All language subtitles for Inside The Milky Way NG 1080p_Subtitles01.ENG

NARRATOR: The Milky Way galaxy...

a vast cosmic city
of 200 billion stars.

We live in a quiet neighborhood,

tucked away in a safe neck
of the woods.

But what if we could take our planet
on a journey across the galaxy?

From the violent graveyards
where stars, billions of years old,

go to die...

to the cosmic cradles
where new stars burst to life.

Dare to travel through
billions of years of space and time

to find out how our galaxy
came to be...

and the dark fate that awaits us.

It's the ultimate journey
to uncover the secrets that lie...

inside the Milky Way.

Modern cities are a testament

to some of the greatest accomplishments
of human civilization--

feats of engineering that dazzle
with millions of lights.

But the bright lights conceal
something even more amazing.

Turn them off and behold...
a great city in the sky.

JAMES BULLOCK: What is this?
Well, this is the Milky Way.

This is our galaxy.

Well, if you'd like, you could think
of the galaxy as a city of stars.

NARRATOR: Our sun is just one
of the 200 billion stars

that make up a vast cosmic city.

A city we're just beginning to know.

BULLOCK: It's really a wonderful time
to be an astronomer,

especially in studies
of the Milky Way.

We're undergoing something
of a revolution.

In fact we can take you places
that are really quite remarkable.

NARRATOR: We're about
to make a major move.

We're picking up the earth

and traveling across
thousands of light years--

relocating to distant neighborhoods
of the galaxy.

From our new address
the sky looks different...

full of wonder and beauty...

lit by a multitude of brilliant suns...

...revealing the power of stars
that lived billions of years ago.

Out here we'll get a glimpse of the future,
when our sun exists no more.

It's a journey to unravel some
of the greatest mysteries of the universe:

how the Milky Way was born,

how it survived for so long

and how it will eventually die.

But before our trip can begin,

we need a map
of where we're headed.

And making one is the job
of astronomers

like Robert Kirshner
and James Bullock.

The first obstacle is simply figuring out
what kind of galaxy the Milky Way is.

The Hubble Space Telescope

gives astronomers the capability
to see billions of other galaxies.

Each one is different.

But it turns out there is a pattern.

BULLOCK: When we look out to study
other galaxies in the universe,

We see that there are basically
two types of galaxies.

NARRATOR: The first type,
elliptical galaxies,

appear as large balls of stars,

and no matter what angle
they're viewed from,

they always look rounded.

The other main class is
the so-called "spiral galaxies,"

because their stars are contained in arms
that spiral out from their centers.

From a distance, a spiral galaxy
looks something like a Frisbee.

The key to correctly
identifying the Milky Way

is written across our night sky.

BULLOCK: The Milky Way,
we believe, is a spiral galaxy.

So what we're really seeing,

when we look up at night at this band,

is we're seeing our place
in the universe.

We're part of a giant disc of stars.

NARRATOR: But that's just
an insider's view.

BULLOCK: Now, of course I can't show you
a picture of the galaxy in all its glory.

We can't fly above the galaxy
and take a picture of it and show you.

We're stuck in the disc of the galaxy,

but we can still image it
from the ground.

In fact, this image is a picture
of our galaxy, the Milky Way,

taken from Earth.

NARRATOR: This is one
of the most detailed images

of our galaxy ever created.

It's made from 800 million pixels

contained in over a thousand
individual photographs,

taken from the darkest
places on Earth.

The photos have been painstakingly
stitched together

to create this breathtaking view.

But impressive as it is,
it's only part of the picture.

ROBERT KIRSHNER:
It's something like a pizza.

And if you were in the pizza,
if you were a pepperoni,

your view would not be a very clear one
of what the whole story was.

In the same way, we don't see
the whole reach of the Milky Way.

NARRATOR: What astronomers
really need is a bird's eye view.

KIRSHNER: You would need
to get out of the Milky Way

to really see what it looks like.

We don't have a way to do that,

but we can look at other galaxies
and see what they look like.

NARRATOR: Hubble's cameras capture
nearby galaxies in amazing detail--

like Messier 74.

Although it's over 30 million
light years away,

it's one of our closest neighbors.

Messier 74 is
a beautiful spiral galaxy.

Its large, starry arms sweep out
from a bright core.

BULLOCK: This is an example
of a galaxy

that astronomers think looks
a lot like our galaxy, the Milky Way.

This is a great representation
of our own star city.

In the central region
we have the downtown.

This is the bulge,
this bright spot in the middle,

and from that we see,
spiraling out, these arms,

these beautiful spiral structures
we see in this galaxy.

NARRATOR: Astronomers compare
Hubble's incredibly detailed images

of other spiral galaxies

with the best images
of our own galaxy

taken from the ground.

Using satellites to measure
the distance and density

of stars in different directions,

astronomers reveal the grand plan
underlying our star city.

At its heart, a bright central region--

the galactic core--

our galaxy's downtown district.

From here two majestic spiral arms,
bright bands of billions of stars,

sweep out--

Scutum Centaurus

and the Perseus arm.

There are also three smaller arms.

From one end to the other,

our galaxy measures
a staggering 600,000 trillion miles.

BULLOCK: It takes light 100,000 years
to cross our galaxy.

This is a big galaxy,

and it's quite amazing,
if you think about it,

that we understand as much
as we do about this system.

NARRATOR: Our sun and the solar system
are located here--

in a quiet neighborhood
nestled between two spiral arms.

This is the galactic home address
that we know so well.

But our surrounding neighborhoods
are wildly different.

Like any large city, there are
dynamic industrial zones...

where heat and pressure
forge new stars

and others die
in violent explosions.

Downtown, in the very heart
of the galaxy,

stars jostle for space,
pulled by mysterious forces.

[people screaming]

Our galaxy also has quaint,
historic neighborhoods

that tell the story of how
our star city was founded.

Now we head to one of the most spectacular
locations in the Milky Way--

a place that holds the clue

to how the 200 billion
stars of the galaxy

were first created--

and it's just around the corner.

We're picking up and leaving home.

We're taking our planet on a journey.

The destination?

A place where stars are born.

It may look close by, but even
traveling at the speed of light--

186,000 miles a second--

the trip takes 1,500 years.

We arrive at a vast glowing cloud
of gas and dust:

the Great Orion Nebula.

Beautiful new colors
fill our evening sky.

But this cloud isn't
just a work of art.

It holds the key to how our sun,
and every star in the galaxy,

came to be.

The Milky Way is filled with
billions of stars in every direction.

From Earth the naked eye

also picks out large, dark,
seemingly starless patches.

To astronomer James Bullock,

in these areas, there's more
than meets the eye.

BULLOCK: Perhaps the most beautiful
part of this image

is that we have this contrast
of dark and light regions

running through the plane
of the disc.

What that really is, it's dust.

There are clouds of dust

that are casting a shadow
from the back of the stars,

and the stars are trying
to shine their light through,

there are dust clouds there
that are blocking the light,

much like a cloud on Earth
would block the Sun.

NARRATOR: These vast clouds
of cosmic gas and dust

stretch thousands of light years
across the Milky Way.

