All language subtitles for How the Universe Works - 6X07 The Quasar Enigma

Mysterious lights

shine out
from the edge of space,

brighter than a trillion suns.

They had to be
the brightest objects

we've ever seen in the universe,
putting out amounts of energy

that we couldn't
possibly explain.

So powerful,
they can incinerate planets

and rip stars to pieces.

These are among
the most mysterious

and most energetic phenomenon
in the universe.

They can destroy galaxies,

but may also be the key
to their survival.

These objects are a hotbed
of all kinds of crazy physics.

These celestial
powerhouses are called quasars,

and we may owe them
our very existence.

For decades,
astronomers have observed

brilliant points of light
in the night sky,

but there was something
strange about them.

They looked like a pinprick
of light, like a star,

and so they were really
mysterious initially.

Are they a new type of star,

or something else entirely?

One of these strange objects
is hiding within

the Virgo galaxy cluster.

From Earth, object 3c 273
looks just like a nearby star,

but scientists studying its
light made a stunning discovery.

It was tremendously far away.

It not only was not
in our galaxy,

it wasn't even in any galaxy
that they could see.

It was over a billion
light-years away.

If it's that far away
and as bright as it was,

this must be the most luminous
known object in the universe.

And this is one of the reasons
these were so mysterious

for so long.

These objects
are so bright that,

despite the incredible distance,

to us, they look
like nearby stars.

They're called quasi-stellar
objects, quasars for short.

But what are they?

With more detailed
observations of quasars,

we found that they don't
originate from any random place.

They always come from
the core of a galaxy.

A quasar
is the ultra-bright core

of an extremely distant galaxy.

The reason we can see them
at all

is the result of
their incredible power.

A single quasar outshines
an entire galaxy,

hundreds of billions of stars'
worth of energetic output,

all concentrated
into a single source.

One of the most energetic events
that human beings

have ever been
witness to on Earth

is the biggest atomic bomb
ever exploded.

We're aware of quasars out there
that are putting out more energy

than 1 trillion trillion
of those massive

atomic bomb blasts per second.

It is just a huge amount
of energy packed into

a very, very, very small volume,

and that is almost
literally unbelievable.

But if they're
so small, how can they produce

such vast amounts of energy?

How can you generate something
that's so bright, so energetic,

but not be very large?

What could possibly power
something like that?

Hiding within a quasar

must be a very powerful engine.

What kind of object can generate
that much energy,

that kind of power
to create these things?

There's only one thing
in the universe

that's both massive enough
and dense enough...

A black hole.

That is the only thing that
we know of in the universe

that could power a quasar.

Eventually, stars
25 times more massive

than our sun lose their
lifelong battle against gravity.

They suffer
a catastrophic collapse.

All their incredible mass
is compressed

into a single point...

Giving birth to a black hole.

Black holes are totally unique.

There's nothing else
like them in the universe.

They're extremely
massive and dense.

They have so much mass crammed
in such a small space

that they distort space
a tremendous amount.

They create regions
called event horizons.

The boundary of these
monsters, the event horizon,

is a point of no return.

Anything that crosses that line
is never coming back,

not even light.

So as light tries to freely fly
through space and time,

that space and time
is bent back in on itself,

and that means light
can never escape.

We see black holes
across the universe.

They range in mass from regular,

three times more massive
than our sun,

to supersized...
Supermassive, in fact.

So we think that quasars
represent

the largest black holes
that we see in the universe.

We're talking billions of times
more massive than our sun.

It is literally almost
at the edge of your ability

to perceive, right,
our ability to even

think about black holes
being so massive,

that they're a billion times
the mass of our sun.

It's this enormous
mass that leads

to their enormous gravity.

We think that only
supermassive black holes

could provide the power,

but quasars outshine
their entire galaxies.

Black holes suck things down.
They're black.

How can they possibly be bright?

Black holes are
voracious eaters.

They drag in gas and dust,

which builds up in a ring of
material around the black hole.

We call this the accretion disk.

