Original subtitles

The fate of our galaxy hangs in the balance.

The Milky Way is dying, and we don't know why.

Our galaxy, like all galaxies, has a limited life-span.

After that, it's lights out.

The race is on to find a smoking gun.

It's safe to say right now there are many ways to kill a galaxy.

It's a cosmic crime scene investigation.

Is it murder most foul?

Or is it death by natural causes?

The suspects are lined up.

The interrogation is underway.

It's another example of this big universe of ours

throwing puzzles at us that now we have to solve.

What is killing the Milky Way?

Captions by vitac... www.vitac.com

captions paid for by discovery communications

Earth.

Our home.

Just one of 100 billion planets

orbiting 400 billion stars

that make up an immense galactic spiral...

the Milky Way.

Galaxies are where stars form,

and, of course, planets form around stars,

so the story of the Earth,

of yourself, of the solar system

has everything to do with the story of the galaxies.

The story of the Milky Way begins 13.6 billion years ago,

just after the big bang.

It's a time when there are no planets and no stars...

Just a vast, lumpy soup of superheated hydrogen gas.

Over millions of years, the temperature drops,

and gravity compresses the lumps down, until eventually

the hydrogen molecules fuse and ignite a star.

In time, billions of stars burst into life.

And the Milky Way begins to take shape.

You can think of a galaxy as sort of like a human being.

When you're young and in your adolescent stage,

you're vibrant and active.

That's a young galaxy forming stars in a crazy way,

and it's not even fully formed yet.

At a certain point, galaxy reaches middle age,

and a middle-aged galaxy really is what it's going to be...

It has its shape...

But in the long run, a galaxy will stop forming stars,

and eventually, just like we all die, our galaxy will die.

So, at what stage of life is the Milky Way?

Is it a healthy, active youngster,

or is it heading for its deathbed?

Scientists can determine each galaxy's stage of life

by its color.

So, we see different colors of galaxies in the universe.

We see galaxies that are tinted blue

and galaxies that are tinted red.

When we see a blue galaxy, that tends to be a younger galaxy

full of bright, hot, newly formed stars.

When we see a redder galaxy,

that tends to be a dimmer, older galaxy

that isn't forming new stars in the present moment.

All its stars are aged and older and redder,

and so the entire galaxy casts a different hue.

So, what color is our galaxy?

It's a simple question, but the answer is hard to come by,

even though we've been looking at the Milky Way

for thousands of years.

The term "Milky Way" is ancient.

It goes back to a time when in the dark sky,

people noticed there was this light band

that actually went from horizon to horizon,

and that band turned out

to be made of thousands and thousands of stars

actually too far away to see individually.

But it took us a long time to realize

what the shape and the scale of the Milky Way galaxy is.

The amazing thing to think about is that we actually don't know

our home galaxy very well at all.

We actually live in the middle of this disk of gas and dust,

and that obscures our view of the larger Milky Way.

Using visible light, we can't even see to the center,

let alone the other side of the Milky Way galaxy.

The solution is to use a form of light

that passes through the gas and dust...

Infrared.

This is the Sloan digital sky survey telescope

at the Apache point observatory in new Mexico.

It's mapping the galaxy using infrared

and giving scientists unprecedented insights.

The first sensitive infrared observations really weren't done

till the last 15 years,

and each of these new windows on the universe

teach us different things.

In the last 15 years,

Sloan has surveyed more than 250 million stars,

analyzing their light to work out the color of the Milky Way.

And what scientists saw shocked them.

Until very recently, we thought the Milky Way

was a young, healthy galaxy,

but now there's evidence

that we may be entering the pathway to death.

The Sloan telescope reveals that star production in our galaxy

is falling through the floor.

The Milky Way is dying.

And when it stops forming new stars,

its time will be up.

Paradoxically, our galaxy still has

star-forming gas in the tank, so it should be healthy,

but something is killing it off.

So, the Milky Way galaxy is this wonderful disk

filled with rich hydrogen gas, lots of dense dust clouds.

It has everything you need there for star formation,

but it seems to be slowing down and even turning off,

and right now, we don't really understand what the culprit is.

