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[MUSIC PLAYING]

If our descendants or any conscious being

is around to witness the very distant future of our galaxy,

what will they see?

How long will life persist as the stars begin to die?

[THEME MUSIC]

For the sake of argument, let's say

that humanity survives the several ends of world

that await us.

We somehow persist through the gradual heating of our Sun

and the evaporation of our oceans.

Our descendants cling to existence

through the countless generations

as we watch the Andromeda Galaxy merge with the Milky Way,

forming a vast elliptical galaxy.

We seek refuge in the outer solar system

as the Sun finally expands into a red giant twice.

And finally, our heirs or successors

find new homes among the stars after the Sun's final death

and transformation into a dim white dwarf.

We covered all of these catastrophes in past episodes,

but what's next?

How long can life survive into the far future?

An absolute requirement for the continued existence of life

is energy or, more accurately, a persistent energy gradient,

as we've also discussed recently.

For life to stave off rising entropy and decay,

energy must flow.

And the deepest wells of accessible energy

in the universe are stars.

When the last star blinks out, life must soon follow.

To know the future of life, we must

understand the life cycles of the longest-lived stars

in the universe.

That would be the red dwarf.

And don't be scornful of this little star.

They have very, very bright futures

and may even spawn a renaissance of life trillions of years

from now.

So let's talk stellar astrophysics.

Stars generate energy, fusing hydrogen

into helium in their cores.

The Sun burns through 600 billion kilograms of hydrogen

every second, generating 4 by 10 to the power of 26 watts

or around the energy equivalent of 20 million times

the Earth's entire nuclear arsenal every second.

This rate will only increase as the core's temperature

increases, and the Sun will burn through the hydrogen

supply in its core in five billion years.

Because the rate of fusion depends

very sensitively on temperature, more massive stars

with their hotter cores burn through their fuel

much, much more quickly.

The most massive stars live only a few million years.

And the relationship goes both ways.

Stars less massive than the Sun burn through their fuel

much more slowly.

This is all astro 101, so let's get a little crunchy

and figure out the lifespan of red dwarf stars,

also known as "M dwarfs."

We observe that a red dwarf with 10% of the Sun's mass

is about 1,000 times fainter than the Sun.

That means it's burning through its fuel 1,000 times less

quickly.

But it also has less fuel to burn, right?

Actually, wrong-- stars like our Sun

can only burn the hydrogen in their cores.

The layer above the Sun's core is what we call "radiative."

All of the energy travels in the form of photons

bouncing their way upwards.

Closer to the surface, the Sun becomes convective.

Energy is transported in giant convection flows

rising to the surface and sinking again.

That radiation zone isolates the Sun's core,

preventing new material from reaching those depths.

As a result, the Sun will only have access

to 10% of its mass for fusion fuel.

But red dwarfs are entirely convective.

Rivers of plasma flow from the core to the surface,

carrying both energy and the helium produced in the fusion

reactions.

That helium gets mixed through the star,

while new hydrogen is brought to the core for fusion.

Over the course of its long life,

a red dwarf will convert all of its hydrogen to helium.

A red dwarf with 10% the Sun's mass has just as much fuel

to burn as the Sun does, yet it burns it 1,000 times slower.

That means it should live 1,000 times longer--

so 10 trillion years instead of the Sun's 10 billion years.

That 10 trillion years assumes our red dwarf keeps

burning at the same old rate.

It doesn't.

Just like the Sun, the cores of red dwarf stars

shrink and heat up over time.

The heating core causes red dwarf fusion rates

to increase by a factor of 10 or more,

particularly towards the ends of their lives.

That shortens their lifespans, but we're still

talking trillions of years.

An interesting thing about red dwarfs

is they don't expand as they brighten,

unlike more massive stars.

If you increase the energy output

but keep the size of the star the same,

then you necessarily increase the surface temperature

of the star.

This is because the light produced by stars

comes from the heat glow of their surfaces.

This is thermal or black-body radiation,

and it obeys a couple of very strict laws.

First, the hotter something is, the more thermal photons

it produces.

So increasing the surface temperature

allows a red dwarf to shed all of those excess photons

produced by its rising fusion rate.

And rule two, the hotter something

is, the more energetic its individual thermal photons.

The black-body spectrum of a hot object

emits relatively more photons at short energetic wavelengths

than a cooler object.

For most of its life, the spectrum of a red

dwarf peaks at infrared wavelengths.

To us, they appear red because they're

producing more red light than yellow, blue, green, et cetera.