You could think of it
as a giant whirlpool of matter

that's trying to fall onto
the supermassive black hole.

Well, it can't all
fall in at once,

and so there's a lot
of friction in this disk.

This friction increases

as the motion of the gas
and dust speeds up.

That could be a significant
fraction of the speed of light.

If you rub your hands together
at a significant fraction

of the speed of light,
they will vaporize.

They will get very hot.

And so the material
in this accretion disk

can actually get heated
to millions of degrees.

When matter heats up,
it produces radiation,

which we see as light.

The center of the galaxy shines,

visible billions
of light-years away.

That's where stuff
gets dense and hot,

and there, you have a quasar.

So even though black holes are
the darkest objects

in the universe, they, in turn,

power the brightest objects
in the universe.

Quasars are so bright,

some are visible
from the edge of the universe,

or 13 billion light-years away.

That means they burst into life
less than a billion years

after the big bang.

How could such monsters exist

so soon after the birth
of the universe?

Across the universe,

we have discovered
stunningly luminous quasars.

They're powerful, brighter
than their whole galaxies,

and that light has been
traveling towards us

for billions of years.

As fast as light goes,
186,000 miles per second,

it takes time to cover the vast
distances between the galaxies,

so if you look out into space,

you see quasars as they
were millions of years ago

or billions of years ago,

and the light has just arrived
at your eyes tonight.

2017, Chile...

Scientists at
the Los Campanas observatory

focus their telescopes

on the most ancient part
of the universe.

They get a huge surprise.

This quasar is only about 600,

700 million years
after the big bang,

and this black hole weighs
about 800 million times

as much as the sun.

It's the oldest
quasar ever found.

It burst into life
around 700 million years

after the big bang.

Back then, the universe
was mostly a soup of hydrogen

and helium gas.

We know quasars are powered
by supermassive black holes,

but discovering a black hole
this big so early

in the evolution of the cosmos,
that's a huge mystery.

One of the big questions
we have in astronomy is,

how did these supermassive
black holes form?

How did they form so early
in the universe?

This is an interesting mystery.

We see quasars about
as far as we can see.

That means these objects existed
in the very earliest galaxy.

That means a million or billion
solar mass objects

were able to accumulate.

I don't think we really have
a good picture

of how that can happen.

The mystery lies
in how fast black holes grow.

They grow by eating
at an astonishing rate.

When you eat, eventually,
you get full.

You've had your last bite.

But black holes, they are
never done being hungry.

They are insatiable.

A black hole eats
everything that gets too close,

growing larger and larger.

But there's a limit
to how fast they can grow.

There's not enough time,

a billion years after
the universe was created,

for them to get to a billion
solar masses in...

It's just too short a time,

so there had to be
another process,

in addition to all this eating,
that caused the seed to form.

To bulk up
to a billion solar masses,

we now think these giants grew
from smaller seed black holes.

We know that black holes usually
form from exploding stars

of around 25 solar masses
or more.

That's pretty small.

To be born supermassive,
you'd need a supermassive star,

a stellar giant born
from the primordial gases

of the early universe.

And so you had these
huge clouds of mostly hydrogen,

a little bit of helium,

and that's basically
all that was there,

and as material cooled,

it would collapse
into these big,

big stars of just balls
of hydrogen in space.

These super-giant
stars lived fast and died young.

When they died, they
formed these seed black holes.

So that's one way you
could get a massive seed,

is that you just have
the earliest stars

collapsing into a black hole
when they die.

But there's a problem.

Seems even these supermassive
stars couldn't have produced

big enough seed black holes.

There has to be another way
to generate a billion-solar-mass

black hole
in less than a billion years.

How else
could the universe create

supermassive black holes

from just thick clouds of gas?

One theory of how you get
these supermassive black holes

is that you just have
one big collapse event.

We call it direct collapse.

It's only a theory,

but we think this is
how direct collapse would work.

Huge, super-dense clouds
of hydrogen gas clump together.