With a galaxy killer at large,

scientists embark upon the biggest murder investigation

in the history of the universe.

Everything in science, when you're exploring a problem,

is a bit like a crime scene.

You've got the evidence laid out in front of you...

and we have to figure out who done it.

Our home in the universe is dying...

Not the Earth, but our galaxy, the Milky Way.

It's been producing stars for billions of years,

but soon, it will stop.

Our own sun formed about 4 1/2 billion years ago

in the Milky Way galaxy,

and we are not the oldest star by far.

And yet, tragically, we actually seem to be

one of the last generations of new stars in the Milky Way.

Current projections suggest that in about 4 billion years,

star formation may have ceased all together,

which is almost just a blink of an eye

in the life cycle of the universe.

To find out why, scientists launch an investigation.

The most crucial question?

How is the milky way dying?

To kill a galaxy, you have to get rid of the cold gas,

because that's what stars form from.

There are many ways you can do this.

You can blast it out from the inside.

You can draw it out from the outside.

You can heat it up so it's no longer cold.

You can use it all up,

and there's even more ways you can stop it.

What we have to do is figure out

which way is happening in our galaxy.

Perhaps the culprit is inside the Milky Way itself.

A clue comes from another galaxy entirely.

This is w2246-0526.

Scientists call it a hot, dust-obscured galaxy,

or "hot dog" for short.

This galaxy is 12 1/2 billion light-years away.

It's the most luminous galaxy we know of in the universe.

It has the light of 300 trillion stars.

The source of the intense light is not its stars,

but a mysterious object at the galaxy's center.

It's a million times smaller than the galaxy itself.

There's only one thing that small and that powerful...

a supermassive black hole.

So, supermassive black holes, as the name suggests,

are indeed supermassive.

These are billions of times more massive than our sun.

These are gigantic objects.

The gravity in the supermassive black hole is off the charts.

It sucks in incredible amounts

of the hot dog's vital star-forming gas.

And as the gas swirls to form a disk,

the intense friction superheats it to millions of degrees

and, in some galaxies, triggers huge jets.

When a lot of material falls onto that black hole,

it creates incredibly energetic jets

that can be tens of thousands of light-years across.

All of a sudden, you have this blowtorch

in the middle of the galaxy.

Black hole jets are bad for galaxies

because they can shut down star formation.

They can heat gas up, blow gas out of galaxies,

and they could really kill them.

A supermassive black hole is cooking the hot dog.

What's going on in our galaxy?

In 2016, scientists at Harvard discovered damning evidence

that may link the Milky Way's supermassive black hole

to the galaxy's demise.

Just like the hot dog,

the Milky Way is surrounded by a vast cloud of blown-out gas,

and the scientists traced the gas back to its source...

Sagittarius a-star, our supermassive black hole.

Well, it turns out our supermassive black hole

had a bit of a hiccup about 6 million years ago.

There's evidence that some matter must have fallen

into that black hole, and if it fell in too quickly,

it would have gotten superheated by its own friction,

and this would have acted, in a sense, like an explosion.

And that event was huge.

Our galaxy expelled an incredible amount of gas...

130 billion times the mass of the Sun.

Large amount of gas.

This event must have been very catastrophic

for the inner parts of the galaxy.

Luckily, Earth is in the outer parts of the galaxy,

where we were able to survive this event.

Is this the smoking gun?

Is our own supermassive black hole killing the Milky Way?

The evidence seems to mount up.

But Sagittarius a-star has an alibi.

It exploded too late.

Sagittarius a-star got very active,

very explosive about 6 million years ago,

but that's so recent, it shouldn't have really affected

the star formation rates.

Something else is going on.

There must be another culprit besides the black hole.

Studies suggest our supermassive black hole must have been active

hundreds of millions of years ago

to stop all star formation in our galaxy.

Sagittarius a-star wasn't active at that time,

so it's no longer a suspect.

The hunt is on for a different galaxy killer,

and scientists are widening the investigation.

Maybe the killer isn't inside our galaxy.

It could be that we suffered a hit-and-run.

Our universe is a crime scene.