Hubble finds them
in most spiral galaxies.

Dark, ghostly bands,
woven through the spiral arms--

and spreading across the entire disc.

But there's something strange
about this gas and dust.

Sometimes it glows.

These bright glowing clouds
are called nebulas.

Each one is unique...
and breathtakingly beautiful.

The Eagle Nebula, with towering pillars
up to four light years in size,

and the Carina Nebula,
with its distinctive green glow.

These vibrant colors reveal
what gases nebulas are made of.

KIRSHNER: So, for example,
if there's oxygen gas,

you get a green glow.

If there is hydrogen gas,
you get a red glow.

So analyzing the light from a nebula
turns out to be very instructive.

It tells us what's there, it tells us
what the physical conditions are,

we can tell how dense it is,
how hot it is

and what it's made of.

We can find out a lot
about the neighborhood

by looking at these clues that come
directly from the glowing gas.

NARRATOR: The gases glow
at thousands of degrees,

heated from a mysterious source
hidden deep within the nebulas.

To figure out what the source is,
we need to peer deep inside.

KIRSHNER: But of course
the gas and dust is in the way.

So it's not so easy.

It's a very mysterious part
of the galaxy.

It's a place that we have to use
these special tricks to look into.

NARRATOR: And Kimberly Weaver
is an astrophysicist

who's got a few tricks
up her sleeve.

KIMBERLY WEAVER: I've got
a really neat way to show you this.

This is a bag that you can't see through
with your eye.

So a normal telescope that
looks at optical light

could not see through this.

In infrared light, a telescope
can see through it.

The infrared camera,

if I put my hand inside,
can see my hand.

I'll wiggle my fingers
to show you.

But you're seeing the heat
from my hand inside the bag,

and this is just like a star that's
hidden inside a cloud of gas and dust,

that infrared astronomers can detect
by using an infrared telescope.

This is a picture of the Orion Nebula
in visible light.

We can see all of the gas here

located in front of what we know
are stars in the background,

and we want to be able to look inside
this nebula and see the stars.

In infrared light, in this image,

we can now pick out the stars
inside the nebula,

and we can see dusty cocoons
around the stars.

NARRATOR: But scientists still need
a way to strip away the remaining dust.

WEAVER: How do we get rid
of all this haze and fog?

The way to do that is
with an X-ray picture.

Now when we transition
into the X-ray image,

you can see just
the stars themselves,

the X-rays coming
from the surfaces of the stars,

and now we can study them
in great detail.

NARRATOR: By analyzing the light
from these stars,

astronomers make
an astounding discovery.

Hidden within the Orion Nebula are some
of the youngest stars ever found--

stars just a few
hundred thousand years old--

a mere heartbeat
in the life of the galaxy.

And it's not just the Orion Nebula.

Nebulas house baby stars
in every spiral arm of the galaxy.

BULLOCK: These regions
are the nurseries for new stars.

There are young stars
in these regions

that are heating up gas clouds
that surround them

and making those
gas clouds glow pink.

Stars are made out of gas, basically,

and our galaxy has gas.

In fact, our galaxy, you can think of it

as having an atmosphere
of gas and dust

that surrounds all of the stars
that we see in the disc,

and it's from this gas
that new stars are born.

NARRATOR: By observing nebulas
at different stages in their evolution,

the story of a star's birth
begins to emerge.

It all starts inside a cold, dark cloud
of dust and hydrogen gas,

where a quiet tug of war begins.

The cloud wants to dissipate,
like smoke in the air,

but gravity wants
to pull it together.

KIRSHNER: They're in
a kind of balance

between gravity pulling in
and gas pressure pushing back out.

Gravity wins, and the material
crunches down into a disc

that is the beginning
of becoming a star.

NARRATOR: As gravity pulls
more and more gas

towards the center of the disc,

it gets denser and denser
and hotter and hotter....

...until finally,
at 18 million degrees,

a miraculous transformation
takes place.

Hydrogen atoms fuse together
to form helium--

and with a burst of nuclear energy,
a star begins to shine.

KIRSHNER: These stars eventually get
their nuclear fires going in the core.

And when they do, they heat up,

they can expel the material
that's around them

so that it kind of clears up
the neighborhood.

NARRATOR: Over the next
few million years,

winds blow the surrounding gas
into spectacular swirling patterns.

KIRSHNER: It blows away the gas,
it blows away the dust

and it lets us see
this beautiful new thing,

this place where the star
has been born.

NARRATOR: A human lifetime
is too short

to witness the wonder of a star's birth
in the spiral arms.

But by speeding up millions of years
of cosmic time into just a few seconds,

we can see one star born
after another.

Here and there are even more
brilliant flashes of light,

coming from some of the most violent
and dangerous neighborhoods

in the entire Milky Way galaxy.

Here stars aren't born...

they die.

We're taking the Earth

from the familiar neighborhood
of the sun

to visit the wonders
of the Perseus Arm,

nearly 6,500 light years away.

Here lies one of the galaxy's
most beautiful sights--

the Crab Nebula.

Although it's made of gas and dust,
this nebula hasn't created stars...yet.

But for Alex Filippenko,

this area does represent
the industrial zone of our galaxy,

where the building blocks of Earth
were manufactured long ago.

ALEX FILIPPENKO: Look at that
molten iron. Holy moly!

The Crab Nebula is a fascinating object.

We see these very rapidly
expanding gases.

NARRATOR: The crab may look static,
but gases are racing out from its center

at over three million miles an hour,

put into motion by a phenomenally
powerful and violent event in the past.

FILIPPENKO: When we examine
the gases of the Crab Nebula,

which are expanding outward,

and we extrapolate that expansion
backward in time,

we find that all of the gases
were at a common point

about a thousand years ago.

NARRATOR: Back on Earth,
a thousand years ago,

early civilizations
watched the heavens.

In 1054, Chinese manuscripts describe
the sudden arrival of a brilliant new star.

It shines brighter than any other star,
so brightly it's visible during the day.

But then it mysteriously disappears.

Today, the Crab Nebula lies
in exactly the same part of the sky

where the Chinese observed
their brilliant star.

What they witnessed was
the moment the crab was born.

FILIPPENKO: The Crab Nebula
was produced

by the colossal titanic explosion
of a star at the end of its life.

It's a supernova remnant.

NARRATOR: The spiral arms
of our Milky Way

are littered with these
colorful remnants.

Tombstones of stars
that died violently

in cataclysmic explosions
called supernovas.

To figure out this mystery, astronomers
need to locate the next victim--

a massive star at the brink of death.

FILIPPENKO: Astronomers
are like detectives.

We have to figure out
what's going on in the universe

sometimes based on
a minimal number of clues,

and in the case of most astronomers,
the clues come from only the light.

NARRATOR: Andy Howell knows
catching light from a supernova

is all about timing.

ANDY HOWELL: Supernovae happen
about once every 70 years

in a galaxy on average,

so about the human lifetime.

So chances are you're not going
to see one in your lifetime.

In fact the last one in our galaxy
that anybody saw

was about 400 years ago.

So it's been a long time,

and, you know, I study
supernovae for a living.

