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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?
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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.