All language subtitles for How.the.Universe.Works.S07E05.Secret.World.of.Nebulas.1080p.WEB.x264-CAFFEiNE

Narrator:
All across our galaxy,

stunning clouds of gas
and dust -- nebulas.

They contain secrets
of the cosmic circle of life,

the birth and death of stars,
planets, and us.

These things are really cradles
of creation.

You are intimately related
to the nebulas.

You are a nebula come alive.

Narrator: The story of how
our solar system formed

starts with a nebula.

If you want to build
a solar system,

you're going to need a nebula.

Plait: Look around you.

Everything you see everywhere
was once inside of a nebula.

Narrator: Now scientists are
pulling back the veil...

Opening our eyes to the true
expanse of our universe.

Narrator:
...Solving the riddles

of these engines of creation.

Thaller: There are mysteries
waiting inside

that we haven't
even guessed at yet.

-- Captions by vitac --
www.vitac.com

captions paid for by
discovery communications

Narrator:
The milky way --

a spiral galaxy
full of regions

of gas and dust called nebulas,

and everyone has their favorite.

Bullock: I really like
the horsehead nebula.

It just looks awesome.

The cat's eye nebula has always
been really captivating to me.

My favorite nebula
is the Orion nebula.

Narrator: The Orion nebula is
perhaps the best place

to understand
the evolution of stars,

and it's right here
in our own backyard.

The Orion nebula is maybe one
of the most famous nebulas

because you can go outside
at night

and see it with your own eyes.

Narrator:
Humans have been observing

this fuzzy patch of sky
for centuries.

The Maya of central America
called it

"the fire of creation".

The Maya were more right
than they knew.

Almost every part of the life
cycle of a star

you can see in a nebula.

Sutter: We can't understand
the life cycle of stars

without understanding
the life cycle of nebulas.

They are intertwined.

Narrator: Orion has it all,

from massive stars
on the brink of death,

to newborn stars
swaddled in gas.

Sutter: You see the intricate
wisps of material,

the thin veils enveloping
newborn stars,

pillars colliding
into each other.

You see stars plowing
through clouds of gas.

You see this frenzied hive
of activity

operating right before our eyes.

Narrator:
In 2018, using new data,

NASA creates a groundbreaking
3-d visualization

of Orion's interior.

For the first time in history,
we have the right tools

to actually explore
the hearts of these nebulas.

Stricker: It was already
beautiful to begin with,

but now we have even more vivid
images to really appreciate

how great of
a structure this is.

Narrator: At Orion's heart lies
a cluster of young stars.

Together, they blast out charged
particles and solar winds,

blowing open a gap
at the center,

creating a window inside.

Thaller: We actually see
the structures and the volume.

We can actually see
the processes happening

right before our eyes.

Narrator:
The cluster's intense starlight

energizes the surrounding gas,

causing it to glow
pink and blue.

Durda:
The pinks come from light

emitted from hydrogen atoms
in the nebula,

glowing like the gas
in a neon tube.

The blues tend to come from
the light from the hot,

new stars reflected
off of dust particles.

Narrator:
These hot, new stars

illuminate the Orion nebula,

but they were actually
born in the dark.

One particular type of nebula
is a dark nebula,

and basically that's when
the concentration of dust

is a lot greater.

Narrator:
Dense clouds of dust

block out visible light
from the stars behind,

creating shadowy shapes
like the horsehead nebula.

This nebula is so large
and dense, it has enough mass

to make about 30 stars
the size of our sun,

and now astronomers
can peer inside.

Only recently have we been able
to start doing this...

...thanks to detectors that
can see light in the infrared.

The infrared allows us
to sort of see

through the dust of a nebula

and see what's going on
deep in its heart.

Narrator:
Humans can't see infrared light,

but we can feel it as heat.

Infrared detectors tell us
these dark, star-forming clouds

are cold, hundreds of degrees
fahrenheit below freezing.

But deep inside are hot spots.

Sutter: If you look at it
with infrared, you see,

"ah, the signature
of incredible densities

and incredible temperatures --

the signs that a new star
is being born."