Gravity builds up,
dragging in more gas,

becoming more and more dense

until the gas collapses
under its own weight.

Instead of forming a star,
it crushes down straight

to a supermassive black hole.

A galaxy starts to form
around the giant.

Gas streams toward the center,
getting hotter and hotter,

until a quasar
explodes into life.

They're so bright,
we can see them today,

13 billion light-years away.

All of these are ideas
right now.

Theorists are working really
hard to make these models work.

Finding more and more
distant quasars

will teach us what those seeds
are that form into the quasars.

It might teach us
some new physics

to create these big
black holes that quickly.

The more
we investigate the quasars,

the more we discover
about the early universe,

but they're also revealing
their destructive nature.

Recently, we've
discovered galaxies

with holes torn out of them.

The culprit?

Death rays

blasting at close
to the speed of light.

The universe is
full of galaxies.

From a distance,
they look calm and peaceful.

Look a little closer, and you'll
see evidence of extreme violence

and destruction...

Scars extending
from their centers

out tens of thousands
of light-years.

What could've caused
such devastation?

This is galaxy cluster hydra a.

Here are the scars, but viewing
it in multiple wavelengths

reveals the culprit.

Two colossal jets of energy

blasting out from
the galactic core,

shooting out from
the heart of a quasar.

They tear through the galaxy
and out into space,

creating voids
in the surrounding gas.

The amount of energy in these
jets is staggering,

soul crushing, mind destroying.

Think about how much energy is
wrapped up in these quasar jets.

Something can take many,
many times the mass of the sun,

accelerate it to speeds
near the speed of light

and throw it out across hundreds
of thousands of light-years.

These jets seem to launch out

from the quasar's core,

supercharged particles twisted
into tight beams of energy,

moving at millions
of miles per hour,

heated to trillions of degrees.

Regular quasars
are seriously powerful,

but quasars with jets,
those are off the charts.

You're taking the power

of billions or trillions
of stars

and focusing them
into narrow jets,

which basically march across
the universe like death rays.

If you're too close to this
thing, and you're in its way,

yeah, you're not going
to be in its way for long.

It's not just the galaxy

and the surrounding gas
that suffers.

These jets coming out are more
powerful than the death star.

They would destroy not just a
planet going through their path

but, like, stars,
whole solar systems.

This is exactly what's
going on in system 3c 321.

Viewed in visible light, all
we see is a pair of galaxies.

But in multiple wavelengths,
we see the larger galaxy

firing out a huge death ray,

ripping through its smaller
neighbor and out into space.

Imagine being in
the direction of a jet.

If a jet's coming at you,
it's over.

You're in for
a deep world of hurt.

Planets would be destroyed.

Stars would explode.

These jets travel incredible
distances through space.

So the largest know jet is about
1.4 megaparsecs in length.

And a megaparsec is about
3 million light-years.

So, we're talking almost
5 million light-years

from end to end.

Eventually, the jets do stop

when they slam into
the intergalactic medium,

the thin film of gas
that surrounds galaxies.

It can go hundreds of
thousands of light-years,

striking the intergalactic
medium and setting it ablaze.

The impact sends out
massive shock waves

like in galaxy Pictor a.

They form these huge,
puffy clouds.

They look like
sort of cotton swabs.

You had a narrow jet with these
big puffy things at either end.

But it seems that
quasars with jets

are a very rare species.

Only 10 percent have them,
and no one really knows why.

The huge jets coming out
of quasars are things

we can see clear
across the universe.

The amazing thing is we don't
even really understand

how they're created.

It's such a complicated process

that it's hard to untangle
the astrophysics going on here.

So we think that
the formation of jets

arises from the accretion disk,

or this spinning disk of gas

that's spiraling close to the
event horizon of the black hole.

This is our best theory.

Gas falls towards
the supermassive black hole.

It moves faster and faster,
getting hotter and hotter.

When you heat gas to extreme
temperatures, it becomes plasma,

full of electromagnetically
charged particles.

We think, in the regions around
supermassive black holes,

you have quickly moving
charged particles.

