All language subtitles for The universe S07E03 Our Place in the Milky Way

Male narrator: In the beginning,
there was darkness,

and then, bang,

giving birth to an endless
expanding existence

of time, space, and matter.

Every day, new discoveries
are unlocking the mysterious,

the mind-blowing,
the deadly secrets

of a place we call
The Universe.

In the universe, it's important
to know your nearest neighbors.

But how much do we really know
about our corner

of the Milky Way?

In just the last few years,

scientists have uncovered
incredible secrets

lurking in our own backyard:

new moons, new planets,
and new mysteries.

- It's like there was a house
in your neighborhood

that you never knew
was there.

Narrator. Meet new neighbors

who are just
passing through...

- There are planets that are
wandering the galaxy aimlessly

without a place
to call home.

Narrator: And old friends

whose days are numbered.

- It's conceivable
that Betelgeuse

will go supernova tonight.

Narrator: Join us for a tour
of the neighborhood

we're only now
getting to know.

This is
"Our Place in the Milky Way."

This isn't your neighborhood,

neither is this

or this

or any of these.

And it isn't even this.

Looked at from
a wider perspective,

your neighborhood
is a big cloud of gas.

Astronomers say the solar system
is moving through

the local
interstellar cloud,

also called
"the local fluff"

because of its low density
and irregular shape.

- The gases are mainly hydrogen
and some helium.

There are trace amounts
of heavier atoms

like carbon and oxygen
and nitrogen

that are just floating around
the interstellar medium.

We know that the heavier atoms
in the interstellar medium

are left over from previous
explosions of stars

as supernovae.

Narrator: The local fluff
is 30 light-years long,

about 180 trillion miles.

It's inside a larger
chimney-shaped gas cloud

called the local bubble,

also the remnant
of an ancient supernova.

The bubble
is 300 light-years long

and lies in the inner edge

of one of the spiral arms
of the Milky Way.

And that's our neighborhood.

At least, we think it is.

- Our exact position
in the Milky Way galaxy

relative to the arms
actually isn't known.

The structure of the galaxy
is not known in any real detail.

Some people think
there are two major arms.

Some people think there are
four major arms.

It's hard for us to determine

the exact structure
of our Milky Way,

where all the arms are
and so on,

because it's kind of like
a mouse being inside a maze.

You don't get
the big picture.

Narrator: In almost any
Earth neighborhood,

you can determine your location
very precisely.

- Turn left in 30 feet.

Narrator: But when you're
dealing with something

as big as the Milky Way,

GPS isn't an option.

- The galaxy is, you know,
100,000 light-years across.

Narrator. Even exploring
our local neighborhood

involves a lot
of uncertainty.

But if we did have
a galactic positioning system,

it would probably locate us
about midway between

the top and bottom
of the Milky Way

and about midway between
the galaxy's outer edge

and inner core.

- Our solar system is about
26,000 light-years away

from the center
of our galaxy.

Narrator: According to one
hypothesis,

we have a very exclusive
location.

- There is one idea that
only stars in a certain range

of distances from the center
of our galaxy

are in the so-called
galactic habitable zone,

that is, able to have life
on planets

surrounding those stars.

- It is just the right place

with a star
of the right temperature

and a planet
at the right distance

for there to be a lot of
liquid water on the surface,

where the chemistry of life
began and evolved into us.

Narrator: The overall range
of the galactic habitable zone

extends from about 13,000
to 35,000 light-years

from the center
of the Milky Way.

The main part, where we are,

ranges from 20,000 to 29,000
light-years from the core.

Inside the zone,

old neighborhoods
have been destroyed

to make a place
we can call home.

- Depending upon
how you look at things,

our local neighborhood,
our local solar system,

is actually
a relatively safe place

compared to what
seems to be going on

if you look at the universe
in the large.

In the early history
of our solar system,

it was a much more violent
place.

And the material that formed
the Sun and the planets

was still sorting itself out.

There were all sorts
of collisions

and violent things happening

that gave rise
to this nice, calm-

or relatively calm place
that we have today.

- We think that
the earliest stars formed

out of hydrogen and helium
alone,

but that, over time, the stars
work as these processors

that create
the heavier elements.

This is important
because when those stars

eventually die and explode,

these supernovae,
or stellar death explosions,

seed the galaxy
and the material around it

with heavier elements.

- So for example,
the carbon in our cells,

the oxygen that we breathe,

the calcium in our bones,

the iron
in our red blood cells,

all those
are heavy elements.

