:34:48,460 --> 00:34:51,527
of there being plenty of gas
in the Milky Way.

576
00:34:51,564 --> 00:34:54,348
But if you use a radio
telescope, you can see it.

577
00:34:54,380 --> 00:34:56,246
You can look at the radiation
coming from that gas,

578
00:34:56,269 --> 00:34:58,125
and that's exactly what
we're doing right now.

579
00:34:58,156 --> 00:35:01,736
This gas is connected
to stars deeply...

580
00:35:01,773 --> 00:35:03,847
It's what stars form from.

581
00:35:03,885 --> 00:35:07,464
If this gas wasn't there,
stars would never have formed.

582
00:35:26,318 --> 00:35:30,989
Hydrogen was created shortly
after the birth of the universe,

583
00:35:31,023 --> 00:35:33,511
and it has always been spread
throughout the Milky Way.

584
00:35:35,567 --> 00:35:38,732
But not evenly.

585
00:35:38,767 --> 00:35:42,085
It clumps together in
dense clouds

586
00:35:42,127 --> 00:35:44,267
that, in this iconic image,

587
00:35:44,303 --> 00:35:46,857
extend up to 30 trillion miles.

588
00:35:49,168 --> 00:35:52,911
Scientists call them
stellar nurseries,

589
00:35:52,944 --> 00:35:57,614
where temperatures are
low enough for gas to condense.

590
00:35:57,648 --> 00:35:59,591
Stellar nurseries

591
00:35:59,632 --> 00:36:02,448
are some
of the largest, coldest,

592
00:36:02,480 --> 00:36:06,278
and certainly among the darkest
regions within any galaxy.

593
00:36:06,320 --> 00:36:10,031
♪♪

594
00:36:10,065 --> 00:36:13,164
If you were to fly through
a stellar nursery,

595
00:36:13,201 --> 00:36:17,937
it would be extremely cold,
and it's an extremely

596
00:36:17,969 --> 00:36:22,705
turbulent and chaotic place,
pervaded by magnetic fields,

597
00:36:22,737 --> 00:36:25,640
and, and charged particles
streaming throughout.

598
00:36:25,681 --> 00:36:28,203
♪♪

599
00:36:28,241 --> 00:36:30,697
It might be glowing
a little bit,

600
00:36:30,738 --> 00:36:33,357
and as you approach
closer and closer,

601
00:36:33,395 --> 00:36:35,817
you would realize that
it's actually heating up a bit.

602
00:36:35,858 --> 00:36:38,226
It's, it's actually becoming
warmer.

603
00:36:38,258 --> 00:36:41,642
You would perhaps surmise that
this is where

604
00:36:41,682 --> 00:36:43,690
a new group of stars is
being born.

605
00:36:43,731 --> 00:36:50,540
♪♪

606
00:36:50,579 --> 00:36:52,106
Hydrogen can be thought of

607
00:36:52,147 --> 00:36:53,642
as the lifeblood of galaxies,

608
00:36:53,683 --> 00:36:55,659
because it's the first building
block of stars.

609
00:36:55,699 --> 00:36:59,279
In the center of a star,
it's fusing hydrogen together

610
00:36:59,316 --> 00:37:01,040
all the time to produce helium.

611
00:37:01,076 --> 00:37:03,019
And that gives off energy,
which allows the stars

612
00:37:03,059 --> 00:37:04,970
to light up.

613
00:37:05,012 --> 00:37:07,534
♪♪

614
00:37:15,796 --> 00:37:18,481
When the Sagittarius Dwarf
galaxy

615
00:37:18,517 --> 00:37:20,242
collides with our Milky Way,

616
00:37:20,277 --> 00:37:24,751
it brings more hydrogen
to these clouds,

617
00:37:24,789 --> 00:37:28,019
triggering a new era
of star birth.

618
00:37:28,053 --> 00:37:30,957
♪♪

619
00:37:30,998 --> 00:37:33,846
When galaxies interact with
one another

620
00:37:33,878 --> 00:37:35,701
and they collide with one
another,

621
00:37:35,734 --> 00:37:39,662
what typically happens is that
you actually get

622
00:37:39,702 --> 00:37:43,248
a big burst of star formation
occurring.