Star production in the Milky Way is breaking down.

Our galaxy is dying,

and astronomers are examining the body for clues.

The Milky Way's disk is made up of three sections...

A nucleus, home to the galaxy's supermassive black hole...

a dense, central bulge 10,000 light-years across,

and the spiral arms...

Full of gas, dust, and billions of stars.

The spiral arms should be flat, but they're rippling.

Is this a clue for the cosmic detectives?

Today, we look at the edge of the Milky Way,

and we see mysterious ripples in its gas,

and we wonder, what's the origin?

Something must have caused it to happen.

Something like that just doesn't happen on its own.

The real question is, why?

Whatever caused the ripples didn't hang around.

Is this evidence of a galactic hit-and-run?

January 2016.

Astronomers studying data from the vista telescope

discover something incredible...

three nearby stars.

On their own, nothing special,

except they've recently left our galaxy,

and they're traveling at 350,000 miles an hour.

So, we've discovered these stars that are careening

out of the galaxy at super-high velocities.

Could these three stars somehow be responsible

for warping the Milky Way's disk?

Well, absolutely not.

The Milky Way is so much more massive than just three stars.

Three stars alone can't warp a galaxy,

but those three stars can be indicative of more stars.

They can be indicative of the presence of, say,

a dwarf galaxy, and that can warp the galaxy.

Dwarf galaxies are abundant.

But a tiny fraction of the size of a major galaxy,

like the Milky Way.

So they're difficult to detect.

But these three bright stars show there's a dwarf galaxy

hiding beyond the edge of the Milky Way.

And scientists can study the trio of stars

to rewind the clock

and track back the past movements of the dwarf galaxy.

Simulations suggest that millions of years ago,

this dwarf galaxy

punched through the plane of the Milky Way.

As the fast-moving dwarf galaxy hurtles towards the Milky Way,

millions of stars seem set on a collision course.

Catastrophe looks inevitable.

But appearances can be deceptive.

When galaxies collide, the first thing you might imagine

is that the stars collide, but actually, that doesn't happen.

Galaxies are mostly empty space.

If you took the Sun, which is really big...

It's a million miles across...

And shrunk it down to the size of a piece of pollen,

the galaxy itself would be twice the size of the pacific ocean,

and the nearest star to the Sun would be a mile away.

Those tiny pieces of pollen are never going to hit each other.

The distances involved are staggering.

And at the moment of impact,

most of the stars from the two galaxies

miss each other entirely.

But that doesn't mean the Milky Way is safe.

Even though the stars just pass each other,

they do gravitationally interact as they come close,

and this gravitational interaction sets them

on a course that is different

than if they were to live by themselves.

In much the same way that taking a stone

and dropping it into a still pond

creates ripples in the water,

a galaxy like this slamming into the Milky Way

can create ripple effects throughout the disk.

The ripples in the Milky Way

stretch across tens of thousands of light-years.

Still, this hit-and-run isn't enough to kill the Milky Way.

It only causes a flesh wound.

But what if this dwarf galaxy isn't acting alone?

What if it has accomplices?

There are a lot of dwarf galaxies out there,

and it turns out collisions between these dwarf galaxies

and big galaxies, like the Milky Way, are common.

They happen all the time.

Right now, there are several dwarf galaxies

that the Milky Way is swallowing up.

In fact, a really fun thing is that we're actually closer

to the core of one of these galaxies...

The Canis Majoris dwarf galaxy...

Than we are to the core of the Milky Way.

So some of the stars that you see around you in the night sky

are actually stars from a different galaxy.

So, what happens when all these dwarf galaxies come together

and start pulling and tugging on a larger galaxy?

Cosmologists believe there could be hundreds of dwarf galaxies

surrounding the Milky Way.

A collision with just one of these dwarf galaxies

may have rippled the Milky Way's spiral arms,

but a gang of dwarf galaxies

could have a far bigger and far more deadly effect.

Dwarf galaxies and the way they interact with big galaxies,

like the Milky Way,

can inflect tremendous change in our universe.

When they slam into a galaxy, they can change its structure.

The Milky Way would not look anything like it looks today

without those dwarf galaxies.