Narrator: A knot of matter
comes together

under the force of gravity.

As it grows,
so does the gravity.

It pulls in more gas,
growing bigger and bigger.

Plait: That gets very massive,
very dense, and very hot.

Eventually it gets high enough
pressure and temperature

in the center of that object
that you ignite fusion.

Narrator: A star is born...

...one of the hundreds
of billions

that make up our galaxy,

the latest in a stellar
production line

going all the way back
to the dawn of time

and the very first nebula.

Bullock: If we want to unravel
the history of the milky way,

we want to start
in the beginning,

and that's the big bang.

Narrator:
13.8 billion years ago,

the universe sparks into life.

At first, it's pure energy.

But over 300,000 years,

that energy cools into hydrogen
and helium gas.

Back then, the entire universe
was one enormous cloud.

The essential ingredients
of our universe spread

as the universe expanded.

And so the universe started
as one giant nebula.

Narrator: Over time,
the primordial nebula

starts to collapse

and fragment
into smaller clumps.

These regions become so dense,
they collapse into discs

with super-hot balls of gas
in their cores.

��

the first stars ignite.

They start out
as nearly pure hydrogen,

but as they age, they make
other, heavier elements.

Stars forge new elements.
That's what they do.

The very definition of a star
is in its core,

it's fusing hydrogen atoms
into helium

and releasing energy.

Narrator: But many of those
first simple stars were massive,

and massive stars
don't live for long.

They burned through their supply
of hydrogen incredibly rapidly,

and they burned themselves out,

and they died after
a few million years.

Narrator:
They go out with a bang...

��

...an explosion that releases
more complex elements

back in
to the primordial nebula.

Straughn: After that first
generation of stars

started to form,

there was this huge burst
of new elements that formed

and that were dispersed
throughout the universe

to be able to form
that next generation of stars.

Narrator:
As the second generation
of stars lives and dies,

it adds even more ingredients
to the cosmic mix.

Sutter: The next generation
of stars fuse more elements,

exploded, died,
spread the material,

new generation of nebula,
new generation of stars,

each generation
having more and more elements

in the periodic table
than the last.

Narrator: And around 300 million
years after the big bang,

our galaxy --
the milky way -- takes shape.

Plait: The galaxies like
the milky way formed out of,

essentially,
a proto-galactic nebula,

some gigantic gas cloud
that collapsed down

and formed our galaxy.

There is a rich cosmic symphony

playing back and forth
between stars and nebulas,

and we now know that we are
a part of that symphony.

Narrator: Eventually, our
element-rich sun is born.

We think that our sun
is a third-generation star,

so it was actually a nebula,
a star, a nebula,

a star, a nebula
before it became our sun.

Narrator: It took around
10 billion years

to create a cosmic mix
of elements

rich enough
to build planets and life.

Carbon, hydrogen, nitrogen,
oxygen, phosphorous, and sulfur.

These are the key ingredients
to life as we understand it,

and those need to be
made in stars.

Narrator:
These elements are created

during the life of a star,

but it takes
an incredibly violent process

to liberate them
into the cosmos...

...an event that can be seen
clear across the universe --

a supernova.

Narrator: The most beautiful
nebulas in our galaxy

are born out
of incredible violence --

the deaths of giant stars.

Thaller: Some of the most
colorful nebulas in our galaxy

are remnants of supernova
explosions,

things like the crab nebula,

cassiopeia a,
also the veil nebula.

Those all happened when
a giant star exploded violently.

Narrator: The crab nebula
was once a massive star,

with around 10 times
the mass of the sun.

��

in its core,
that star crushed atoms together

to form heavier elements,

a process that releases
huge amounts of energy.

Plait: A massive star can fuse
heavier elements,

and those heavier elements
into even heavier elements

until it gets to iron,
and when it gets to iron,

that's when things go bad
really fast.

Narrator: Iron atoms are so big
that fusing them

takes up more energy
than it produces.

The core starts
to collapse on itself,

setting off
a catastrophic explosion...

��

...blasting elements
out into space.