623
00:37:43,286 --> 00:37:45,075
And that's primarily because

624
00:37:45,110 --> 00:37:48,046
you are essentially bringing
in a new source

625
00:37:48,086 --> 00:37:51,917
of star-forming fuel into the
Milky Way.

626
00:37:51,959 --> 00:37:57,841
♪♪

627
00:37:57,879 --> 00:38:02,703
This era coincides with
the birth of our own sun,

628
00:38:02,743 --> 00:38:05,427
4.6 billion years ago.

629
00:38:05,463 --> 00:38:09,326
♪♪

630
00:38:09,367 --> 00:38:12,435
The jury's still out,
but we think that the sun

631
00:38:12,472 --> 00:38:15,059
could have formed in that first
enhancement in star formation.

632
00:38:15,096 --> 00:38:18,774
♪♪

633
00:38:18,808 --> 00:38:21,656
The timing of the collision
between the Milky Way galaxy

634
00:38:21,688 --> 00:38:24,656
and the Sagittarius Dwarf galaxy

635
00:38:24,697 --> 00:38:27,567
coincides with a peak in
star formation

636
00:38:27,608 --> 00:38:29,136
that we see happen in our
Milky Way.

637
00:38:29,177 --> 00:38:31,032
♪♪

638
00:38:31,065 --> 00:38:33,914
And we know that
the age of the gas

639
00:38:33,945 --> 00:38:36,150
in which our solar system
was formed

640
00:38:36,185 --> 00:38:41,008
lies very close to this spike
in star formation.

641
00:38:41,049 --> 00:38:43,123
♪♪

642
00:38:43,161 --> 00:38:45,399
It is certainly possible, right?

643
00:38:45,433 --> 00:38:46,928
That our own solar system

644
00:38:46,970 --> 00:38:49,622
is anchored around a star

645
00:38:49,658 --> 00:38:53,401
that was born from gas that
did not originate

646
00:38:53,434 --> 00:38:57,592
in our home galaxy...
It was taken, it was pulled,

647
00:38:57,626 --> 00:39:00,856
or consumed by the Milky Way

648
00:39:00,890 --> 00:39:02,964
when it ripped apart
a satellite galaxy,

649
00:39:03,003 --> 00:39:05,273
maybe even
the Sagittarius Dwarf.

650
00:39:09,179 --> 00:39:14,843
For a small galaxy, Sagittarius
Dwarf has had a big impact,

651
00:39:14,876 --> 00:39:17,462
and not just by triggering
star birth.

652
00:39:17,500 --> 00:39:21,462
It plunges back and forth
through the Milky Way

653
00:39:21,500 --> 00:39:24,349
as the galaxies become enmeshed,

654
00:39:24,380 --> 00:39:28,659
which likely contributed to the
formation of the spiral arms.

655
00:39:28,700 --> 00:39:32,858
But its influence is
fast fading.

656
00:39:32,892 --> 00:39:35,045
The question as to whether the
Sagittarius Dwarf galaxy

657
00:39:35,068 --> 00:39:38,550
is still around
kind of depends on

658
00:39:38,588 --> 00:39:41,273
what you kind of end up thinking
of as being a galaxy,

659
00:39:41,309 --> 00:39:43,220
after a certain point.

660
00:39:43,261 --> 00:39:45,204
It is really a galaxy that is

661
00:39:45,245 --> 00:39:48,443
in the process of being
totally disrupted.

662
00:39:48,477 --> 00:39:49,820
And one day it will end up
merging

663
00:39:49,853 --> 00:39:52,723
with the center of our galaxy.

664
00:39:52,766 --> 00:39:55,003
So in some sense, it's only the

665
00:39:55,038 --> 00:39:56,839
sort of memory of the galaxy
that is left behind.

666
00:39:56,862 --> 00:40:01,467
♪♪

667
00:40:01,502 --> 00:40:03,674
When we look up
at the night sky,

668
00:40:03,710 --> 00:40:07,191
it's easy to think of
the Milky Way as static.

669
00:40:07,231 --> 00:40:12,218
♪♪

670
00:40:12,254 --> 00:40:16,893
But we now know it's evolved
through a turbulent history

671
00:40:16,927 --> 00:40:21,598
of collisions and mergers.

672
00:40:21,632 --> 00:40:25,277
I think that Gaia opened up this
whole new vision for us.