- We know that the Sun

is at least a second-
or a third-generation star

because there are planets
around it.

There are things
made of iron and carbon

and other heavier elements.

Narrator: But the processes
that led to life on Earth

don't seem to exist
outside the zone.

Closer to the edge
of the galaxy,

fewer massive stars
have exploded,

producing fewer
heavy elements.

- Further out in the galaxy,

you don't have as many atoms
like carbon, nitrogen, oxygen—

The atoms that are so important
for the chemistry of life.

So the habitable zone
of the galaxy

cuts off at a distance
where you just won't have

the heavier atoms
to make life.

Narrator: If the outer galaxy
is a bad neighborhood,

the inner area
is even worse.

Gravity from massive
gas giant planets

could tear us apart.

And there are other dangers
the closer you get

to the galactic core.

- Back in the times
of Copernicus,

we thought that we were
the center of our universe,

and even as we started
to learn more about the heavens,

eventually, we still thought
that we were the center

of the galaxy.

Now that we know even more,
though,

it actually turns out
that we're lucky

we're not in the center
of the galaxy.

Narrator: At the center
of the Milky Way,

sucking matter
and even light into it,

is Sagittarius A-Star,

a black hole
nearly 14 million miles across

with a mass 3.7 million times
that of our Sun.

- If the galactic center
has a black hole in it,

it gives off
a lot of radiation—

Enough to fry life
as we know it—

So you can't be too close
to that.

Then there are other regions
in the galaxy

that are also probably
not so great for life

because there's just
so much radiation

from nearby
really hot O-type stars.

Narrator: O-type stars
are giants.

They're hotter than the Sun,

10 to 50 times as massive,

and throw out titanic amounts
of ultraviolet radiation.

With these stars,
you don't worry about sunburn

but extinction.

- It's probably not easy
to survive in an environment

where you're in a tight cluster
with a lot of O-type stars.

Narrator: O-type giants
can destroy planets

before they form.

- The radiation from these stars
is so strong

that it actually sweeps
the material away

from these newly forming,
would-be planetary systems

and rips it out of the orbit
of their stars.

Narrator: If you want proof,
look at the Rosette nebula.

It's 5,200 light-years away,
far outside the local bubble,

but it shows what O-type giants
could do to our neighborhood.

A 2008 study by
the University of Arizona

of 1,000 stars in the nebula

found star after star
had been made barren

by being too close
to a blue giant.

So what's a safe distance
from the radiation

of an O-type giant?

- Well, if you ask me,

you can never be
too far away from a giant.

If you're life
like us here on Earth—

We're used to
our fairly tame Sun—

You want to be probably at least
tens of light-years away,

maybe more than that.

Really,
just don't get too close.

Narrator: Like a city between
a desert and an ocean,

our corner of the galaxy
thrives

between two different
inhospitable regions.

With the elements of life

and without the threat
of intense radiation,

it seems like our neighborhood

is literally the only place
to live.

But is our place in
the Milky Way

really so exclusive?

- The idea of the galactic
habitable zone is that

if you're too close
to the center of the galaxy,

there's all these crazy things
going on,

and it tends
to kill off life.

To be honest, I'm personally
skeptical of the idea

because I think
that life can happen

in all sorts
of environments,

or at the very least,
we don't know,

so we should be open-minded.

It's possible
that our kind of life

can only live in this
galactic habitable zone,

but elsewhere, there could be
other kinds of life

that we would call
extremophiles.

On the other hand, they would
call us extremophiles.

Narrator:
One thing is certain.

In our neighborhood,
we have a sun that,

unlike a blue giant,

protects us from danger
and destruction

in ways that
we're still learning about.

That protection
may be invisible,

but if we lose
the Sun's protection,

our neighborhood
could be doomed.

Our place in the Milky Way
seems pretty peaceful

because, like a lot
of communities,

we don't give much thought
to the 24/7 security systems

that keep
the bad stuff away.

Many cities on the edges
of rivers or oceans

have dams and levies
to protect them from floods.

If the dams and levies fail—

Disaster.

We've got threats and defenses
on a galactic scale too.

- Space is filled with radiation
known as cosmic rays.

Cosmic rays are bad for us
in the same sense

that nuclear radiation here
on Earth would be bad for us,

because high-energy radiation

tends to dissociate
carbon bonds,

which is what
we're made of.