Thaller:
When a star goes supernova,

it violently rips itself apart,

and all of the material
of the star

can be spread across
light-years.

We call this, rather obviously,
a supernova remnant nebula.

Now you have a nebula filled

with all of these
interesting chemicals.

All of those are illuminated

by the energy
of the supernova explosion.

Narrator:
Supernova remnant nebulas

glow brightly
in many different colors.

Plait: The colors in a nebula
are kind of like a fingerprint

or a DNA test
of the elements inside.

Durda:
Every atom has a shell,

a cloud of electrons
that orbits around its nucleus,

and as those electrons
change energy levels,

the frequencies of light

associated with those
energy changes

are emitted into space
and contribute

to the broad spectrum
of colors that we see.

Sutter: So, we can look at a
distant nebula, and we can say,

"it's this much hydrogen,
this much helium,

a little bit of platinum.

Oh, we got a lot oxygen
in that one."

Narrator: The colors of a nebula
reveal the elements created

during a star's life and death.

But a nebula's shape can reveal
what happens after a star dies.

Thaller: You would think that
one exploding star

would be pretty similar
to every other one.

They would make the same
sort of nebula.

And then you see
the crab nebula,

with this beautifully
complex shape --

all of these different arcs
and whirls of gas and dust.

Something must be shaping it
from the inside.

Narrator:
Within the crab nebula

lurks a stellar corpse
called a pulsar.

Pulsars are a kind of
neutron star,

a ball of super-dense matter.

They're born from the death
of massive stars.

Plait: This is the leftover core
of the star that exploded.

This collapsed down and formed
a very tiny ball of neutrons

and a little bit of normal
matter that's very, very hot

and has a very,
very strong magnetic field.

Narrator:
This pulsar is spinning

at around 30 times a second...

...blasting out
beams of radiation

that sweep through space

like a frenzied
cosmic lighthouse.

And the pulsar in the crab
nebula doesn't just emit light.

It's also blasting out a wind
of charged particles.

The gas cloud itself around it
is the pulsar wind nebula.

So, it's taking all that
leftover stuff

from the supernova

and blowing it out
into that expanding cloud.

Narrator: The pulsar winds plow
through the surrounding gas,

creating the twists
and folds of the crab nebula.

��

supernovas create the elements.

Their winds spread them
throughout the cosmos,

forming new nebulas,

nebulas that might form
a solar system like ours.

The nebula is essentially
the starting point

of the recipe
for the solar system.

So, it's got
all the ingredients,

all of the chemicals,
all the gasses,

all that we see
in our solar system today.

Think about the major elements
that make up the planet earth.

What happened to bring
all that together?

Narrator: How did a gassy cloud
of elements

become our planet and our sun?

What turned a nebula
into our solar system?

Narrator: Once upon a time,

there was no sun,
no solar system, no us.

Just a cloud of gas and dust --
a solar nebula.

Sutter: We are here today
because billions of years ago,

there was a nebula containing
all the necessary ingredients.

Wadhwa: Everything that we see
in our solar system today,

that was all part, originally,
of the cloud of gas and dust

that was our solar nebula.

Narrator:
Almost five billion years ago,

a solar nebula was prepared
to give birth to our sun.

So, we have,
billions of years ago,

our solar nebula
cloud of gas and dust,

and it's hanging out,
but it's unstable.

Narrator:
What tips the balance

to turn a cloud of gas
into solid objects?

Thaller:
Something has to change.

Inside a nebula, something has
to trigger the formation

of stars and planets,
and that remains a mystery.

Narrator: So, what's the answer

to this
5-billion-year-old mystery?

��

there are two theories.

Both start with fossils --

fossils that make their way

from the edge of
the solar system towards earth,

break through our atmosphere,
and find their way to us.

Meteorites are really important
for us to understand and study

because they're time capsules

to when the solar system
was basically first forming.

Narrator: Meena wadhwa curates
one of the largest collections

of meteorites on the planet.

These rocks hold
a pristine record

of the very early history
of the solar system.

Wadhwa: There was nothing else
around in the solar system

before these rocks were formed.

There was no earth,
there were no other planets.