673
00:40:25,312 --> 00:40:28,280
Our Milky Way is not static.

674
00:40:28,319 --> 00:40:33,077
It is dynamic and it has such
a rich, dynamic history.

675
00:40:33,120 --> 00:40:36,088
But none of it is random.

676
00:40:36,128 --> 00:40:39,391
♪♪

677
00:40:39,424 --> 00:40:42,774
The force that causes galaxies
to form,

678
00:40:42,816 --> 00:40:47,771
merge,

679
00:40:47,809 --> 00:40:51,552
and evolve

680
00:40:51,585 --> 00:40:55,481
is gravity.

681
00:40:55,521 --> 00:40:57,016
The thing that ultimately
sculpts

682
00:40:57,057 --> 00:40:58,846
how those galaxies look
is gravity.

683
00:40:58,881 --> 00:41:02,231
♪♪

684
00:41:02,273 --> 00:41:04,063
It's not the collisions.

685
00:41:04,097 --> 00:41:09,663
It's the stars within those
galaxies tugging on one another.

686
00:41:09,698 --> 00:41:11,935
And it's the underlying
dark matter halos

687
00:41:11,970 --> 00:41:14,360
of those galaxies
merging together.

688
00:41:14,402 --> 00:41:17,403
♪♪

689
00:41:24,098 --> 00:41:26,914
So we're actually at a really
exciting time now in astronomy.

690
00:41:26,947 --> 00:41:28,857
Because we can tell the story

691
00:41:28,899 --> 00:41:30,842
not only of how our galaxy
came to be,

692
00:41:30,883 --> 00:41:32,772
and how everything led up
to now,

693
00:41:32,803 --> 00:41:34,593
but we can also start to peer
into the future

694
00:41:34,627 --> 00:41:36,417
and see what's in store,
what's yet to come

695
00:41:36,452 --> 00:41:37,762
for the evolution of our galaxy.

696
00:41:37,795 --> 00:41:42,074
♪♪

697
00:41:52,164 --> 00:41:54,784
The more we learn about
the Milky Way

698
00:41:54,820 --> 00:41:58,978
and its dynamic history,

699
00:41:59,012 --> 00:42:03,869
the more incredible
it seems that we ourselves,

700
00:42:03,909 --> 00:42:07,325
orbiting just one star
among billions,

701
00:42:07,365 --> 00:42:13,728
have been able to figure out
our galaxy's story,

702
00:42:13,765 --> 00:42:15,108
written in the stars.

703
00:42:15,141 --> 00:42:17,957
♪♪

704
00:42:17,990 --> 00:42:24,004
And we are now poised
to map out its ultimate fate.

705
00:42:28,647 --> 00:42:32,445
The Milky Way is no stranger
to galactic collisions.

706
00:42:32,487 --> 00:42:34,560
As we look around the night sky,

707
00:42:34,599 --> 00:42:36,967
we see evidence that our
Milky Way galaxy

708
00:42:36,999 --> 00:42:39,237
has had these interactions
with galaxies before,

709
00:42:39,271 --> 00:42:40,963
but what's coming next
is something

710
00:42:40,999 --> 00:42:42,788
on an entirely different scale.

711
00:42:42,823 --> 00:42:46,402
This faint smudge of light
that you see right there

712
00:42:46,440 --> 00:42:48,350
in the center of the image,

713
00:42:48,391 --> 00:42:49,985
it's not some condensation
on the lens

714
00:42:50,216 --> 00:42:52,355
or a cloud in the sky above us.

715
00:42:52,392 --> 00:42:56,320
This is an entire other galaxy,
a huge galaxy,

716
00:42:56,360 --> 00:42:59,295
two-and-a-half million
light years away from us.

717
00:42:59,337 --> 00:43:00,864
To put that into units

718
00:43:00,904 --> 00:43:02,366
that humans can try to
understand,

719
00:43:02,409 --> 00:43:04,995
this faint smudge of light
is about

720
00:43:05,033 --> 00:43:06,943
15 billion billion miles away.

721
00:43:06,984 --> 00:43:09,407
♪♪

722
00:43:11,112 --> 00:43:14,496
This galaxy is called Andromeda.

723
00:43:14,537 --> 00:43:17,288
♪♪

724
00:43:17,322 --> 00:43:20,967
And is set to play a defining
role in our galaxy's future.