Repeated dwarf-galaxy collisions

could have radically altered the shape of the Milky Way itself.

Their gravitational disruptions could have created a distinctive

and possibly fatal feature in the middle of our galaxy...

The galactic bar.

The center of the Milky Way is elongated.

Instead of it being shaped like a sphere,

it's more shaped like a bar, and the bar is made by stars

actually orbiting in this sort of elongated way.

And this bar can be bad for the health of the galaxy

because what they do is help to funnel gas

into the core of the galaxy.

The loss of this gas could be a way of stopping star formation.

The bar-shaped bulge at the center of the Milky Way

sweeps our galaxy's star-building gas

into the galactic nucleus.

Here, it gets gobbled up

by our galaxy's supermassive black hole.

Without the star-building material, no new stars can form,

and the galaxy dies.

So, is it case closed?

Are dwarf galaxies killing the Milky Way?

Is the murder weapon a galactic bar?

So, it's possible that the formation of these bars

helps turn off star formation in the very core of the galaxy,

but that's just the central regions of the galaxy.

That doesn't explain what's going farther out

in the spiral arms.

So, if star formation really is shutting down in the Milky Way,

it's not really the fault of the bar.

Dwarf galaxies cause the Milky Way grievous bodily harm

by creating the galactic bar.

But they're off the hook for attempted galactic murder.

The investigation continues,

and it could be about to take a dramatic twist.

It might not be that the galaxy's being murdered.

It could just be eating itself to death.

The Milky Way is being killed off.

And the perpetrator remains at large.

Scientists investigating the crime

are running out of suspects.

But the hunt for clues continues,

so astronomers are examining the dying body of the Milky Way.

Our galaxy is a hazy disk of stars

surrounded by a halo of superheated gas.

It's over 100,000 light-years across.

But it hasn't always been so big.

When you think about things so vast,

so gigantic and ancient as galaxies,

you're kind of tempted to think

that they're very stable objects,

that they don't change much over time,

but we now know that our own galaxy

is the product of many smaller galaxies

that came together over time,

and there are other galaxies still colliding with us.

We see galaxies eating each other all the time.

They collide, and if one galaxy is very big

and one galaxy is very small,

the little galaxy falls into the big one, gets torn apart,

and becomes a part of that bigger galaxy.

The Milky Way might be dying,

but it's still a monster foraging through the universe,

swallowing smaller galaxies whole.

It consumes their stars.

But it also has a taste for their star-building gas.

And it doesn't have to collide with other galaxies

to feed off of them.

Now, the lifeblood of a galaxy is hydrogen gas.

That's what actually creates new stars.

So as a dwarf galaxy passes by the Milky Way,

the tremendously massive halo of the Milky Way,

all of that gas, can draw off material from the dwarf galaxy,

adding it to the Milky Way.

So in this way, the Milky Way drains away

the lifeblood of other galaxies.

In some sense, you could say it's a vampire

because a vampire sucks the life out of other things

so it can remain young.

In its 13 billion-year life, our vampire galaxy has feasted.

Consuming the lifeblood of its galactic victims,

the Milky Way has grown fat.

But could this monstrous feeding frenzy be a factor

in the Milky Way's demise?

Once again, crucial evidence

comes from the Sloan digital sky survey.

Their telescope maps the stars in our galaxy,

but it also maps the galaxies in our universe.

Looking at distant galaxies is like looking back in time.

Because the farther away they are,

the longer their light takes to reach us.

We see the most distant galaxies not as they are now,

but as they were... Billions of years ago.

So, when you look at these galaxies,

you're seeing them as they were when they were very young,

and you're seeing these galaxies as they are more recently,

so you can actually look at the evolution...

How galaxies change over time as the universe ages.

While studying the data,

scientists make a dramatic discovery.

They find spiral galaxies, just like the Milky Way,

dying all over the universe.

And what connects them is their mass.

There seems to be an upper weight limit

for the sizes of spiral galaxies.

Up to about a trillion times the mass of the Sun,

we see spiral galaxies that continue to form stars,

but once they pass this threshold,

galaxies tend to die and run out of stars.