I couldn't do this if I had to just
wait for one in our galaxy.

NARRATOR: But thankfully
for Howell and Filippenko,

there's no shortage of galaxies.

HOWELL: So what we do is we look
at other galaxies, more distant galaxies.

There are billions
of galaxies out there,

and we see the supernovae
that happen in those galaxies.

And if you look at 70 galaxies,
on average you'll find one a year.

If you look at 700 galaxies,
you'll find ten a year, and so on.

FILIPPENKO: There's power in numbers.

If we look at thousands of galaxies,
we improve our odds tremendously.

NARRATOR: This is a supernova

that Filippenko and his colleagues
are lucky enough to catch--

an exploding star on the outskirts
of a galaxy 55 million light years away.

It briefly outshines
the entire galaxy--

the light of a billion suns
distilled into one dying star.

HOWELL: It takes supernova light

a million, or even
a billion years to get here

if they're millions or billions
of light years away.

But they only shine
for about a month,

so we have this little tiny window
to study these things

before that light is gone forever.

NARRATOR: In the workshop,
Howell and his team

are busy preparing
their telescopes.

HOWELL: Pretty cool.
WOMAN: That's right.

HOWELL: We're building
a network of telescopes

so that we can study supernovae
in greater numbers,

in greater detail, than we've
ever been able to before.

Let me show you the telescopes
we're building.

These are the 0.4 meter telescopes

and there are four of them here,

and we're building them,
20 of them in total,

and putting them
all around the world.

So some of these first ones
will go to Chile,

we have some in Hawaii already.

So let me show you one
of the bigger telescopes

we're building here.

Here we have
the one meter telescope.

We're building about fifteen.

The mirror's not here yet, but this
is where it's going to go.

That will reflect the light
we gather from the supernova.

We have to be able to point
anywhere in the sky,

and so you can see that the telescope
pivots along this axis,

and this C ring moves.

The great thing about
this kind of observing

is that it's totally robotic,

and I can just sit here
in Santa Barbara

and have a beer and pizza
while the telescopes do their work.

All new discoveries about supernovae
from all different places in the universe.

NARRATOR: Once they've caught
the light of a dying star,

the detective work begins.

FILIPPENKO: We collect that light
and we analyze it in great detail

in order to determine
what's going on,

what's the chemical makeup
of the star,

what's the pressure inside,
what's the temperature,

what kind of nuclear reactions
are going on,

how does a star explode.

All of these things we figured out
through the analysis of light.

NARRATOR: Astronomers deduce
that only stars with a huge mass

go out with a bang.

FILIPPENKO: A massive star has
a very interesting and vigorous life.

Initially it fuses hydrogen
to form helium,

and that produces energy.

That makes the star shine.

Then the ashes of that reaction,
the helium,

fuse together to form
carbon and oxygen,

releasing yet more energy.

Then the carbon and oxygen
can fuse into still heavier elements,

magnesium and sodium and neon
and things like that,

and then silicon and sulfur,
and finally iron.

NARRATOR: When it starts to make iron,
the giant star is doomed.

In the core a fierce battle
takes place:

energy pushes outwards,
holding it up,

while gravity wants
to crush it inwards.

The battle continues as the star
makes heavier and heavier elements--

producing energy while
fending off total collapse.

But once it starts to form iron,
the battle is lost.

FILIPPENKO: Fusion of iron nuclei
into heavier things

does not release energy,
it absorbs energy.

So an iron core builds up,

but finally it becomes
so massive that gravity wins.

The iron core collapses.

In less than a second
the outer layers collapse inward,

then rebound and get blown
to smithereens.

NARRATOR: But from this death
comes new life.

[train horn blows]

[horn blows]

FILIPPENKO: We're at a foundry here,

and they're pouring molten iron
from old machinery,

and they're going to make parts
for new machines out of that iron.

So they're recycling it.

But all that iron was created
and ejected into the cosmos

by gigantic stars that exploded
as supernovae.

Those explosions created the iron,
ejected it into the cosmos,

and then it got incorporated
into planetary systems like ours.

But ultimately the atoms of iron
were created by exploding stars.

NARRATOR: Supernovas are
the industrial zones of our star city--

cosmic foundries
that forge new elements.

In catastrophic explosions

heavy elements are spewed out
into our galaxy,

enriching it over billions of years.

FILIPPENKO: So if some stars
were not to explode

in the industrial zones
of galaxies like our Milky Way,

then we wouldn't have these
industrial zones here on Earth.

It all is linked.

We're all linked to the cosmos.

NARRATOR: Our lives today
are only possible

because of events that happened
thousands of millions of years ago

in the hearts of supernovas.

[horn blows]

FILIPPENKO: It's fascinating to realize
that the heavy elements in our bodies,

the carbon in our cells,
the calcium in our bones,

the oxygen that we breathe,
the iron in our red blood cells,

all of those heavy elements
were synthesized,

created through
nuclear reactions in stars

and ejected into the cosmos
by supernovae.

NARRATOR: But only a handful of stars
are massive enough to die as supernovas.

Most stars, like our sun,
suffer a more gentle death.

FILIPPENKO: Most stars don't die
in a cataclysmic explosion.

Our own sun, for example,
a typical star,

will die with a whimper,
not a bang.

NARRATOR: Death comes
when the gravity pulling in

finally succumbs
to the nuclear energy pushing out.

When this happens, any star,
even our sun, will die.

FILIPPENKO: In about
four or five billion years

it'll grow into a much bigger star,
a star called a red giant,

and the outer atmosphere of gases

will be held so loosely
by the sun at that time

that the gases will be blown away gently,
in what I call a cosmic burp.

NARRATOR: These cosmic burps
leave behind dying stars

that litter the spiral arms as they
slowly shed layers of elements.

HOWELL: Some layers are oxygen
and some layers are silicon

and some layers are sulfur,

and those are
the different colors we see

in the Hubble Space Telescope images.

NARRATOR: Not far from our sun
is a place where a star is dying:

the Helix Nebula.

It sheds light on how
most stars end their lives.

Our sun is destined to follow
a similar path when it dies,

five billion years from now.

But in other neighborhoods
in the galaxy,

stars suffer a fate
worse than death.

At the center of the galaxy

lies a place where stars
disappear altogether.

[people screaming]

We're taking the Earth
from the safety of home

to go downtown,
to the heart of the Milky Way.

It's a dynamic, exciting district,
but it's also a risky place to hang out.

[screaming]

Andrea Ghez has spent over 15 years
exploring this neighborhood.

ANDREA GHEZ: If we were to take a trip
from the spiral arms,

out where we are by the sun,

down to the center of the galaxy,
it would be an interesting trip.

It would be very much like
moving from the suburbs

into the heart of a very busy
metropolitan area.

NARRATOR: As we head downtown,
the number of stars increases.

GHEZ: So the density of stars is
tremendous at the center of the galaxy.

It's about a billion times higher
than out here by the sun.

NARRATOR: Here, at the center
of the galaxy,

there are so many stars in the sky

that the Earth is bathed
in perpetual light.

It's a stunning but dangerous
sight to behold.

The stars aren't just close together.

They're moving at super speed.