This creates magnetic fields.

As the particles swirl
around the black hole,

they build up
a powerful magnetic field.

The field builds up intensity
and surrounds the black hole.

That strong magnetic field

can wrap itself
around the black hole,

and any charged particles
in the accretion disk

will follow the paths
of those magnetic fields.

And where there is a body
with a magnetic field,

there are magnetic poles,

and that's where things
can escape.

It gets wound around,
spiraled up,

and shoved up into a jet.

The pressures in that
discuss are incredibly high,

and these magnetic fields
are incredibly strong,

and they're wound up tightly
by the spinning black hole,

and what you end up with
are jets that are collimated

and incredibly powerful.

The jet blasts out
from the poles of the black hole

at 99 percent
the speed of light.

These quasars
are big generators.

They convert gravitational
energy into magnetic energy,

and that magnetic energy gets
converted into kinetic energy

in the launching of these jets.

They carry so much energy,

they can be detected
clear across the universe.

Quasars continue to
confuse astronomers.

Now, they're presenting us
with another problem.

Quasars should take millions
of years to fire up.

But recently, we've detected
one that switched on

in a cosmic heartbeat.

So far, we've
discovered over 200,000 quasars.

And in June of 2016,
we discovered another.

But this one is different.

It ignited in just 500 days...

In cosmological terms,
just a blink of an eye.

That's incredibly weird,

because remember what
we're talking about here.

We're talking about
the consumption

of galaxy-sized
quantities of gas.

How could that be fast?

That's a very rapid process.

Usually, when we think
of astronomy,

we think of astronomically
long time scales

where we don't get to see
things happen in real time.

So when we see these incredible
engines in our universe

changing their complete nature

by turning on over the course
of a few months or a few years,

that's a little bit frightening.

What triggers a quasar
to ignite?

Scientists now theorize
the quasars switching on

may be a natural part
of a galaxy's life.

Galaxies are not fixed things.

They're constantly
changing, evolving.

It's also true that suddenly
we see these things switch on,

and it's kind of mesmerizing
to imagine the situation.

Within a year's time, a galaxy
could go from being normal

to being active.

It's thought
that most, if not all,

galaxies go through
a phase of development

that includes forming a quasar,

that it's a normal part
of a galaxy growing up.

Quasars can act
a lot like a teenager,

tantrums and all.

This was when they were
shining brightly,

eating mass at
an incredible rate.

It's almost like
your teenage years, you know?

You ate everything in sight,
and things were pretty messy.

You might have undergone
a lot of mood swings.

It's kind of like that
with a quasar.

When a quasar gets hungry

and raids the refrigerator,
it can switch on.

So what flips the switch?
What turns a quasar on and off?

It fundamentally has to do
with gas

getting into
the centers of galaxies.

When stuff gets in there,
a black hole is waiting,

and then it gets bright.

So where does a quasar
find enough gas to feast on?

To turn on a quasar,
you need certain conditions.

You need a supermassive
black hole,

and you need a lot
of material dumped on it.

What does this?
A galaxy collision.

Galaxies are not
locked in place.

They move through space.

Sometimes, they collide.

We think that the supermassive
black holes,

at their cores,
will collide and merge,

and the gas supply from
the combined galaxies

streams towards the new
supermassive black hole.

It's almost like, when you have
these two galaxies merging,

that they have all new food.

It's a brand-new dinner plate,

a brand-new buffet
of food to eat.

Well, that's a feeding source,
right?

You're going to start
a feeding frenzy

on the black hole
that's sitting there.

The gas spirals in,
heats up to millions of degrees.

The galactic core lights up.

And a quasar is born.

It seems galaxy mergers provide

the best conditions
for quasar formation,

but when astronomers studied
the rapid ignition

of these unusual quasars,

they didn't find evidence
of a galaxy merger.

Something else
must switch them on.

When we think about the size
scales of a typical quasar,

we think it must take years
to really turn on.