725
00:43:24,170 --> 00:43:26,178
The Hubble Space Telescope

726
00:43:26,218 --> 00:43:29,351
has taken extraordinary images
of Andromeda.

727
00:43:29,578 --> 00:43:31,946
♪♪

728
00:43:31,978 --> 00:43:34,697
Compared to the disk of the
Milky Way,

729
00:43:34,730 --> 00:43:37,960
Andromeda seems tiny,

730
00:43:37,994 --> 00:43:41,378
when in fact, it's anything but.

731
00:43:41,419 --> 00:43:48,490
♪♪

732
00:43:48,523 --> 00:43:51,371
It's our largest neighbor
in the local group.

733
00:43:55,980 --> 00:44:01,382
With the same spiral structure
and the same long history

734
00:44:01,420 --> 00:44:04,421
of feeding on smaller galaxies.

735
00:44:04,460 --> 00:44:10,212
♪♪

736
00:44:10,252 --> 00:44:12,085
This image right here
is actually ridiculous

737
00:44:12,108 --> 00:44:14,051
when you think about it.

738
00:44:14,092 --> 00:44:16,777
It's an observation of part
of the Andromeda galaxy

739
00:44:16,812 --> 00:44:18,756
taken with the
Hubble Space Telescope,

740
00:44:18,797 --> 00:44:21,962
and the level of detail here
is incredible.

741
00:44:21,997 --> 00:44:26,406
This image contains about
100 million stars

742
00:44:26,446 --> 00:44:28,748
that we can see
in another galaxy.

743
00:44:28,781 --> 00:44:30,091
It's just mind-blowing.

744
00:44:30,126 --> 00:44:32,680
When we look at it,

745
00:44:32,717 --> 00:44:35,336
we start to be able to
understand its structure.

746
00:44:35,374 --> 00:44:39,882
And what strikes me immediately
is that it's kind of familiar.

747
00:44:39,918 --> 00:44:41,959
If you zoom in on the
spiral arm,

748
00:44:41,998 --> 00:44:43,191
it's exactly the same as what
we see

749
00:44:43,214 --> 00:44:44,143
when we look into our own
Milky Way.

750
00:44:44,174 --> 00:44:47,492
And when we look at the
Andromeda galaxy,

751
00:44:47,534 --> 00:44:50,503
we see this history, we see that

752
00:44:50,543 --> 00:44:52,649
it's been cannibalizing these
satellite galaxies

753
00:44:52,686 --> 00:44:54,695
in a similar way to the
Milky Way,

754
00:44:54,735 --> 00:44:56,624
growing into this beast,
this giant

755
00:44:56,655 --> 00:44:58,893
that's a match
for our own galaxy.

756
00:44:58,927 --> 00:45:04,144
♪♪

757
00:45:04,175 --> 00:45:07,079
We now have many beautiful
images of Andromeda.

758
00:45:07,120 --> 00:45:09,871
We've studied it with a huge
range of telescopes,

759
00:45:09,904 --> 00:45:11,448
and in many ways, you know,
it's a lot like the Milky Way,

760
00:45:11,471 --> 00:45:13,414
this beautiful spiral galaxy.

761
00:45:13,456 --> 00:45:16,490
So you might think that they're,
you know, going to be

762
00:45:16,528 --> 00:45:19,398
very similar galaxies
with a very similar history.

763
00:45:19,440 --> 00:45:22,191
But what we've learnt through
studying Andromeda over time

764
00:45:22,225 --> 00:45:23,866
is that actually,
they're not quite the same.

765
00:45:23,889 --> 00:45:27,272
♪♪

766
00:45:27,313 --> 00:45:32,878
In fact, Andromeda is 50% bigger
than the Milky Way.

767
00:45:32,913 --> 00:45:36,875
And that's not all.

768
00:45:36,913 --> 00:45:39,914
The Andromeda galaxy
is actually heading towards us

769
00:45:40,145 --> 00:45:45,133
at about 250,000 miles per hour.

770
00:45:45,170 --> 00:45:48,106
In about four-and-a-half billion
years' time,

771
00:45:48,146 --> 00:45:50,384
that faint smudge of light
we saw in the sky

772
00:45:50,418 --> 00:45:52,688
will collide with
the Milky Way galaxy,

773
00:45:52,722 --> 00:45:55,057
changing our galaxy forever.