It's mind-blowing.

Narrator:
The first solid objects

form out of a cloud of dust
surrounding our newborn star.

Asteroids and meteorites forming
at the same time contain

the chemical fingerprints
of our solar nebula.

They actually contain some
of the oldest materials,

oldest solids that condensed
from the cloud of gas and dust

as our solar system was forming.

And so, they came together
and formed

this big rock that you see here.

Narrator: In 2017,

researchers analyzing
the composition of a type

of rocky meteorite
called chondrites

find a clue about
how our solar system was formed.

There might be, in fact,
a smoking gun

somewhere in the chemistry
of these rocks

that could tell us about
what exactly happened

and how our solar system
was formed.

Narrator: This smoking gun is
a radioactive element

called iron-60,

and it's thought to be created
only in supernovas.

Plait: If you have a nebula
which is about ready

to start forming stars

and a supernova
goes off next to it,

that supernova is going to dump

all those heavy elements
into that gas cloud,

but it's also going to trigger
the formation of stars

by slamming into that gas
and compressing it.

Narrator:
A nearby star goes supernova.

The shock wave strikes
our solar nebula,

injecting it with iron-60.

But the collision starts a
runaway gravitational collapse

in the core of the nebula.

The gas cloud clumps together,
becoming hot and dense.

Our sun is born.

Stricker: As the sun is forming,

there's basically a cloud
of junk all around the sun,

and as it orbits the sun,
it kind of accretes

or sticks together
and grows into these balls.

Narrator: Over the next
hundred million years,

these balls get bigger
and bigger,

forming asteroids, moons,
and planets.

Lanza: All the planets
in our solar system

seem very different.

Some ice giants,
some gas giants,

some rocky bodies.

But, in fact,
all of these planets

came from the same
pre-solar nebula.

Narrator:
And on one small planet,

the right cocktail of elements
gave rise to us.

Everything -- every atom
in our bodies --

was once part
of the pre-solar nebula.

Narrator: The theory
that a supernova

nudged our solar system
into existence is compelling,

but not everyone agrees.

Sometimes, the biggest arguments
among scientists

are caused
by the littlest things,

and in this case,
I'm talking about little,

tiny radioactive atoms.

Narrator: In 2017,
studies reveal

other meteorites contain

a different radioactive
signature --

a rare isotope of aluminum
called aluminum-26.

Thaller:
That's a rather odd atom

that is not formed
very easily in supernovas,

so that had to come
from somewhere else.

Narrator: That somewhere else
is a rare type of giant star

40 to 50 times
the mass of our sun --

a wolf-rayet star.

Oluseyi:
Stars can be very weird,

and the very massive stars
are incredibly weird.

The largest type of star
that we've seen

is what's known
as a wolf-rayet star.

Narrator: Wolf-rayet stars burn
the hottest of all stars,

producing heavy elements

like aluminum-26
during their short lives.

Plait: These are massive
and hot and luminous,

and they blow off
a tremendous wind.

Narrator: This stellar wind

ejects tons of matter
from the star

into the surrounding space,
creating a bubble structure.

Scientists see this process
at work in the bubble nebula,

7,000 light-years from earth.

Thaller:
In the middle of the nebula

is one of these giant stars

with a massive stellar wind,
high-energy particles,

radiation,
and just like the name suggests,

it's blowing a bubble
in the larger nebula around it.

Narrator:
The walls or shell of the bubble

are dense and full of matter.

The stellar wind pushes
more and more matter

into the shell...

...until this material collapses
under its own gravity

and condenses into stars.

Plait: It's entirely possible

that what we're seeing
in the bubble nebula

is what happened here 4 1/2
or more billion years ago

to form the sun and the planets.

Narrator:
If our solar system formed

within
a wolf-rayet bubble nebula,

it would explain why
so much aluminum-26

is present in meteorites.

But the jury is still out.

What we do know
is that our story

began with the collapse
of the solar nebula.

But one day, our star will die.

Will the sun turn into
a stunning nebula,

or will it just fade to black?

Narrator: Nebulas make stars.

Stars make nebulas.