And if you're thinking
about a flow of gas

passing through
the central parts of a galaxy

to encounter the black hole,
to activate the dynamo,

to launch a jet,

this should take
a healthy amount of time.

This... this is big stuff.

So what could speed this up?

One idea is that something
catastrophic happens

inside the accretion disk,

the ring of gas
surrounding the black hole.

An active quasar means that a
black hole is eating something,

so when you see
a quasar turn on,

you know that something

went very wrong
around the black hole.

Maybe some of the disk fell in.

Maybe even a star
got a bit too close.

Accretion disks
are not just gas and dust.

Black holes will drag in
anything and everything.

There are stars.
There's gas.

There's gas clouds.
There's stars in formation.

There are stars dying.

A lot is happening there,
but basically, what happens

is that star veers too close,

and it is basically stripped.

It's pulled apart.

The enormous gravity
of the black hole

could tear a star to pieces,

giving rise to a sudden
surge of energy

within the accretion disk.

Or perhaps, an exploding star

could be the catalyst
for quasar ignition.

So it could be that a supernova
going off in the accretion disk

triggers an avalanche
of material

suddenly being able to fall in
on the black hole.

In both cases,
a sudden rush of hot gas

would heat up
the accretion disk,

switching on the quasar.

You need to do something
violent to a galaxy

to let it really feed enough
to create a quasar activity.

Once activated,

a quasar can shine
for millions of years,

blasting out energy
and destructive jets.

We see them across the universe.

It seems the basic ingredients
are quite common,

even in our own neighborhood.

Let's put all this together.

What do you need for a quasar?
You need a galaxy.

You need a central
supermassive black hole.

You need gas, and you need stuff
falling into that black hole

to create the quasar phenomenon.

Well, we live in a galaxy,
the milky way,

and it has a central
supermassive black hole,

and there's gas orbiting
around near there.

That doesn't sound good.

Jets ripping through
the milky way,

heat and cosmic winds
bursting out from the core,

posing the question...
Would we survive?

We're used to thinking
of our galaxy as peaceful.

But what if it isn't?

What if it's hiding
a violent past?

What's been discovered recently
is kind of fascinating.

There's two large
evacuated bubbles

emanating from
the center of our galaxy.

The bubbles are full
of superheated gas,

and they're moving
out of our galaxy

at 2 million miles per hour.

These bubbles are huge.

They're on the scale
of the galaxy itself.

They're 50,000 light-years long.

They're so big that if you were
to map them out on the sky,

they would stretch
from horizon to horizon.

These gas bubbles look
similar to those seen

within distant galaxy clusters
like hydra a.

One big question is,

what could've put
that gas there?

What could have made
this gas so hot

that it's able to puff out
so far from the galaxy?

One possibility is that this was
driven by an active phase

in our galaxy's history.

Far away,
in the heart of our galaxy,

hidden by gas and
clouds, lies a giant...

A supermassive black hole
called Sagittarius a-star.

Now, it's quiet.

Is it dead or just sleeping?

The only thing that
could have created

these vast lobes of material
above and below the milky way

are giant jets streaming out
of the core of our galaxy.

Well, doesn't that sound
a lot like a quasar?

But unlike hydra a,
the expelled gas

doesn't date from hundreds
of millions of years ago.

These were blown out
within the last 6 million years.

Almost all quasars we see are
in the distant universe,

which means they're
in the distant past.

But here we have
recent activity.

Just 6 million years ago, our
central black hole was feeding.

No one really
expected that because,

for as long as we can remember,
we've thought about

our milky way galaxy
as being quiescent,

that is to say
not really eating too much,

sort of like a black hole
on a diet.

Something must have
broken its diet.

Maybe an unfortunate group
of stars strayed too close.

Whatever the delivery method,

the sleeping Sagittarius a-star
gorged

on its new meal
and woke up explosively.

Its jets blasted over a trillion
trillion trillion tons of gas

out of the galaxy.

Thankfully, our black hole

is much smaller
than a typical quasar.