What you're really doing
is damaging your DNA,

and there's a potential there

that you could start to have
mutations based off of that.

Narrator: Some mutations
can help a species survive

or lead to extinction.

- There's no evidence
that cosmic radiation

has really negatively impacted
Earth in the past,

but it's nothing that you want
to play around with.

Narrator. Our neighborhood's
prime defense

against cosmic rays:

magnetism.

- We have this zone of
protection in our neighborhood.

Of course, the Earth
has a magnetic field

due to how things move around
in the core of the Earth.

The Sun also has a powerful
magnetic field,

and it also has
the solar wind.

These phenomena actually
generate ways of protecting us

from things that come from
outside the solar system.

Narrator. The Sun's magnetic
field is twisted

by the solar wind,

streams of charged
protons and electrons

that shoot out of the Sun
at a million miles an hour.

- And then the particles
that live in the solar system

between the planets

actually stretch the lines
of the magnetic field around

in complicated patterns.

Narrator: The solar wind
carries the magnetic field

more than three times
farther out

than the orbit of Neptune.

But 9 billion miles away,

at a place called
the heliopause,

the solar wind runs out of steam
and slows to almost nothing.

As it slows, it twists
the Sun's magnetic field

into a barrier against
cosmic rays

from interstellar space.

This is the heliosheath.

- If it wasn't
for the heliosheath,

these cosmic rays would actually
pour into our solar system

all the time.

The heliosheath acts
as a kind of shark cage

for these
incoming cosmic rays

that might otherwise
influence our planet.

- Some do come through,

but they don't come through
as strongly as they would

without that protection.

Narrator: It used to be thought
that the heliosheath

was a rather elegant barrier

made of flowing curtains
of magnetic force,

but recently,

enter Voyager 1
and Voyager 2,

probes sent out from Earth
in 1977.

In the early 21st century,

these disco-era devices
sent back information

indicating that the Sun's
magnetic field lines

don't flow smoothly
together;

they break up and reform
into violent magnetic froth,

and each bubble in that froth
is 100 million miles wide.

- We used to think that
it was a smooth, nice barrier

between them, but in fact,

it's a roiling place
with all sorts

of bubbles and patterns.

I think there's always been
people who think the universe

is more elegant than it is

and people who think
it's more violent than it is,

and we're always surprised
one way or the other.

The truth is that some aspects
of the universe

are quite elegant,
and in other aspects,

it's quite a mess.

Narrator: Where are
the elegant areas

of our galactic
neighborhood,

and where are
the rough parts?

It can be hard to tell

with all that gas and dust
in the cosmos.

- It's sort of like looking here
behind me at Hollywood.

There's even a landmark there,

the Capitol Records
building.

You can barely make it out.

And even beyond that,

there are some hills
that are even hazier.

It's because there's stuff
in the air that blocks the view.

The hill itself obscures
my ability to look beyond it,

and that's kind of like the dust
in very dense molecular clouds.

When you hit a big wall
of a dense molecular cloud

filled with dust,
you can't see anything.

Narrator: When we explore
our galactic neighborhood,

what we see depends on
how we look at things.

- One of the things
we've learned

through the history
of looking at the sky

is that every time you look
in a different way,

you see new things,

and looking at the sky
a different way

often simply means
looking in a different part

of the electromagnetic
spectrum.

As we look up into the sky
with our eyes

or with the aid
of optical telescopes,

in the visible part
of the spectrum,

we see the night sky,

and there's a lot to see,

but it's only a fraction
of what's out there.

Narrator:
Some space telescopes

see through cosmic dust
with infrared vision,

similar to that used
by commercial infrared cameras.

- I've brought with me
this plastic bag,

and when I put my hand
inside of the bag,

you can't see how many fingers
I'm holding up.

With the infrared camera,

you can see the heat
coming off my body,

so my face, which is warm,
is red and white,

but my hair, which is cool,
shows up as blue.

So with the infrared camera,
you should be able to make out

how many fingers
I'm holding up,

even though you can't see
through this bag.

That's how astronomers
peer through cosmic dust

when they want to see things
that are hidden from sight.

For example, stars being born
are very warm,

but they're obscured
by dust shells.

With infrared,
we can see them.

- Certain objects
are transparent or opaque

depending upon the frequency
of the light

that's trying
to get through them.

And so, in fact, something
that's getting in the way,

like a lot of interstellar dust
or gas,

is getting in the way of what
your telescopes can see,

are actually invisible
in another part of the spectrum.