GHEZ: Going to the heart
of the galaxy

might not be dissimilar to going
to an amusement park.

The rides are somewhat similar

to how the stars orbit
the center of the galaxy.

Ten million miles per hour,
compared to, say, our sun,

is about a factor of 50 times faster.

So something has to be going on
at the center of our galaxy

to make that happen.

NARRATOR: But figuring out what
is no small task.

The heart of our galaxy lies
26,000 light years away.

It's difficult to observe

through the vast amounts
of stars, gas and dust.

And there's another problem
even closer to home:

the Earth's atmosphere.

GHEZ: The atmosphere
is great for us.

It allows us to survive
here on Earth,

but it's an absolute headache
for astronomers.

It's very much like the problem

of looking at a pebble
at the bottom of a stream.

The water in the stream
is moving by and it's turbulent

and it makes it very difficult
to get a clear vision.

In the same way, looking
through the Earth's atmosphere

prevents us from getting clear pictures
of the stars at the center of the galaxy.

NARRATOR:
So astronomers like Ghez

turn to a technique
called adaptive optics

to get a better view.

By measuring how a laser beam
is distorted in moving air,

it's possible to compensate
for the atmosphere's blurring effect.

GHEZ: So let me show you
an example

of how powerful
adaptive optics is.

The stars that we want to see

are the ones that are
at the very center,

and we think the heart
of the galaxy

is right within the center of this box,
which is panned out here.

Without adaptive optics, this region
looks completely blurry.

You don't see the individual stars.

With adaptive optics
you see the individual stars.

NARRATOR: For 15 years Ghez
has taken infrared images

of the stars at the heart
of the galaxy

to produce an extraordinary
time-lapse movie.

GHEZ: So if we zoom in
to the very heart of the galaxy

we can actually see
the data that we've taken

over the last 15 years,

and you can see the stars

and you can see the tremendous motion
that they've gone through.

in particular SO-2,
which is my favorite star--

every astronomer
has a favorite one--

so you can see SO-2
goes around

and in particular you can see,
as it gets to the center of the frame,

it moves much more quickly.

So something's interesting
as it goes through that region.

So putting everything together,
all the measurements that we've made,

we've been able to make
an animation

that shows how the stars have moved
over the course of 15 years.

Each star goes whipping
around the center of the galaxy.

in particular the most striking thing
that you'll notice

is the motion of SO-2.

So SO-2 goes on an incredible
roller coaster ride.

it comes whipping around
and then back out.

NARRATOR: For an object to have
enough gravitational pull

to send SO-2 on rapid orbit
around the center of the galaxy...

it must also have a huge mass.

GHEZ: SO-2 goes around
once every 15 years,

and what it tells us is that there
is four million times the mass of the sun

confined within its orbit.

NARRATOR: Astronomers know
of only one contender

that has a giant mass
but is so small.

GHEZ: So that's an incredible
amount of mass

inside a very small volume,

and that's the key
to proving a black hole.

NARRATOR: And so at the center
of our galaxy

lies a massive black hole,

an object whose gravity is so strong
not even light can escape it.

This is a real image
of the center of our galaxy.

We can't see the black hole--

but we can see bright clouds
of dust and gas spiraling toward it.

We're nearing the black hole.

It's at the center of a stream
of dust and gas...

...the debris of stars blown apart
after straying too close.

GHEZ: Black holes grow with time,

and that happens
by material falling onto it,

accreting onto it,

and that material can come
in the form of either gas

or stars that get torn apart
by the black hole itself.

NARRATOR: At the center
is the invisible black hole.

This is the material it feeds on.

The glowing region
is the accretion disc.

Here star debris falls inward

and whips around
at astonishing speed.

Friction heats the debris up
to such high temperatures

that it glows white hot.

GHEZ: So at the center of our galaxy
we do have a black hole.

We now know that today,

but it's not producing
a tremendous amount of energy.

So it's perhaps, we could say,
it's a black hole that's on a diet.

It simply doesn't have
a lot of material to feast on.

NARRATOR: But what would happen
if SO-2 and the other stars

were pulled inward
by the black hole?

GHEZ: What happens when
that material falls onto the black hole

is that the black hole,

there's radiation associated
with the black hole

and it can generate these jets,

squirting out from the center
of the galaxy.

NARRATOR: Spewing out subatomic particles
close to the speed of light,

the beams are like
vast cosmic searchlights.

This is Messier 87,

a large elliptical galaxy that has
a super massive black hole at its heart.

It's feasting on its own stars.

Shooting out from its bright core

are jets that travel
over 5,000 light years.

GHEZ: I like to call these
the prima donnas of the galaxy world.

These are the ten percent
of galaxies that are showoffs.

NARRATOR: Astronomers believe
that the massive black hole

at the heart of the Milky Way

has been there from the very start.

But in order to get back
to where the galaxy first began,

we have to travel out to the oldest
neighborhood in our star city.

We're traveling upward,
away from our solar system,

out of the spiral arms
of our Milky Way.

Up ahead lie vast clusters of stars
that orbit the heart of our star city.

There are over 150 of them.

These satellite towns,
called globular clusters,

hold the answer to one of
the greatest mysteries in astronomy:

the true age of our galaxy.

BULLOCK: Globular clusters are
really fascinating groups of stars.

They contain about
a million stars each,

and the thing that's
really cool about them

is the stars are
really tightly packed.

KIRSHNER: If you could visit
a globular cluster,

the night sky would be
spectacular,

where many of the stars would be
as bright as the full moon.

And the nighttime sky in all directions
would be filled with bright nearby stars.

There'd be like fireworks all the time.

NARRATOR: Besides the sheer
number of stars,

there's something even more intriguing
about these clusters.

BULLOCK: One of the very interesting
aspects of globular clusters

is there's no sign of young stars.

NARRATOR: Stars are like people.

Look at them,
and you can guess their age

and the lives they've led.

With people, gray hairs and wrinkles
are the telltale signs.

With stars, it's color and size.

BULLOCK: So the biggest stars,
the most massive stars,

the ones with the most gas,
live life in the fast lane.

They live very short
amounts of time.

But they burn very brightly
and they're very, very hot,

and so they tend to be blue.

KIRSHNER: On the other hand
you have the red stars,

which use their energy
very conservatively,

last for a long time,
don't glow too brightly.

And those stars last
for a very long time.

BULLOCK: So by measuring
the brightnesses

and the colors of the stars
in a globular cluster,

we can figure out
how old they are.

And here's the remarkable thing.

They're very old.

Globular clusters, at least the stars
in globular clusters,

in many cases are almost
as old as the universe itself.

NARRATOR: Globular clusters
are living fossils.

They're like discovering
a community of people

who've been around
since the stone age.

Some stars here have been shining
for 12 billion years--

more than twice as long as the sun.

And that's a helpful tool in placing
an age on the Milky Way.

BULLOCK: Globular clusters
are part of our galaxy.

They orbit our galaxy.

In some sense they're tracers
of our galaxy itself.

And so by the fact that
the globular clusters are so old,

it suggests that the galaxy is old.

NARRATOR: And our galaxy
isn't just old--

it's very old.