774
00:45:55,090 --> 00:46:00,492
♪♪

775
00:46:07,251 --> 00:46:09,489
The Milky Way as we know it
today

776
00:46:09,523 --> 00:46:14,762
is not eternal.

777
00:46:14,803 --> 00:46:19,212
And Earth will witness
the final act.

778
00:46:22,740 --> 00:46:28,491
♪♪

779
00:46:28,532 --> 00:46:34,229
Two galaxies in a single sky,

780
00:46:34,260 --> 00:46:40,045
gradually, but inevitably,
merging into one.

781
00:46:40,085 --> 00:46:43,021
There is absolute evidence
that Andromeda

782
00:46:43,061 --> 00:46:46,892
is going to collide with the
Milky Way one day,

783
00:46:46,933 --> 00:46:49,651
because they are pulling each
other closer

784
00:46:49,686 --> 00:46:51,475
and closer over time,
and one day,

785
00:46:51,510 --> 00:46:53,311
they're just gonna come so close
that they will collide.

786
00:46:53,334 --> 00:46:55,244
♪♪

787
00:46:55,286 --> 00:46:56,945
Andromeda and the Milky Way,

788
00:46:56,981 --> 00:46:59,470
when they come together,
sparks fly.

789
00:46:59,510 --> 00:47:02,959
It's going to be
an incredible time.

790
00:47:02,998 --> 00:47:05,999
If we were able to view
this collision happening,

791
00:47:06,038 --> 00:47:08,657
it would be amazing to watch
the night sky change over time.

792
00:47:08,695 --> 00:47:11,696
♪♪

793
00:47:11,734 --> 00:47:13,279
It'd be a really nice sight,
actually.

794
00:47:13,302 --> 00:47:15,442
You know, just watch it coming.

795
00:47:15,479 --> 00:47:16,341
I mean, there's nothing
you can do about it

796
00:47:16,375 --> 00:47:17,408
except sit back
and enjoy the view.

797
00:47:17,431 --> 00:47:22,386
♪♪

798
00:47:22,423 --> 00:47:25,239
We'll end up smashing these
two galaxies together.

799
00:47:25,272 --> 00:47:29,102
♪♪

800
00:47:29,144 --> 00:47:32,014
There may be a huge burst
of star formation initially,

801
00:47:32,056 --> 00:47:34,577
which will sort of light up the
night sky with fireworks.

802
00:47:34,616 --> 00:47:38,709
♪♪

803
00:47:38,744 --> 00:47:40,534
And then over time,
that will sort of

804
00:47:40,568 --> 00:47:43,090
burn off all the remaining gas
we have in those two galaxies.

805
00:47:43,128 --> 00:47:46,904
♪♪

806
00:47:53,785 --> 00:47:56,786
But unlike in previous
collisions,

807
00:47:56,825 --> 00:48:00,754
this time, our galaxy is the
smaller of the two.

808
00:48:00,794 --> 00:48:05,399
♪♪

809
00:48:11,514 --> 00:48:13,882
Andromeda and the Milky Way

810
00:48:13,914 --> 00:48:16,785
pull at each other's
spiral arms...

811
00:48:24,987 --> 00:48:26,963
Scattering stars...

812
00:48:29,851 --> 00:48:35,220
Until no trace of the original
structures remain.

813
00:48:35,259 --> 00:48:40,246
♪♪

814
00:48:40,284 --> 00:48:44,114
Two spiral galaxies,

815
00:48:44,156 --> 00:48:47,769
merged into one colossal mass
of stars.

816
00:48:54,237 --> 00:48:56,245
Watching the motion of galaxies

817
00:48:56,284 --> 00:48:59,417
is like looking at a really,
really exquisite ballet

818
00:48:59,453 --> 00:49:01,908
in really, really slow motion.

819
00:49:01,949 --> 00:49:04,950
When that dance
is finally complete,

820
00:49:04,989 --> 00:49:07,990
the structure of the Milky Way
will be forever altered.

821
00:49:08,029 --> 00:49:10,452
While this collision will
extinguish the Milky Way

822
00:49:10,493 --> 00:49:11,771
and Andromeda as we know them,

823
00:49:11,805 --> 00:49:14,589
it will also create
a whole host of new stars,

824
00:49:14,621 --> 00:49:17,143
and around those new stars,
there'll be new planets,

825
00:49:17,182 --> 00:49:19,288
and maybe another generation
of people

826
00:49:19,326 --> 00:49:20,967
asking the same questions
that we're asking now.