This was only a small outburst,

but it's possible
our sleeping giant

could wake up again, much bigger

and much more dangerous
than ever before.

There's something coming up
that means that one day,

you may walk outside,
look at the night sky

and realize
the quasar has turned on.

We know quasars can be triggered

by galactic collisions,

and we know a galaxy
is heading our way.

Public safety announcement...
The milky way galaxy

is on a collision course
with Andromeda.

It's traveling at us
at about 70 miles a second,

and in about 4 billion years,

these two galaxies
are going to collide.

Both of us have
supermassive black holes.

Ours is 4 or 5 million times
the mass of the sun.

And Andromeda's black hole
is 20 times larger.

Eventually, those black holes

will start
spiraling around each other,

and they're destined to merge.

The new black hole
will be much bigger

than Sagittarius a-star,

and this supergiant will have
a fresh load of gas to consume.

This will be an incredible
explosion for the milky way,

maybe even the biggest explosion

it's ever had
in its whole life cycle.

A quasar could be born
far bigger

than anything
we have experienced before.

In the chaos of the merger,
our solar system could migrate,

moving closer to the galaxy's
core, closer to the quasar.

We will have a front-row seat
to not only colliding galaxies,

but in fact, something even
more amazing and terrifying,

a quasar being switched on.

The closer we are,
the bigger the spectacle.

It would look like a single
bright source in the sky,

almost like a second sun.

Along with the beauty
will be incredible heat,

quasar winds, and,
perhaps, even jets.

What will all this
mean for Earth?

The atmosphere is going
to be stripped away.

Oceans are going to boil.

Maybe the rock
under your feet will melt.

I mean, we're talking about

a tremendous amount
of energy here.

There would absolutely be
no life on Earth.

The newly ignited
quasar may be so energetic

it blasts trillions of tons
of gas

out of the milky way.

Even the basic ingredients
for future stars and planets

could be at risk.

That's where stars form,
and that's where planets form,

and so if you kick all
the normal matter

out of the galaxies,

you no longer get
stars and planets

and humans and civilizations.

That doesn't sound like
such a good idea to me.

With their terrible
destructive force,

quasars seem like
bad news for galaxies.

But we are now discovering
there's another side to them.

Quasars are putting out
so much energy,

they appear to be so disruptive
to their environment,

but it could be that, without
them, we wouldn't be here.

For all their ferocious power,

quasars might be the ultimate
cosmic creators.

Kicking out unimaginable power,

quasars can cause chaos
in their local environment,

but it's possible that
this might be essential

for a healthy galaxy.

It might even create
the conditions for life.

We're discovering that for all
their destructive properties,

quasars also have
a creative side,

so it very well may be that
the universe we see around us

was shaped by quasars.

Stars are the essence
of a galaxy,

but everything in moderation.

Too many stars
can actually be a problem.

Having a lot of new
star formation

is kind of a good thing,

but too much of it
can be a bad thing.

When new stars are born,
many of them are going to be

hot, large, blue stars,

but eventually, those young, hot
stars are going to start to die,

and when they do, they're going
to explode violently

as supernovae.

There will be new black holes.

There will be jets.

There will be shocks
that actually go

through the gas of the galaxy.

All of that is, effectively,
killing the galaxy.

Large bursts of star formation

make the galaxy
violent and chaotic.

Stars and planets are
wiped out by intense radiation

from supernovas and black holes.

Left unchecked,
the galaxy burns itself out.

But some galaxies
have a cosmic guardian

creating quiet,
peaceful neighborhoods.

Something is controlling
star birth.

In order for stars to form,

you need cool gas,
molecular hydrogen,

and a quasar
is anything but cool.

They're cool to think about,

but they're really hot
if you're near one.

If you're a galaxy,
and you're ready to just

form a bunch of stars,

but then a quasar turns
on in your heart,

that's going to impact
how those stars form.

So the fate of cold gas in
a galaxy is incredibly tightly

coupled to the fate
of star formation

throughout that galaxy,

and we think that
quasars deposit so much energy

into their ambient surroundings

that they can prevent
the formation

of very, very cold gas
by heating it up.