In fact, the Milky Way is one
of the oldest objects in the cosmos.

It's been around almost since
the beginning of the entire universe--

at least 12 billion years.

Globular clusters also show
that the chemistry of the galaxy back then

was very different
from how it is today.

KIRSHNER: We can measure
the chemical properties of those stars.

Turns out they have very low abundances
of the heavy elements.

Things like iron are very rare
in globular cluster stars,

compared to a star like the sun.

NARRATOR: That means the early galaxy
was a far less colorful place.

Without heavy elements
there weren't the beautiful hues

we see in nebulas and
supernova remnants today.

Even more importantly--
it was a galaxy without life.

It took billions of years for stars
to form enough heavy elements

for the evolution of life to begin
anywhere in the Milky Way...

...leaving many to wonder

how the galaxy has managed
to keep going for so long.

BULLOCK: One of the puzzles
about our galaxy

is that we know that it's had
stars forming continuously

for about the last
ten billion years.

But at the rate it's eating up
its gas now,

it's forming new stars,
it should burn out that gas soon.

It should run out of fuel.

And so there has to be
some source for new fuel.

NARRATOR: That source must be
outside the galaxy.

And recently astronomers
made a startling discovery:

Globular clusters aren't the only
groups of stars orbiting the Milky Way.

There are other tiny galaxies
circling our galaxy

called ultra faint dwarf galaxies.

BULLOCK: The reason
why we haven't known

about these dwarf galaxies
for very long,

these so-called
ultra faint dwarf galaxies,

is that they contain
just a few hundred stars,

a thousand stars.

So you try to find a clump
of a thousand stars

while looking through a mass
of a billion stars.

It's not easy.

This is a needle
in a haystack problem.

And it's only because
we have the precise maps,

it's the precision
of modern astronomy

that's allowed us to discover

these extremely interesting
dwarf galaxies.

NARRATOR: These elusive bodies
may help solve the mystery

of what's fueling the galaxy.

BULLOCK: So these dwarf galaxies
are whizzing around our galaxy.

They're in orbit around it.

Now sometimes they get too close,

and when they get too close
they get ripped apart.

In fact they get eaten,
in some sense, by our galaxy.

NARRATOR: This computer model
shows dwarf galaxies as colored discs

with our galaxy in the center.

Over time, our galaxy
pulls dwarf galaxies in,

devours them, and uses
their gas and dust

to eventually form new stars.

BULLOCK: So in much the same way

that a large city might sort of
cannibalize its neighbors,

the Milky Way is cannibalizing
its dwarf galaxy population.

NARRATOR: Globular clusters
and dwarf galaxies

provide crucial insight
to just how old our galaxy is...

and how it's managed
to survive for so long.

These bodies were once thought
to mark the Milky Way's city limits,

the very outer reaches
of our star city.

But today astronomers
are rethinking all that.

Our galaxy might be bigger
than what we can see,

spreading out further
than we ever imagined.

We're picking up our Earth

and moving from our quiet suburb
to a new neighborhood

in the outer spiral arm
of our galaxy.

Here we'll uncover the mystery

of what holds all the stars
in the Milky Way together.

From our new address, the night sky
looks a little different.

The Milky Way is smaller
and the sky darker.

Here, tens of thousands of light years
away from the center of our galaxy,

we're still bound
by the force of gravity.

BULLOCK: Gravity is the force
that makes any two objects

want to move towards each other.

NARRATOR: On Earth, cities are built
with iron girders and concrete beams--

an invisible scaffold which holds
buildings up against the pull of gravity.

Without this scaffolding, skyscrapers
would crumble and bridges collapse.

Gravity governs Earth
and the entire universe.

Anything that has mass
has a gravitational pull.

The more the mass,
the stronger the pull.

With 200 billion stars,
the Milky Way has a huge mass--

and a tremendous
gravitational attraction to match.

So, like a building, our galaxy
also needs propping up

against the force of gravity.

BULLOCK: Imagine the disc
of our galaxy.

If you just took a disc of stars

and put it there,

gravity would tend to make

this disc collapse in on itself,

and it would immediately
just fall together.

That's not what we see
with the galaxy.

What's actually going on is the stars
are orbiting around the center,

and that's what keeps them
from falling in,

in much the same way that the Earth
is orbiting around the sun.

NARRATOR: The planets in our solar system
are in a delicate balance--

gravity pulls them towards the sun

while their orbital velocity wants
to fling them out into space.

In order to stay balanced,

planets further from the sun
must orbit more slowly.

BULLOCK: If you go
to more distant planets

at the edge of the solar system,

they're going around the sun
much more slowly than the Earth is,

and that's because
the gravity is weaker.

NARRATOR: The same should hold true
for stars in the Milky Way.

They all orbit the center
of the galaxy,

but the stars in the outer arm
should be traveling more slowly

than those closer
to the galaxy's heart.

BULLOCK: What's interesting
is that's not what's going on.

The stars in the outer parts
of the galaxy

are spinning around just as quickly
as those in the inner parts.

NARRATOR: And they're not
the only ones.

BULLOCK: It's not just our galaxy;
it's every galaxy we look at.

Every galaxy we look at seems to be
spinning too fast in its outer parts.

NARRATOR: These speeding stars

should be flung out
of the galaxy altogether.

But they're not.

BULLOCK: That is a puzzle.

This means that there's
a lot more mass there

that we just can't see.

NARRATOR: Mass that produces
the gravity

that holds these stars
in their orbits.

But when astronomers
look for the mass,

there appears to be
nothing there...

leading cosmologists like Joel Primack
to an astounding conclusion.

JOEL PRIMACK: All of the galaxies,

all of the stars and gas and dust
and planets and everything else

that we can see with
our greatest telescopes,

represent about half of one percent
of what's actually out there.

The rest is invisible.

It's mostly some mysterious substance
that we call dark matter.

BULLOCK: You can't see dark matter.

The reason why you
can see normal matter

is because light shines on it
and reflects off of it.

That's how you can see me.

Dark matter doesn't work that way.

The light goes right through
the dark matter.

The way we detect dark matter
is because it has mass.

Anything with mass affects
other things via gravity.

That's the golden rule of mass,
that's what mass does,

it tugs on other things
because of gravity.

NARRATOR: Without dark matter,
the Milky Way couldn't exist.

BULLOCK: So the galaxy is spinning.

The galaxy is spinning fairly rapidly.

The reason why it can spin so rapidly
is because it has so much dark matter.

The dark matter has a lot of mass
and therefore it has a lot of gravity,

and that's what keeps the stars
whizzing around.

If you were to magically take
all of the dark matter

away from our galaxy,

it would fly apart.

The stars would just
keep going straight

and in a very short amount of time
the galaxy would just be gone.

PRIMACK: There'd be just a mess
of stuff flying every which way.

And that's not just true
of our galaxy,

it's true of every galaxy

and every cluster of galaxies
in the universe.

They're all held together
by this invisible stuff

that we call dark matter.

BULLOCK: So we need the dark matter.

It's the glue that holds
galaxies together.

NARRATOR: The discovery
of dark matter

has revolutionized our picture
of the Milky Way.