While devouring the star-building gas

of smaller galaxies,

the Milky Way may have grown obese,

and now it could be choking to death on its own dinner.

But how?

Once a spiral galaxy is sufficiently big,

it's going to have an incredible gravitational force,

so any gas that it pulls to itself

is going to come in at an incredibly high speed.

That gas is going to be superheated.

The superheated gas moves so quickly

that it's prevented from falling into the Milky Way.

The gas is too energetic for our galaxy's gravity to pull it in.

Instead, it stays in the halo around the Milky Way,

and our galaxy's food supply is choked off.

Eventually, our galaxy will starve.

This will only happen if the Milky Way

is over the star-building weight limit.

But how exactly do you weigh a galaxy?

One basic way we can weigh a galaxy

is measure how fast the stars are moving within it.

So the faster the stars orbit around the center of the galaxy,

the more massive the galaxy is.

This method of weighing the Milky Way relies on gravity.

Fast-moving stars need more gravity to hold them

in their orbits,

and more gravity means more galactic mass.

When scientists use this information to run the math,

the horrible truth is revealed.

We've passed kind of a critical level.

The Milky Way is far too massive for its own health,

and we've entered the beginning of the end.

We're running out of gas, and I mean this literally.

Gas clouds form stars,

and as they form stars, they're used up,

and so our gas tank is getting closer and closer

to empty every day.

The investigation into the killing of the Milky Way

is closed.

The verdict?

The greedy Milky Way is killing itself.

Over millions of years, star formation grinds to a halt,

and the galaxy dies.

But could the galaxy be resurrected?

We seem to be telling a very sad story.

We're talking about the demise of the Milky Way galaxy...

The end of star formation...

But maybe it's just a little bit too soon

to write the death announcement yet.

Hope could be just over the horizon.

In space and astrophysics, really anything is possible.

The shocking case of our dying galaxy has been solved.

There was no killer.

Turns out, the Milky Way is eating itself to death.

But is this really the end?

Could salvation be heading our way?

Even if star formation is turning off

in the Milky Way now,

we know that it's on a collision course

with the Andromeda galaxy.

They're moving toward each other

at hundreds of thousands of miles per hour.

A collision sounds like something

that's always destructive,

but that's not necessarily the case.

The Milky Way's collision

with our giant galactic neighbor Andromeda

won't happen for another 4 billion years.

By then, star formation in both of these galaxies

will have stopped completely.

But a giant meet-up could change all that.

As an isolated galaxy,

the Milky Way is already in its wind-down phase.

It's not producing as many new stars as it used to.

But there is one way to generate a new round of star formation,

and that's through a galactic merger event.

When Andromeda gets close enough,

the mutual gravity between the two galaxies

will start to stretch them out, pulling them out like Taffy.

Stars will be pulled out into these long, looping streamers,

and then the galaxies

will physically pass through each other.

Eventually, the two galaxies will draw back together again

and merge into one gigantic galaxy,

and at that point, all of these gas clouds

will flash into star formation.

As the galaxies merge, they'll be reborn.

Two dying spiral-shaped galaxies

become a single living elliptical galaxy

called Milkomeda.

Imagine you're living in the far future of the galaxy

and you see the night sky

while the Milky Way and Andromeda are colliding.

It will look like a very different place.

Rather than one band across the night sky,

you might have two as the two disks come together.

It will be a miraculous sight,

but a very, very different place than we have today.

Our sky will light up for the first time in billions of years.

Star formation will flare across the galaxy.

But is it too soon to celebrate?

This new round of star formation

during the merger of our two galaxies...

While it's very cool for a little bit,

once it's over, that kind of sends the new galaxy

into a death spiral.

When new stars are born in this new galaxy,

many of them are going to be hot, large, blue stars.

Eventually, those young, hot stars are going to start to die,

and when they do, they're going to explode

violently as supernovae.

And those supernovae are going to start

blasting gas out of the galaxy.

All of the gas is gone.

There's no more stuff to form stars.

And that's what kills a galaxy.

It'll take hundreds of millions of years

for Milkomeda to run out of star-building gas.