You can see what's behind.

Narrator: With infrared

and a multitude of other
wavelengths at our command,

we've discovered
a lot of neighbors

we didn't know we had.

- There's a whole array
of instrumentation

which is exploiting
that lesson

that if you look

in a particular part
of the spectrum,

you see the sky
in a very particular way.

Narrator: From what
we've observed,

it looks like some old neighbors
might have helped

life form on Earth,

while some newer neighbors
may be planning

to wipe us out.

As we've explored our place
in the Milky Way,

we've met a lot of interesting
new neighbors,

but there are good neighbors
and bad ones.

[dog barking]

- Good neighbors are,
for example,

objects that are
in predictable orbits,

moving around,
doing their own thing,

minding their own business.

We can look over
and wave to them,

but they're not gonna
do something sudden

or dangerous to us.

The bad neighbors, then,

would be things that may do
something unstable.

They may do something
that could affect us

in a way that we can't predict
when it's going to happen.

So that might be when a star
dies and explodes...

Or it might be when something
collides and bounces off

something else and comes
spinning in our direction.

So classifying things roughly
into good neighbors

and bad neighbors
is really a classification

into predictability
and unpredictability,

or violence and nonviolence
if you like.

Narrator: Sometimes
a good neighbor

will bring a moving-in gift.

That might have happened to us,
billions of years ago,

as the Earth was still cooling
and forming

out of recycled material
from a recycled sun.

We might have received a gift
that changed everything.

- The early Earth
was very hot,

and probably any original
surface water evaporated away,

so we think that
quite a bit of the water

may have come from either comets
or icy asteroids or both.

- One of the theories about
how we might have gotten

so much water here on Earth

is from icy bodies
in the outer solar system,

left over from the formation
of the Sun and the planets,

crashing into our inner
solar system where Earth lives

and deliver
some of that water.

Narrator: According
to one recent theory,

about 4 billion years ago,

the gravity of gas giants
like Jupiter

sent icy asteroids slamming into
Mars, Earth, and Venus.

But only on Earth did the ice
penetrate into the mantle.

The water softened the Earth

and initiated a titanic process
of plate tectonics,

which led to the emergence
of continents and oceans.

And what of the life
that formed in the oceans?

Did organic compounds
necessary for life

also splash down
from space?

In rare meteorites called
carbonaceous chondrites,

scientists have found
organic compounds

like those that helped
form life on Earth.

These compounds are similar
to what's been collected

from many different sources,

including Antarctic
micrometeorites,

interstellar dust,

and comet samples acquired

by NASA's Stardust mission
in 2005.

- The origin of life
involves a long series

of reactions with many different
organic molecules,

organic molecules being
just ones with carbon in them,

and it's possible that different
circumstances are needed

to make the different
organic molecules.

Some of them might be made
here on Earth,

but others might be easier
to make out in space

and then bring them here
to Earth

on asteroids or comets.

Narrator: It's possible that
without extraterrestrial gifts

from our neighbors in space,

life on Earth
might never have happened.

Milky Way neighbors
may have helped nurture us,

but the Milky Way has things
that can kill us as well,

with something like this:

an orange dwarf
named Gliese 710.

It's about 60% as massive
as the Sun

and is currently just
63 light-years from Earth

and getting closer.

- Gliese 710
appears to be heading

pretty much straight toward
the solar system.

- As an orange dwarf
approaches the solar system,

it becomes more and more
significant.

When it's about
a light-year away or less,

then it becomes
very important.

Narrator. Almost exactly
1 light-year away from Earth

is a huge region of icy objects
called the Oort cloud.

- The Oort cloud objects
could turn into comets

if they were to come
close enough to the Sun,

but usually
we don't see them at all

because they're so far away
from the Sun.

Narrator: Billions
of potential comets

are waiting for something to
give them a gravitational push,

something like Gliese 710.

- It'll start intersecting
the Oort cloud

or at least
gravitationally disturbing it

in something like
1.3 million years.

Narrator: If Gliese 710
gets close enough,

its gravity could turn
harmless chunks of ice and dust

into rampaging comets
launched at us.

The results for Earth
could be devastating.

- At that point, there could be
a huge onslaught of comets

into the inner solar system

that could lead to another
mass extinction.

We don't know
that that'll happen,

but it could happen.

Narrator: Astronomers say
there's an 86% chance

that Gliese 710 will barrel
right through the Oort cloud.