But as these stars heat up, their spectrum shifts.

First, they shine white as their black-body spectrum

spans the visible range, just like our Sun.

In the final few billion years of their lives,

some red dwarfs may even become hotter than our Sun,

developing a faint blue tinge.

Finally, with the last hydrogen fuel spent,

the entire star will become composed of helium

and will quietly contract into a helium white dwarf, supported

by quantum mechanical electron degeneracy pressure.

It will slowly radiate away its internal heat

for another several billion years before turning black.

So what does this mean for the future of our galaxy

and for any life that exists then?

Well, long before the first red dwarfs approach

the ends of their lives, there will be no other living

stars left in the galaxy.

Many new Sun-like stars will be born in the Milky Way/Andromeda

collision four billion years from now,

but they will have expired, leaving their own white dwarfs.

And those white dwarfs will have faded

long before the first red dwarf passes away.

At that point, the night sky will be dark,

and only a powerful telescope could reveal the trillion faint

red dots scattered across the sky.

As these brighten one by one, the most massive

will shine brighter than the current Sun.

Individual points of white light will appear in the night sky,

shining for up to a few billion years before winking out.

That dark future is inevitable, but for several trillion years,

red dwarfs will be the last warm places in the universe.

That's an awfully long time at many times the current age

of the universe, Red dwarfs will surely

be the places our own starfaring descendants will wait out

eternity.

But what about new life?

We know that red dwarfs do have planetary systems.

Just look at TRAPPIST-1 with its seven terrestrial worlds,

two of which are at the right distance

from the star to have liquid water.

We don't know yet whether life can

evolve around red dwarf stars.

They're violently active when they're young,

but perhaps ancient red dwarfs will have the stability needed

for new life to take hold.

This may be especially true right near the end.

Red wharfs in the middle range of mass, around 15%

of the Sun's mass, are predicted to enter

a period of relatively constant brightness

right at the ends of their lives.

This period could last for up to five billion years,

during which the star will shine almost as bright as the Sun

and quite a bit hotter.

Those stars will have long-frozen worlds

in the outer parts of their solar systems.

Those planets will thaw as their star

brightens and may enjoy billions of years of stable warmth.

So could life begin from scratch in a trillion years

right as the red dwarfs begin to die?

It's very possible that most of the life in the universe

is yet to evolve.

Perhaps the descendants of humanity

or some other pre-merger species from the old Milky Way

will be there to witness this, one last long renaissance

of life as we huddle in the warmth of the last stars

to burn in the darkening end of space time.

Last week, we talked about a swarm

of black holes recently discovered

in the core of the Milky Way.

But before we jump into comments,

I just want to let you know about a new PBS Digital Studios

show, "Hot Mess."

"Hot Mess" is a deep dive into the real science of climate

change, along with the implications for the future

and the technology we'll need to fix it.

We'll put a link in the description

so you can join the conversation after we finish talking

about black hole swarms.

Joshua Hillerup asks whether dynamical friction

leads to less dark matter near the centers of galaxies

since dark matter's not very dense.

Good insight, Joshua.

Yeah, dark matter is expected to be more evenly spread

through the galaxy than things like stars and black holes.

And that's what we see.

Dark matter exists in a puffy sphere some 200,000 light years

in radius surrounding the Milky Way,

compared to the 100,000 light years of the Milky Way

stellar disk and the much smaller and denser stellar

call.

OXFFF1 asks how we'd be able to tell

that the supermassive black hole in our galaxy center

is in itself a dense swarm of smaller black holes

in a shared orbit amounting to the same total mass.

Well, the answer is that we can constrain the size of the Milky

Way central black hole, Sagittarius A*,

because we can see stars in orbit around it.

They get way too close to allow anything

but a single black hole to exist in that tiny space.

There certainly couldn't be millions

of stellar-mass black holes.

Also, the Event Horizon Telescope has now detected

radio emission from pretty close to the event horizon of Sag A*,

which confirms it as a single black hole.

Lucas James noticed that during minute seven of the "Black Hole

Swarms" episode, the plot only shows 12 blue dots, not the 13

that I claimed.

Yeah.

I noticed that but decided to gloss over it, hoping

no one else would notice.

But who am I kidding?

Of course, you guys are going to pause the video and count dots.

I mean, hell, I did--

peer review by YouTube.

Anyway, as Gareth Dean points out, two of those dots

were almost on top of each other, so we're all good.

But thanks for keeping us honest,

and we'll see you next week.

The Description of The Star at the End of Time | Space Time