The stars of the galaxy represent
just a fraction of its mass.

The rest is made up of an invisible halo
of dark matter--

surrounding every single star
and every creature in the galaxy.

PRIMACK: The stars are
just the central region.

The halo is at least ten times bigger

and weighs much more
than ten times more

than all the stars and gas and dust
that we can see.

It's that whole structure
that's the real Milky Way galaxy.

And that's not just true
of our galaxy,

it's true of every galaxy
we've ever studied.

NARRATOR: But dark matter does more
than simply hold galaxies together.

Astronomers now think
it binds the Milky Way

into an extraordinary structure
with billions of other galaxies--

a structure that reaches
to the very edge of the universe.

We've left our home galaxy
to take the earth

across billions of light years
of space and time.

BULLOCK: One of the great things
about telescopes

is they're time machines.

Because light travels at a finite speed,

when we look at distant objects

we see them as they were
when the light left them.

NARRATOR: As astronomers look back
over billions of years,

they see a universe
teeming with galaxies.

But these galaxies aren't scattered
randomly through space.

They cluster along delicate filaments
woven in an intricate structure--

a vast cosmic web
that holds the answer

to the birth of galaxies
themselves.

It's a story shrouded
in darkness.

Look back far enough and gradually
all the galaxies disappear.

We've reached a mysterious
period of time,

12.5 billion years ago.

BULLOCK: There's this time period
that we can't see

because nothing's formed yet.

It's this epoch that's called
the dark ages.

NARRATOR: During the dark ages,
the universe was a very different place

than the one we live in today.

It's filled with dense clouds
of hydrogen gas.

Just as gas obscures stars
in the Milky Way today,

these clouds of hydrogen block the view
inside the early universe.

BULLOCK: It's extremely frustrating
because this region,

this time period, holds within it,
in some sense,

the Rosetta Stone
of galaxy formation.

NARRATOR: But there is one clue
to what's happening

inside those dense
hydrogen clouds.

Look back further in time

to a moment just 380,000 years
after the big bang.

The universe isn't filled
with darkness...

but with light.

Its faint afterglow is still visible
to astronomers today.

BULLOCK: In fact,
this picture is amazing.

This is a picture
of the early universe.

This is an image of the afterglow
of the big bang.

NARRATOR: The universe is filled
with a hot atmosphere

of matter and radiation.

But already the seeds of change
are being sown.

BULLOCK: Everywhere we look
around us in the universe

we see structure; we see galaxies
all over the place.

Where do these galaxies
come from?

There's a big clue to this
buried in this picture.

If you look closely,
you can see

that there are red spots
and there are blue spots.

These red regions are regions
where there's basically more stuff,

and the blue regions are the regions
where there's less stuff.

NARRATOR: This image reveals
tiny variations

in the density of the gas
that fills the early universe.

Minute ripples
that will grow with time.

BULLOCK: We think that these ripples,
these primordial ripples,

are the seeds
to all future structure.

These ripples eventually grew
into what became the first galaxies.

NARRATOR: It takes
a powerful force

to grow something so small
into something so big.

BULLOCK: It's gravity
that amplifies these ripples,

and in fact we need
an additional source of gravity

to amplify those ripples to form
galaxies like we see today,

and that additional gravity comes
in the form of dark matter.

PRIMACK: What happens is that first
the dark matter forms the structure.

The ordinary matter
then follows the dark matter.

The ordinary matter is hydrogen
and helium at this stage.

And the hydrogen and helium
fall to the center

of the dark matter halos
that are forming,

and that's going to become
the galaxies.

NARRATOR: Dark matter may be
the missing link

between these minute ripples
in the early universe

and the vast cosmic web
that now fills space.

But dark matter is invisible.

So there's no way to actually see it
creating the cosmic web.

But the process can be simulated

in one of the world's
most powerful super computers.

PRIMACK: Here we are at NASA Ames,

the research center where we have
the Pleiades super computer.

Each one of these cabinets
contains 512 processors.

Let me show you.

So that's half a terabyte
in each one of these cabinets.

There's 110 of these cabinets

to make up the entire
Pleiades super computer.

So this is a really big
super computer.

This is NASA's biggest.

NARRATOR: The challenge
is equally big--

to develop a virtual universe--

from its early beginnings
all the way to the present day--

to see what role dark matter might
have played in shaping the cosmos.

If you tried to do this
on a home computer,

it would take over 680 years.

PRIMACK: If we're doing
our job right,

we can put the pictures
into a video, if you like,

that shows the whole structure
of the universe.

NARRATOR: And this is the end result.

It's called Bolshoi--

an amazing visualization

of what the structure
of dark matter might look like

in the universe today.

PRIMACK: So what we're looking at

is a region about 200 million
light years across,

which is actually just a small part
of our really big simulation

that we call Bolshoi,
which is Russian for "big."

Everything that you see here
is actually completely invisible.

It's not the visible universe
that you're seeing.

The bright spots are dark matter.

They're the halos of dark matter
within which galaxies form.

And each one of these little blobs
would represent probably one,

or at most a couple
of Milky Way size galaxies.

And you can see that the galaxies
are in long chains,

filaments we call them.

Basically all the structure is forming
along these filaments of dark matter.

NARRATOR: Now comes
the real test of success:

Primack compares
the Bolshoi predictions

with the actual structure of galaxies
scientists see in the universe.

PRIMACK: As far as we can tell,

these simulated universes
that we make in the super computers

look just like the observed universe.

There don't seem to be
any discrepancies at all.

This is exactly the way
we see the galaxies distributed

in the observed universe.

NARRATOR: The Bolshoi simulations
are astounding.

They match the pattern of galaxies
seen in the cosmos today perfectly.

It's persuasive evidence

that dark matter has been sculpting
the universe for billions of years.

PRIMACK: No, I'm really
impressed with this

because we stuck
our necks way out

when we made
these first predictions,

and they turned out to be right.

And they keep turning out
to be right.

And, you know, this is, of course,
great joy for a theorist.

NARRATOR: By going back
to the beginning of the universe,

astronomers
have uncovered the origin

of the underlying structure
of the entire cosmos.

But our time travel
is far from over.

The question of how the first galaxies
kindled the very first stars still remains.

We're taking the earth
inside the dark age--

a time over 12.5 billion years ago.

The sight is spectacular.

Our skies are lit by the first stars
of the Milky Way.

Their light pierces the hydrogen fog--

bathing the earth in strong
ultraviolet energy.

These first stars will change the way
we see the universe forever.

Tom Abel studies the life and death
of these early stars.

TOM ABEL: The beautiful thing
is that we now have computers.

We program them
with the laws of physics,

put in some gravity,
hydrodynamics,

how gases move around,
some of the chemistry,

and as we evolve it
all together,

we gain an intuition
of how stars come about,

and in the case of the very first stars,
this is absolutely crucial.

NARRATOR: Abel begins
with the basic ingredients

available during the dark ages:

hydrogen, helium,
dark matter and gravity.

Using computer models,

Abel recreates the lives
of these early stars.

ABEL: Here we see one of
the first stars in the universe.

It's a hundred times
as massive as the sun,

a million times as bright.