- So if the orange ball

was like an orange dwarf
like Gliese 710

and the pins
were the Oort cloud,

this is one thing
that could happen.

- All right, but here's

something else
that could happen.

There's a 14% chance
that Gliese 710

is just gonna pass right by,

outside the Oort cloud,
not coming inside it at all.

Narrator: But even without
a direct hit,

the effect
of the star's gravity

could disrupt
at least some comets

and send them
straight for us.

- So the star could knock
just a few comets

toward the inner solar system.

- And all it takes is one comet
to hit Earth

to cause a catastrophe.

Narrator. We've got more than
just Gliese 710 to worry about.

There are more than
150 stars

close enough to disturb us
within the next 2 million years.

- The stars
in our Milky Way galaxy

are all gravitationally bound
together,

so they're moving
in various directions,

overall a rotation around
the center of our galaxy,

but not all the orbits
are exactly the same.

That means,
from our perspective,

a given star might be going
away from us or toward us.

Narrator: And NASA estimates

there are more than 20,000
near-Earth asteroids

more than 300 feet across...

Like 2005 YUSS,

which, in November 2011,

came closer to the Earth
than the Moon.

It might come even closer
in 200 years.

How bad would it be
to get hit by a rock like that?

Think about Nagasaki
at the end of World War I will

and multiply by four.

As we've searched our corner
of the Milky Way

for other neighbors,
bad and good,

we've found
some very unexpected things.

We now have evidence of stars
cold enough to touch

and planets straight out
of science fiction.

Exploring our place
in the Milky Way

has turned up one surprise
after another.

- It's like there was a house
in your neighborhood

that you never knew
was there

and that you've suddenly
discovered,

but it's just down the block.

Narrator:
Take Alpha Centauri,

the brightest star
in the constellation Centaurus

and, after the Sun,
our nearest neighbor star,

4.3 light-years,
or 25 trillion miles, away.

In the 17th century,
astronomers announced

that Alpha Centauri
was really two stars.

Then, in the 20th century,

it turned out to be
a triple system.

- Alpha Centauri A
is very much a sun-like star,

nearly exactly the same mass
as our Sun.

Alpha Centauri B is a little bit
less massive.

The third star,
Proxima Centauri,

is an M-type star.

It's a very low-mass star,

having perhaps only
12% the mass of our Sun.

It's so faint that we can't
see it with our unaided eye.

Narrator: It turns out

that other very well-known
neighbor stars

are also multiple systems.

Sirius,
just 8.6 light-years away

and famed
for thousands of years

as the brightest single star
in the sky,

is really a binary star.

- Most stars are less massive
and smaller than our Sun,

and most stars
are in binary systems.

In both respects, our Sun
is a little bit of an exception.

- The majority of stars
are red dwarfs or brown dwarfs.

Red dwarfs make up
70% of the stars

not only in our galaxy
but in the universe,

and so even though
we orbit our Sun

and we tend to think of it
as the iconic star,

really, the red dwarfs
are far more common.

Narrator:
As for the brown dwarfs,

these are neighbors
we weren't sure existed

until the 1990s.

They're not quite stars,
but they're not planets.

Oh, and they're not really brown
either.

- The brown dwarfs are some
of the most mysterious denizens

of the solar neighborhood

because they're
really very, very cold,

and they're very dark,

and that means that they don't
give off a lot of light,

and they're very difficult
to see.

Narrator: In 2011,
one of NASA's space telescopes,

the Wide-Field Infrared
Survey Explorer, or WISE,

found a series of brown dwarfs
right in our neighborhood,

between 9
and 40 light-years away,

with surface temperatures
once considered impossible.

- One of these brown dwarfs that
we found is actually so cool

that you could touch it
with your hand.

It's only 80 degrees
Fahrenheit,

the same temperature
as a really lovely day

out here on Earth.

And so who knows
what else we'll find.

The more we look,
the more we see.

Narrator. Why are stars
so many different colors?

That's what Anna K.
of Baton Rouge, Louisiana,

texted to ask The Universe.

- Anna, that's a really
interesting question.

Basically, stars have
slightly different colors

because they have different
surface temperatures.

Cool stars like Betelgeuse
look reddish,

and they have temperatures

of only 6,000 or 7,000 degrees
Fahrenheit.

The hottest stars, like Rigel,
appear bluish,

and they're upwards
of 20,000 degrees.

