[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.
Yes, but when ending ends does the beginning begin again?
I can't buy the heat death of the universe. Everything we know exists is a result of emergent phenomena, and given enough time, intelligent life will acquire the knowledge to recreate the progenitor forces of the universe. It basically boils down to a single belief: do you believe in the ability of humanity to give rise to this agency, or do you doubt our and our descendant's ability? Remember, our beliefs determine what we can do, and remaining optimistic despite facing potential failure can give us the ability to succeed by remaining vigilant.
But great video, thank you for sharing. I'm enjoying everything you're posting today. :)