NARRATOR: The first stars are huge--

swollen by the massive amounts
of hydrogen gas

pulled in by the gravitational force
of dark matter.

ABEL: And so even though
they have all this fuel to burn

you'd think they could live
for a long time.

They run through it so quickly

that even after a few million years
they're already dead.

NARRATOR: The first stars
in our Milky Way

are fierce, high octane stars--

burning their hydrogen fuel
at tremendous rates--

racing through their life cycle.

ABEL: They're like the rock stars.

They live fast and die young.

They run through
their fuel very quickly

and even afterjust a few million years
they already die.

NARRATOR: They die in some
of the most violent explosions

ever to rock the universe--

gigantic supernovas
that shine brilliantly.

The energy given off

during the life and death
of these massive stars

leads to a miraculous transformation.

ABEL: In the first billion years
of the universe's history,

galaxies start to form
in a dark hydrogen fog,

their light not being able
to get to us.

But as time progresses

and their most massive stars
put out ultraviolet radiation,

it's that radiation itself
that changes the fog around them,

and the universe becomes
transparent in those regions.

These galaxies in here are clearing out
the fog around them.

NARRATOR: The blue voids
are where energy from the new stars

is clearing the dark hydrogen fog.

ABEL: But towards a billion years
after the big bang

the entire fog has cleared

and we now see all the galaxies,

and the dark ages end.

NARRATOR: As the hydrogen fog lifts,

we get our first glimpse
of newborn galaxies...

including our very own Milky Way.

RICHARD ELLIS: This remarkable image
is the Hubble ultra deep field.

It's the longest exposure
that's ever been taken

with the Hubble Space Telescope.

It's a truly remarkable image,

probably the most famous
to professional astronomers.

NARRATOR: For over eleven days
Hubble pointed at a tiny patch of sky

about the width of a dime
held 75 feet away.

Every faint smudge of light is a galaxy.

For Richard Ellis,
it's a treasure trove.

ELLIS: So much like an archaeologist
would piece together history

by digging into deeper
and deeper layers,

so a cosmologist like myself
uses this image

to look at the history of the universe,

how the most distant galaxies,
seen as they were a long time ago,

evolve and grow to the bigger systems
that we see around us today.

NARRATOR: This image gives us
a sense of the dawn of our Milky Way.

ELLIS: When we look
at these early galaxies,

they don't resemble the star cities
that we see today.

They're lumpy, they're irregular,

they appear to be interacting
with their neighbors,

they're physically very, very small.

So clearly the universe was
very different in those early times.

NARRATOR: 12 billion years ago
the universe is a much smaller place.

It hasn't yet expanded
to the size it is today.

Our young Milky Way
is jostling for room.

ELLIS: So it's very difficult
for these early galaxies

to establish themselves.

These early galaxies are struggling
to survive at this very early time.

NARRATOR: It's survival of the fittest--

the largest galaxies grow bigger
by devouring the smallest.

ELLIS: So it's tough
for these early systems to form,

but clearly they do, and they eventually
merge with their neighbors

and form the bigger systems
that we see today.

NARRATOR: These collisions
in the early universe

created the beautiful spiral galaxy
we live in today...

...and they've never stopped.

Astronomers believe
there's still one final collision

in store for the Milky Way.

One that will change it forever.

We've transported the earth
three billion years into the future.

The sky is dominated by
a massive galaxy called Andromeda.

The view may look peaceful,

but one of the greatest calamities
in the universe

is about to take place...

...and clues to the impending disaster
lie in these mysterious Hubble images.

Galaxies unlike any other...

distorted...

deformed.

Astronomers rely
on computers for help

in decoding what these
mysterious objects represent.

PRIMACK: What we do is
we make galaxies

that look just like the Milky Way
and similar galaxies.

And we let them evolve
in the computer,

they develop
the spiral structure,

they look quite realistic.

We then put them
on a collision path.

NARRATOR: Freeze frame
these simulations

and match them with real images

and suddenly the picture
becomes clear:

It's the greatest clash
in the cosmos--

galaxies in collision.

Like cities, galaxies
tend to cluster.

Our Milky Way
belongs to a cluster

called the local group,

made up of at least 50 galaxies.

The largest in the pack
is Andromeda--

a spiral galaxy that's
even bigger than ours.

Today Andromeda lies
2.5 million light years away.

But astronomers like Abraham Loeb
believe that distance is closing in.

ABRAHAM LOEB: When I started
working in astrophysics

I noticed that most
of my colleagues

are thinking about other galaxies
interacting with each other,

colliding with each other,

and I was wondering why
aren't they examining

the future of the Milky Way
and the Andromeda Galaxy

as they will come together.

NARRATOR: Trouble is brewing
for our star city.

PRIMACK: Our galaxy is rushing
toward the great galaxy Andromeda,

they're rushing toward each other,

and they're going
to encounter each other

in a couple billion years.

NARRATOR: Loeb and his colleagues

decide to simulate this
intergalactic clash of the titans.

LOEB: This was the first
simulation of its kind.

Initially the two galaxies
plunge through each other

producing these beautiful tails of stars,
due to the force of gravity.

They run away, turn around
and come back together,

to make one big
spheroid of stars,

which I called
the Milkomeda Galaxy.

NARRATOR: When the Milky Way
merges with Andromeda,

almost one trillion stars
will come together.

KIRSHNER: The beautiful
spiral structure

of our Milky Way galaxy

is not something that's
going to last forever.

It's going to be a mess,
for a while.

The collision will not be one

in which these two things
are destroyed,

but it is one where the gas
in each system

will collide with the gas
in the other.

That it'll have a burst
of star formation.

LOEB: And the formation
of these new stars

will mark the rebirth
of a new galaxy.

NARRATOR: This spectacular
Hubble image

shows the Antennae Galaxies--

a grand cosmic collision
between two spiral star cities.

The galaxies are
in a frenzy of star birth--

a multitude of nebulas
glow pink in the darkness--

one final flare of stellar activity

before the galaxies merge
to become one.

This is the fate that awaits
our Milky Way

when it merges with Andromeda
three billion years from now.

KIRSHNER: When they collide

there will be a lot of new
star formation that takes place,

there will be a kind of rejuvenation
of the Milky Way for a little while

and then eventually
this combined system

will settle down
to become a new thing,

probably a bigger galaxy

than either of the galaxies
out of which it was made.

NARRATOR: But the real surprise
is the shape of this new galaxy.

PRIMACK: A new galaxy is formed

where instead of the discs
that the original galaxies had,

where all the stars are going around
more or less on a plane,

instead the stars are going
every which way,

just like the elliptical galaxies
that we see.

And so we're pretty sure
that this process

must be a large part
of how elliptical galaxies form.

NARRATOR: The collision
of the Milky Way with Andromeda

will leave behind
a giant elliptical galaxy.

But before that happens
there'll be one final sight to behold.

LOEB: The image of Andromeda
will be stretched across the sky,

looming as big
as the Milky Way itself,

and it's conceivable that there
would be human beings like ourselves

looking at the sky and seeing
this spectacular image.

NARRATOR: We might not be
the only beings enjoying the view.