Then there are stars
like the Sun,

with temperatures of 10,000
or 11,000 degrees,

and they look white.

Now, the Sun looks yellow
when it's setting,

but that's because of
atmospheric effects.

Its true color is white.

Narrator: There are more than
stars out there.

We've discovered hundreds
of neighboring planets

inside and outside
the local bubble.

- We have discovered a lot
of exoplanet candidates

through a mission
called Kepler

that is looking at essentially
little eclipses,

when a planet moves
in front of its parent star

and then out,

and the light dips a little
and then goes back up.

- It's very difficult to see,

analogous to watching
for a single light bulb going out

on the Vegas Strip.

But Kepler is capable

of doing these measurements
so precisely

that it's able to find
even planets

as small as our own Earth
around stars like our Sun.

- The first exoplanet
we discovered

was only about 15 years ago,
and it was very much

like we were the only house
on the block,

and we saw the first neighbor
putting up their house,

and ever since then, the entire
neighborhood has grown.

You built up communities
of other exoplanets

out in our local neighborhood.

Narrator: As far as we know,

our nearest planetary neighbor
outside the solar system

is just down the street,
10.5 light-years away,

orbiting the orange star
Epsilon Eridani.

- This planet isn't exactly
something that we could go visit

and expect to find life.

We think that this planet
is more equivalent

to a planet like Jupiter
in our solar system—

A big ball of gas,

which, as we understand it
anyway,

isn't a great place
to look for life.

Narrator: A little farther out,
about 200 light-years,

is another surprise—

A planet that looks like
something

out of a Star Wars movie.

- Just recently,
the Kepler telescope discovered

a planet that orbits
two suns,

and this is a planet called
Kepler-16b.

So this planet, even though
it has similarities to Tatooine,

isn't exactly like
Luke Skywalker's home world.

It's actually a planet
that's icy and gassy,

more like Saturn
than our own Earth.

Now, we were never sure,
prior to this discovery,

whether you could have a planet
that actually has two suns,

and so now that
we've found one,

we know that
these are possible,

and that's really interesting,

because it means that
these binary systems

are good places
to look for planets.

Narrator: In 2011,
astronomers unveiled

a new kind of planet
in our neighborhood—

The homeless.

- There have been
some indications

that there are planets
to be found

that are not in orbit
around their parent star.

They started out in orbit
around their parent star

but somehow got ejected
from their solar system,

and now they're wandering
the galaxy aimlessly,

without a place to call home,

so one wonders if pretty soon
we'll have another

new definition that encompasses
those bodies

that used to be planets

and no longer
have a parent star.

- I think it's still valid

to refer to these ejected bodies
as planets,

because "planet' is
the Greek word for "wanderer,"

and they are certainly wandering
through deep space.

Narrator: We've even learned
new things

about our solar system
neighbors.

In the summer of 2011,

the Hubble Space Telescope
took the first pictures

of the dwarf planet Pluto's

previously unsuspected
fourth moon.

- Now, you might wonder,
Pluto is not all that distant.

Why did it take us so long
to find a fourth moon?

Well, it's because
it's very, very small,

only 10 to 20 miles in diameter,
so it's very faint.

It reflects only a little bit
of the Sun's light.

Narrator: The new Moon
is probably

a frozen, lifeless world
like Pluto itself.

So far, all of our
newly discovered neighbors

have been too hot or too cold
to have any possibility

of our kind of life,

but the search goes on.

- So even though
we haven't done it yet,

we're at this point
where our technology

has caught up to our needs,

and we're actually
going to be able to start

finding those planets like Earth
in the really near future.

However, being able to determine
whether they would be

supportive of life
is a much more difficult task.

Narrator: None of the exoplanets
we've discovered

in our corner of the Milky Way
pose any threat to us,

but what about some of the stars
out there?

Could some of them die
and take us with them?

Our place in the Milky Way
has a lot of plusses.

We're right in the zone
for life to form,

our closest star protects us
from dangerous cosmic rays,

and most of our neighbors
don't disturb us.

But neighborhoods can change.

[siren wailing]

If a fire destroys
a nearby home or business,

your home could also
be damaged,

so imagine what might happen
when a star

goes out of business
as a supernova.

- That means they'll explode

and throw all their innards
back out to the galaxy.

Narrator. Exploding stars
created us,

most of the heavy elements
in the stars around us,

and the gas clouds
the solar system dwells in,

but it's a bad idea
to be too close

when a dying star explodes.