The most massive stars
do so in violent supernovas.

But 99% of stars aren't big
enough to go out with a bang.

Some will just
burn themselves out.

But others can create
beautiful nebulas

with the misleading name
planetary nebulas.

From a distance,
they look like planets,

but really, they're
the ghosts of stars.

When stars like our sun
begin to die,

they bloat up into what we call
red giant stars.

Narrator: As a sun-sized star
reaches the end of its life,

its core gets hotter and hotter.

As it heats up,
the surrounding gas expands,

transforming the star
into a red giant.

It gets so big, its outer layers

are no longer held
in place by gravity.

The outer layers of that star
begin to drift away.

They kind of lose touch with
that central core in the middle,

and they just begin to blow
into beautiful shells,

beautiful colors,
beautiful shapes.

We call these dying stars
planetary nebulas.

Narrator: We've discovered

over 3,000 planetary nebulas
in our galaxy.

Some look like an hourglass
or it looks like an owl

or a clown
or a sphere or a doughnut.

Narrator:
But if they're all the ghost

of the same type of stars,

why do they look so different?

If you have a star that's just
sitting there, no planets,

nothing else around it,

it's going to blow off
its wind in a spherical shell.

And so, if you see
a planetary nebula like that,

it looks like
a soap bubble in space.

Narrator: But only 20%
of planetary nebulas

have this perfectly symmetrical
bubble shape.

Most of them have these
weird shapes.

They can be two loaves

that looks something
like two squids kissing.

All kinds of different shapes
to these things.

Narrator: Experts think

the strange shapes
of these planetary nebulas

may be linked
to how a star dies.

And now new research may reveal
the fate of our own star.

Will we be a beautiful,
bright planetary nebula,

or will we just fade away
into darkness?

For the first time now,
we think we may have the answer.

Narrator:
It's a long-running debate.

Is our sun big enough to form
a spectacular nebula?

Plait: It's kind of
a funny coincidence.

The model shows that you need
a certain mass

to make a planetary nebula.

By coincidence, the sun is
pretty much right on that limit.

Narrator:
The new data suggests

that our sun
is going to go out in style.

As the sun dies,
it'll expand into a red giant,

filling up the sky.

Thaller: We're used to our
gentle yellow sun

coming over the horizon,

so imagine a giant, bloated,
brilliant red glowing ball

coming over the horizon
for the sunrise.

Narrator: The expanding sun
engulfs Mercury,

then Venus.

It'll cook the surface
of the earth,

turning it into a molten hell.

Straughn:
So, the sad news is,

is that once the sun expands
as a red giant,

it will absolutely boil away
the oceans on the earth,

life will no longer
be sustainable.

It's like sticking your head
in an oven set to broil.

It's not like it's going to be
a fun time on the earth.

Narrator:
Some think it could even mean

the destruction of the planet.

Thaller: We think the sun will
eventually become large enough

to swallow up
where the earth is now.

So, instead of there being
a sunrise and a sunset,

we're going to find ourselves
inside the sun.

��

narrator:
The sun sheds its outer layers,

ejecting over half
of its total mass,

revealing the stellar core.

And so, when we look
at this core,

which is now called
a white dwarf

about the size of earth,
they're very hot.

Like hundreds of thousands
of degrees.

Narrator: This white-hot core
radiates U.V. light and x-rays.

These hit the outer layers
of gas

and turn them
into brightly glowing rings --

a planetary nebula that will
shine for about 10,000 years.

Plait: One thing is for sure,
and that is the solar system,

when the sun turns into
a planetary nebula,

is going to look a whole
lot different than it does now.

It'll be unrecognizable.

Narrator:
The planetary nebula will mean

the end of the solar system
as we know it.

The sun will eventually die away
and unravel itself

back into space.

But then the cycle begins again.

This is not just an ending,
it's also a new beginning.

It's going to provide
the ingredients

that will foster yet
a new solar system.

As one solar system dies,
another solar system is born.

So really, this is the cosmic
cycle of life.

Narrator: Nebulas always signal
change in the universe,

intimately linked
with star birth and star death.

