Original subtitles

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.