- A supernova explosion

is an incredibly powerful
explosion.

The core of the star
bounces out

and smashes into
the outer layers

and blows them out
into the galaxy.

So what actually happens
is that material

gets thrown out
in a shock wave that,

if you're near enough
to the shock wave,

would be destructive.

- If it's 10 light-years away
or so,

then high-energy radiation
like from X-rays and gamma rays

can harm us.

They can, for example,
destroy part of the ozone layer.

What happens is,
the radiation comes in,

disrupts nitrogen molecules.

The nitrogen atoms
then combine with oxygen

to form nitric oxide.

That nitric oxide, NO,

disrupts ozone molecules, O3,

and forms nitrogen dioxide,
NO2.

The nitrogen dioxide can then
combine with atomic oxygen,

forming more nitric oxide,

which then disrupts
more ozone,

which leads to
a snowball effect.

So within a few weeks, you can
destroy much of the ozone layer,

allowing the Sun's ultraviolet
radiation to come in,

and that would then
kill life

that's on the surface layers
of an ocean or in ponds.

Narrator: That death toll
would include the phytoplankton

that are the foundation
of the marine food chain

and provide 50%
of the Earth's oxygen,

and that would spell doom
for most larger forms of life,

including us.

One candidate for
stellar extinction lies

outside the local bubble,
although it's been

a familiar sight
for thousands of years:

the red super giant Betelgeuse.

The star, between 500 and 800
light-years away

and 20 times
the mass of the Sun,

forms the right shoulder
of the constellation Orion.

- Betelgeuse is getting near
the end of its life.

Narrator:
Between 1996 and 2011,

Betelgeuse shrank by 15%

for reasons
that are still not understood.

The red giant may go supernova
in half a million years,

or it may have already
happened.

- It's conceivable
that Betelgeuse

will go supernova tonight
or tomorrow night or next week,

but it's much more likely
to become a supernova

in 100,000 years
or in a few 100,000 years.

Given that Betelgeuse

is at least a few hundred
light-years away,

it's possible that
it's already blown up,

and we just don't know it

because the light
hasn't reached us yet.

Narrator:
The good news is that

even if Orion
does dislocate its shoulder,

Betelgeuse is too far away
to harm our neighborhood.

But then there's HR 8210,

about 150 light-years away,

in the constellation Pegasus.

It's not one star but two—

A star and a white dwarf
in binary orbit

around each other.

The white dwarf is about
15% more massive than our Sun-

not at the supernova
tipping point yet.

- HR 8210
is this binary system,

two stars that are orbiting
one another,

one of which has actually
already died

and is a white dwarf.

Now, this system
has the potential

that when the star
that's very hot right now

starts to go through
its death throes

and stars to puff up
as it dies,

it might start to pour material
onto that white dwarf.

Essentially these systems
are like zombie stars

eating their companions.

- When that normal star
starts to expand,

the white dwarf will start
stealing material

from its companion,
becoming more massive,

and if it reaches
a certain unstable limit,

it'll blow up
as a type la supernova.

Narrator:
Are we far enough away

to avoid being collateral damage
when HR 8210 explodes?

- If you want to be completely
safe from a supernova,

you should be at least
100 light-years away.

10 light-years
might be enough,

but it might not.

It depends on what effect
kills you first.

- But that won't happen
for a really long time,

and by then
we will have moved off,

and it will have moved off

because everything
in the galaxy

is really on its way
somewhere.

So over time,
that might happen,

but at the point that it does,

it probably won't be very close
to Earth at all anymore.

Narrator: But don't feel
too comfortable.

The threat of HR 8210
was only discovered in 2002.

How many more potential
supernovas are out there?

How close are they to us?

And how soon
will they explode?

To possibly
make matters worse,

some astronomers say that
there are a lot more supernovas

in our neighborhood's future.

- Our solar system orbits
our galaxy at a different rate

than the spiral arms do.

That means, eventually,
we're gonna enter a spiral arm,

and because there is
a lot more massive stars there,

some of them will be
ending their lives,

creating supernovae
and posing a greater threat

to life on Earth.

Narrator: Still, our place
in the Milky Way

is secure for tonight

and for at least
a few million nights to come—

Plenty of time
for more exploration

and more surprises.

- We live in a pretty diverse
neighborhood, actually,

and things are changing.

The galaxy
is not a static place,

so it's gonna be
an interesting place to see

in a billion years.