Now new observations reveal

that some of our favorite
nebulas are also dying.

Could the famous pillars
of creation be dead already?

Narrator:
Deep inside the eagle nebula

is a dense region
of cold molecular gas,

perhaps the best-known image
in all astronomy --

the pillars of creation.

Thaller: One of the images
that really changed things

was the pillars of creation,

and it was an image
that was very evocative.

It really made me feel
very emotional.

Narrator: The pillars
are five light-years across

and silhouetted by the light
from a nearby star cluster,

and it was these stars

that carved out
the shape of the pillars.

The surface of these stars
are energetic and boiling

and constantly streaming
particles off of them.

Narrator: 10,000-mile-an-hour
stellar winds ravage

the surrounding gas clouds.

Sutter: Eventually,
they completely dissipate

their surrounding nebula.

Narrator:
As the nebula disappears,

columns of thicker,
denser clouds survive,

but for how long?

When you look at
these beautiful hubble images

of the pillars of creation,
the eagle nebula,

you see some blue,
very diffuse gas

around the pillars themselves,

and this is a clue
as to how the pillars formed

and how they're going
to change over time.

Narrator: This hazy blue gas is
actually super-heated material

evaporating off
the pillars themselves.

Nearby stars are slowly
eroding the pillars.

This is similar to how weather
erosion works here on earth.

Thaller:
Think about monument valley.

You have these amazing
stone pillars

and really unlikely shapes
coming up out of the ground.

Well, those are denser areas
of rock

that used to be
covered up by soil and sand.

Over millions of years,

that lighter material
was blown away,

exposing the denser rock
underneath,

and that's exactly the same
thing that's happened here.

Narrator:
This process is ongoing.

Nebulas like the pillars
are constantly evolving.

Bullock: The thing
you keep in mind

about the pillars of creation

is this is actually
a pretty transient feature

in the life of the galaxy.

It's not going to last forever,

and in fact,
over the course of time

even that we've taken images
with the hubble space telescope,

we've seen it change.

Narrator: When astronomers
compared new data

to the original
hubble image from 1995,

they discover a jet
blasting out of the nebula

at 450,000 miles an hour,

extending 100 billion miles
into space.

What could be the source
of all this energy?

These jets are associated
with the moment a star turns on.

Plait: The stars being born
inside of the pillars

are basically
eating their way out.

They're eating up this material,

and then they're
going to blast it away.

Narrator: Newborn stars
are a lot like little kids

on a sugar rush.

They gorge on gas,
then spin out of control.

But stars also have
a magnetic field.

That magnetic field
is rapidly rotating.

It's sweeping up this material
around it

and shooting it out in two jets

going out of the poles
of the star.

Narrator:
Jets and stellar winds

are destroying the pillars
of creation from the inside out.

What's more, some of these baby
stars are growing so fast,

they could soon reach the end
of their short, violent lives.

When stars die,
they send shock waves,

high-energy radiation,
particles.

Narrator: Supernova explosions
like these could blow

the pillars to pieces.

Some have already suggested
that the pillars may have

already been destroyed
thousands of years ago.

Straughn: Eagle nebula is about
7,000 light-years away,

and so we are literally
seeing the eagle nebula

as it was 7,000 years ago,
not as it is today.

Narrator:
It's a sad fact of life.

Nebulas are destroyed
by the stars they create.

Thaller:
That's happening all the time.

Everything changes.

Our most famous, favorite
nebulas don't exist forever.

And it might seem really sad,

but this is just
how the universe works.

Oluseyi:
It's a transitory state.

It's something in the act
of changing.

Where today we see pillars
of creation,

in the future, they'll just be
clusters of stars.

Narrator: But this eternal
recycling of gas and dust

into stars
can't last forever.

Nebulas across the universe
are disappearing.

Is our galaxy
running out of gas?

Narrator: New research shows
that across the universe,

the birthrate of stars
is falling fast.

Researchers predict that 95%
of all the stars

that will ever exist
have already been born.

Sutter: In order for a galaxy
to be healthy,

to keep making stars,

it needs to keep collecting
new reservoirs of gas,

of raw material.