And then our new elliptical galaxy will starve.

But the final blow is still to come.

Another issue to consider is what happens

to the two supermassive black holes

at the cores of the two galaxies.

Well, initially, they're going to orbit each other,

stirring up a lot of turbulence, and they're going to combine.

And because there's a lot of new, hot, fresh gas,

our new galaxy is going to be a quasar.

And that quasar is going to turn up the heat,

it's going to turn up the turbulence,

and this means star formation is going to be shut off.

The combined power of the supermassive black holes

help create a quasar that tears through the galaxy.

It releases ferocious beams of radiation

that blast through Milkomeda's star-forming gas.

It's only just been reborn,

but our newly enlarged galaxy is once again dying.

Vast galaxies, like Milkomeda, seem doomed from the start.

Their size creates too many problems for star formation.

Or... so we thought.

The more galaxies we see, the more we realize

there's a lot out there we haven't discovered,

and there's a new class of galaxies

only recently identified.

These galaxies are more than 10 times the mass of the Milky Way.

And, intriguingly, they're still forming stars.

Apparently we've missed something.

4 billion years from now, the Milky Way is no more.

After colliding with Andromeda, it's reborn

as a giant elliptical galaxy called Milkomeda.

Scientists thought galaxies this big were doomed.

But is hope on the horizon?

The Sloan digital sky survey has spent a decade

studying over a million galaxies.

It's discovered a rare but enormous kind of galaxy...

A super spiral.

These super-spiral galaxies are spiral galaxies

that are incredibly super,

and by "super," I mean they have four times the size,

10 times the mass,

and they're weird because they exceed

the supposed weight limit for spiral galaxies.

So they shouldn't have new stars, but they do.

They're very healthy galaxies.

Scientists have found just 53 super spirals.

Super-spiral galaxies show that in rare situations,

massive galaxies continue to produce new stars.

So, is this a lifeline for Milkomeda?

When we think about two galaxies colliding,

a lot of our computer models suggest

that they really mess each other up.

Things get very chaotic.

But over time, could they settle back down into a spiral shape?

And, in fact, that may be what happens with super spirals.

One of the clues is that many super-spiral galaxies

have double cores.

Instead of there just being one supermassive black hole,

there are actually two orbiting each other.

The fact that we see spiral galaxies with two cores

makes it possible that you could have a collision

and still survive as a spiral galaxy.

So maybe there's hope that even the Milky Way will be a spiral

once it collides with Andromeda.

Picture the scene... 6 billion years in the future.

Milkomeda drifts through the universe...

not as an elliptical galaxy, but as a super spiral.

This shape means the galaxy is far more stable.

The damaging heat and turbulence

generated by Milkomeda's supermassive black holes

can't disrupt star-building gas way out in the spiral arms.

Far from dying off, our galaxy lives on...

Larger than ever before.

But that isn't the end of the story.

Tens of billions of years from now,

could the galaxy continue to grow?

Our local group of galaxies...

Milky Way, Andromeda, Triangulum...

And then a collection of dwarf satellite galaxies...

Is gravitationally bound together,

and eventually, we're all glued together

into a single massive object.

What does this mean?

This means we might be part of one of the largest structures

in the universe.

During its billion years of life,

the Milky Way changes beyond recognition.

It suffers countless collisions,

feasts on many smaller galaxies,

and gives birth to innumerable stars.

We talk about the life cycle of galaxies...

How they're born, how they live healthy lives making new stars,

and eventually how they die away.

It's really not as depressing as that.

Everything in the universe changes.

Galaxies like ours are in a constant state of flux.

So when it comes to the Milky Way,

death really isn't the end.

What we see in our universe

is that there's always a process of birth and rebirth,

so the future of the Milky Way

is that it's going to keep on doing what it does.

Galaxies are ever-changing.

10 billion years ago, the Milky Way

was nothing like what it is today,

and certainly, 10 billion years in the future,

it'll be a very different place.

Look, I live in this galaxy.

I hope that it can find a way to rejuvenate itself

through collisions or some other process

because that gives me some hope

that it'll go on for a long, long time.

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