827
00:49:20,990 --> 00:49:22,649
Where have they come from?

828
00:49:22,878 --> 00:49:24,221
What's their place in the
galaxy?

829
00:49:24,254 --> 00:49:27,103
And what's going to happen
in their future?

830
00:49:27,135 --> 00:49:29,032
We will not be able to see the
beautiful galaxy

831
00:49:29,055 --> 00:49:31,773
that we see right now,
but the universe will carry on.

832
00:49:31,806 --> 00:49:37,045
♪♪

833
00:49:41,823 --> 00:49:46,080
As we look even deeper
into the future,

834
00:49:46,111 --> 00:49:49,080
all of the galaxies in our
local group

835
00:49:49,120 --> 00:49:53,628
will eventually merge into one
enormous entity.

836
00:49:57,440 --> 00:49:59,863
Floating in isolation.

837
00:49:59,904 --> 00:50:04,346
♪♪

838
00:50:04,385 --> 00:50:07,615
As the universe expands,

839
00:50:07,649 --> 00:50:13,247
the distance between all the
galactic groups will increase

840
00:50:13,281 --> 00:50:17,723
and the other galaxies will
simply disappear from view.

841
00:50:17,761 --> 00:50:23,098
♪♪

842
00:50:23,137 --> 00:50:25,527
Knowing that we can sort of
look into the future

843
00:50:25,569 --> 00:50:27,545
many billions of years
and understand

844
00:50:27,585 --> 00:50:29,387
what will happen to our galaxies
is mind-blowing.

845
00:50:29,410 --> 00:50:31,516
♪♪

846
00:50:31,554 --> 00:50:34,970
And all of this
we have determined

847
00:50:35,010 --> 00:50:39,266
by looking up at the skies from
one tiny,

848
00:50:39,299 --> 00:50:44,188
unremarkable outpost in the
Milky Way.

849
00:50:44,227 --> 00:50:48,538
Even though we as humans have
such an insignificant role

850
00:50:48,579 --> 00:50:50,719
in the grand scheme of things,

851
00:50:50,755 --> 00:50:53,473
there is so much about the
vastness of space

852
00:50:53,507 --> 00:50:55,996
that we can understand just
from our

853
00:50:56,035 --> 00:50:58,076
unique perspective here on
Earth.

854
00:50:58,115 --> 00:51:00,833
♪♪

855
00:51:00,868 --> 00:51:03,040
Earth is a tiny little rock

856
00:51:03,076 --> 00:51:06,972
in a really indescribably vast
cosmic ocean, right?

857
00:51:07,012 --> 00:51:11,748
We are just a tiny little planet
spinning in the void.

858
00:51:11,781 --> 00:51:16,484
But the story of our night sky
is far from being complete.

859
00:51:16,517 --> 00:51:22,082
And there is so much more
to discover.

860
00:51:22,117 --> 00:51:24,900
Is there life in the universe?

861
00:51:24,933 --> 00:51:27,301
And has there been life in the
universe

862
00:51:27,333 --> 00:51:28,032
from the very beginning?

863
00:51:28,069 --> 00:51:31,037
♪♪

864
00:51:31,078 --> 00:51:33,021
What is dark matter?

865
00:51:33,062 --> 00:51:34,021
What is dark energy?

866
00:51:34,054 --> 00:51:36,543
How does it affect our universe?

867
00:51:36,582 --> 00:51:39,266
Particularly, how does it affect
our Milky Way

868
00:51:39,302 --> 00:51:40,547
and even our own solar system?

869
00:51:40,582 --> 00:51:43,649
♪♪

870
00:51:43,686 --> 00:51:47,102
We want to know where
we come from.

871
00:51:47,142 --> 00:51:50,940
We want to understand
our origins and our destiny.

872
00:51:50,983 --> 00:51:53,667
And also, we just love
a good story.

873
00:51:53,703 --> 00:51:55,078
We love mystery.

874
00:51:55,111 --> 00:51:57,054
And the story of the universe is

875
00:51:57,095 --> 00:51:59,584
the greatest story of all.