Could our galaxy be home
to other civilizations?

Unknown life yet to be discovered
inside the Milky Way?

There are around 200 billion stars
in our galaxy.

But there's only one neighborhood
we know for sure that sustains life:

Earth.

GEOFF MARCY: The sun powers almost
everything here on the Earth.

It's the energy source; it's the engine

of life and many other processes.

And life here on Earth
is based heavily on water.

And it's liquid water that's the key
to life as we know it.

And it's because liquid water
serves as the solvent,

the cocktail mixer,
for the biochemistry of life.

NARRATOR: Earth is the only planet
in our solar system

with abundant liquid water.

As with any prime real estate,

it's all about location,
location, location.

MARCY: Venus is closer to the sun,

Mars is farther from the sun,

and there's a zone in between
the blazing hot furnace of Venus,

the frigid Mars,

that zone in between
we call the habitable zone,

and the Earth lies
smack in that thing,

where water would be
in liquid form,

not in steam, too hot,
not in ice form, too cold.

But rather a temperature that,
as Goldilocks said,

is just right for life.

NARRATOR: The location
of a habitable green zone

depends on the star.

With hot blue stars,
the green zone is further out.

With cooler red stars,
it's closer in.

Every star in the Milky Way
has a habitable zone.

But not every star has
planets within that zone.

MARCY: In 1995 something happened
that was extraordinary.

I got a call from my collaborator,
Paul Butler,

and all he said was, Geoff,
come over here.

And it was a moment
that I will never forget.

I was silent, Paul was silent,
and we were just stunned.

There on the computer screen

I saw the unmistakable
signature of a planet.

NARRATOR: Marcy had discovered
the first planet around another star.

But he couldn't actually see it

because the planet
was too small and dim.

MARCY: Any planet orbiting a star
is lost in the glare of that host star,

that outshines it
by a factor of a billion.

And so instead, to detect planets,
we watch the stars.

And in fact a star
will wobble in space

because it's yanked on
gravitationally by the planet,

or planets, orbiting that star.

And by watching the star alone

we can determine whether
the star has planets

and how far out those planets are
from the host star.

NARRATOR: So far astronomers
have found over 400 planets

orbiting stars in our galaxy.

But none of them seem to be
in habitable zones.

MARCY: One type of giant planet
orbits very close to its star.

We call them hot Jupiters,

because these Jupiter-like planets
are so close

that they're blow-torched
by the intense heat from the star.

The other sort of planet
we've found is also bizarre.

We've found planets that orbit
in elongated orbits,

elliptical, stretched out orbits,

but then the planets
go very far from the star

where they would be quite cold.

And so the planets
that we've found so far

are a little too weird
for us to imagine

that life would have
a good chance of surviving.

MAN: Power on.

External.

NARRATOR: But all that may be
about to change.

Recently NASA launched
a powerful new telescope called Kepler,

to hunt for Earth-sized planets

that may orbit habitable zones
around nearby stars.

MARCY: Kepler works
in the most simple way.

All Kepler does is monitor
the brightness of 100,000 stars

with such exquisite precision

that it would detect a planet
as small as an Earth-like one

as it blocks the starlight.

NARRATOR: We see
the same thing from Earth

when Venus and Mercury
are silhouetted against the sun.

But Kepler's task is far more difficult.

MARCY: It's a little bit like
having a searchlight

in which you're trying to detect
any dust on that searchlight

by noticing a dimming
of the searchlight

when one dust particle falls
on this massive searchlight.

NARRATOR: From this tiny dimming,

the size of the planet
can be measured.

And together with the way it
causes its host star to wobble,

Marcy can work out its density.

MARCY: And of course
this is glorious

because by these measurements

we'll be able to distinguish
gaseous planets,

probably not suitable for life,

from the rocky planets that may have
a surface covered by liquid water.

NARRATOR: Astronomers aren't sure
how many planets Kepler will find--

but with 200 billion stars
in the Milky Way,

the odds look promising.

Seth Shostak has done the math.

SETH SHOSTAK: You know,
the indications are

a lot of those stars have planets,
maybe half of them do.

And since planets, you know,
being like kittens,

you don't just get one,
you get a couple.

There are probably on the order
of a million million planets out there.

NARRATOR: A trillion planets.

It's an unimaginably vast number.

But what are the chances
of them being in a location

where life can flourish?

MARCY: We can expand the idea
of a habitable zone around a star

to a habitable zone within
our entire Milky Way galaxy.

NARRATOR: The search for life begins

with the search
for a galactic habitable zone,

the safe haven that
allows life to flourish.

MARCY: In close, at the hub
there is an extraordinary amount

of X-rays, harsh radio waves,
even gamma rays

that would certainly destroy
fragile single-celled life

just getting a start
toward evolution.

SHOSTAK: Downtown is dangerous.

There's a super massive
black hole down there.

You get too close to that, all sorts
of bad things can happen.

There are also a lot
of stars down there

and, you know, a lot of stars
sounds good,

but on the other hand
if you have too many nearby stars

they tend to shake up all the comets
in your solar system

that are constantly
pummeling you

with these collisions that,
just ask the dinosaurs,

are not always good for you.

NARRATOR: The spiral arms may offer

the safest neighborhoods
in the galaxy.

But even here, danger
may lurk around the corner.

SHOSTAK: If you happen to be
on a planet near a supernova,

that explosion could ruin
your whole day.

Life might get started,
and then, you know,

another couple of hundred
million years later it gets wiped out.

So these areas are sort of
no-go zones, no man's land.

Well, no alien's land, perhaps.

NARRATOR: The outer reaches
of our Milky Way are quieter.

But here life would still
find it difficult to take root.

MARCY: At the outskirts
of our Milky Way galaxy

there aren't very many
heavy elements

of which the cells of our bodies
and life as we know it

are composed.

And so we may not have
the essential building blocks of life

at the outer edges
of our own Milky Way.

NARRATOR: So it's not an accident
that we are where we are.

Our neighborhood, tucked away
between two spiral arms,

is prime real estate.

It's remained relatively unchanged
for billions of years,

giving life time
to establish and evolve.

Other advanced civilizations,
if they exist,

are likely to live
in similar neighborhoods,

cocooned from the dangers
of the galaxy.

We haven't found them yet.

But then again,
our galaxy's a big place.

SHOSTAK: We haven't found
any life elsewhere,

we haven't found pond scum,

we haven't found
dead pond scum anywhere else,

not convincingly,
and why is that?

Well, fewer than a thousand stars
have been looked at carefully

for planets that might have
intelligent life.

So you know, it's sort of
like going to Africa

looking for mega fauna, you know,
elephants, giraffes, something like that,

and you land in Africa

and you look at the first
square yard of real estate there

and you say no elephants here,
then you give up.

Well, we shouldn't give up,
we're just beginning.

MARCY: Well, if we do find life,

it's amazing, if we find life
elsewhere in the universe,

I think the stock market
won't budge one bit.

But we humans will know,
for the first time in human history,

that we're not alone.

That we have kindred spirits
out among the stars,

and that our destiny may well be
to venture to the stars,

communicate with them

and become members
of a great galactic country club.