Bullock: Our galaxy is running
out of gas,

and in fact, galaxies
all across the universe

are slowly running out of gas.

That cycle is winding down,
and someday it will stop.

Narrator: More and more gas
is locked up in low-mass stars

that never go supernova,

and the massive stars
that do go out in a blast

push the gas away.

Sutter:
Galaxies eject material.

Supernova winds and fountains
are constantly sending streams

of gas and particles
outside the galaxy.

Narrator:
But stars are not acting alone.

They're in cahoots
with something even bigger.

Experts think the main culprit

lies at the center
of every galaxy --

a super-massive black hole.

In the past, even the milky way
has experienced this gas loss.

Bullock: Just a few hundreds,
millions years ago,

the central black hole
was pretty massive,

and it gobbled up
some material.

In this process,
it released a lot of energy.

It sort of burped up
a lot of energy.

It released gas,
and some of that

probably escaped
the galaxy altogether.

Narrator: Right now our galaxy
is still forming stars.

But the gas tank
needs refilling.

Thaller:
Galaxies run on hydrogen.

It's what creates nebulas,
it creates stars.

So, it looks now like
we may have reached a bit

of a refueling stop.

In space, we've discovered
a giant cloud of hydrogen

heading right for us.

Narrator:
This hydrogen cloud is massive.

10,000 light-years long
by 3,000 wide.

Scientists call it
Smith's cloud.

Plait: The thing is, it's
orbiting our milky way,

and in about
27 million years,

it's going to slam
into the disc of our galaxy.

Narrator: The collision will
re-energize the galaxy,

jump-starting star formation.

Plait:
There's a lot of gas in there.

There's about a million times
the mass of the sun.

You could make a million suns.

Narrator:
But this is only a snack.

To keep forming stars, the
galaxy needs regular feeding.

Oluseyi: Here in the milky way,
we're still forming stars,

and that's because
our galaxy is a cannibal.

It's surrounded by dwarf
galaxies, and it's eating them,

and it's stealing their gas
and their dust.

Thaller:
We're the product of mergers,

many small galaxies
coming together and colliding.

When a new galaxy
collides with the milky way,

it brings with it new gas,
new dust,

the potential
to form new nebulas.

So, by eating its own kind,
the milky way is pulling out

a few billion
more years of star formation.

Narrator: But our galaxy

is always looking
for its next meal,

and in a few billion years,

it will feast
on its next-door neighbor,

the Andromeda galaxy.

When Andromeda merges
with the milky way,

it's almost certainly going to
deliver a fresh amount of gas.

Plait: Although this is
a catastrophic train wreck

on a galactic scale,

it's actually
kind of a good thing

because when it happens,

more stars will be born
inside of the milky way.

That's going to extend
the life of our galaxy,

if you want to
think of it that way.

Narrator: But there are only
so many galaxies nearby

for the milky way to feed on.

Plait: Eventually,
over the long run,

the nebular gas
is being used up.

When that gas is gone,
that's it.

You can't form any more stars.

And so, whatever happens
at that point,

that will be
the last generation of stars.

Narrator:
With no gas to replenish them,

nebulas will disappear
across the universe.

Thaller: The universe really
is winding down.

Nebulas themselves are
being depleted and dying away.

Narrator: The last stars
will eventually blink out.

From here on out,
everything goes dark.

Narrator: Nebulas --

one of the most
spectacular features

of the universe.

One of the things that make
nebulas so appealing

is that they're just
so beautiful.

But it's more than
just beautiful.

Narrator:
They are cradles of creation.

Nebulas are quite literally
the starting point

and the ending point of stars,
and therefore planets and life.

I think it's incredible that
we can learn about the cosmos,

and I think in the end, we're
really learning about ourselves.

Narrator:
They are our connection

to the cosmic circle of life.

Oluseyi: Nebulas are almost like
an analogy for our own lives.

They're incredibly beautiful,
but yet, they're transitory.

They're not going to be here
forever.

And that's the story
of our universe.

It's a story of change.

So, seize the day.