Quasars pump
huge amounts of heat

into their environment.

One way they do this is through
an extreme cosmic hurricane

called a quasar wind.

You have a wind of light.

There's just so much light
that's being powered

by this spinning disk
of material

around this supermassive
black hole

that that light can actually
push on the dust

and the gas in the core
of the galaxy

and push it outwards
very rapidly,

and these winds
can be really fast.

It's not
the kind of wind you and I

are used to thinking about.

It's actually a stream
of high-energy particles.

In some cases, these particles
can be traveling

at hundreds of millions
of miles an hour.

The cool, star-forming
material in the galaxy

becomes hot and turbulent.

Instead of having these nice,
tranquil clouds

of molecular hydrogen

collapsing under gravity,
now here comes this big,

giant nasty quasar
wind disrupting everything.

It's going to basically
warm the galaxy too much,

and that galaxy is going to have

what we call quenched
star formation,

so it's going to have a lower
rate of forming new stars

than we would expect it to.

Quasars have another
star-suppressing weapon

in their cosmic armory.

There's another mode
that we think exists,

and that is mechanical,
or kinetic, feedback,

and that is literally this.
Right?

That is momentum injection.

That is a freight train
plowing through a galaxy

in the form of a jet launched by
the supermassive black hole,

and that jet, just like a truck
plowing a snowy street,

pushes material out of the way.

They bulldoze the gas
to the outskirts of the galaxy.

They blow out
tremendous amounts of energy.

They spit out gas,
and what happens

is it stops the process
of star formation.

There's less fuel
for new stars to be made.

Quasars suppress star formation

with winds and jets.

But if they continue
ejecting gas,

they would permanently
shut off star birth,

killing their galaxy.

Luckily, they stop
before causing fatal damage.

Eventually, the gas
in the center of that galaxy

is going to run out,

and when it does,
it's like a light switch,

and you're turning
the quasar off.

Quasars are fueled by cool gas.

Having blown away its fuel,
the quasar itself will die.

Gas clouds cool, and stars
begin to form again.

But the cooling gas
also falls back

towards the
supermassive black hole,

providing new fuel
for the quasar to reignite.

And so the black hole, in a lot
of ways, can shut itself off,

just like a thermostat
in your living room,

where, if the room gets too
cold, your thermostat kicks in.

The heat turns on
and gets to a point

where the room is now too hot,
and the thermostat kicks off.

When the quasar turns on, it's
like you stop star formation,

and then when it turns off,

it's like you've turned
star formation back on.

Quasars regulate star formation
so that not too many stars

are formed all at the same time.

By regulating star formation,

quasars control the future
star formation of the galaxy.

So we think as quasars as these
incredibly violent phenomenon,

but in actuality, they are
much more subtle and elegant

than you might think.

In a sense, they're a force
for creation, as well,

because they are mitigating how
stars are forming in galaxies.

It's very likely that
quasar activity

is essential
for galaxy development,

so in that sense, we have
an equilibrium generated

by quasar activity.

You could keep them
on an even keel.

Quasars are a rite of
passage for young galaxies,

graduating from chaotic
star-forming adolescence

into mature,
stable galaxies like our own.

The existence of quasars

may be essential to
the health of a galaxy.

But we think they play
an incredibly important role

in the evolution of galaxies
throughout cosmic time.

Quasars are the most
energetic objects

in the universe.

They seem to almost defy
the laws of nature,

but they're also tools.

They're tools that allow us
to understand our own origins.

We can use quasars
to help understand

the origins of galaxies,
the origins of stars.

Quasars help create
stable galaxies,

the kind of galaxies
that could sustain life.

We are connected to the universe

in a beautiful
and fundamental way.

So quasars, even though
they're incredibly violent,

they're a part of galaxies,

and they're a part
of galactic evolution.

Sometimes, in order to
create, you have to destroy.

Quasars have shaped
the universe.

Without them,
we might not even exist.