The Last Star in the Universe – Red Dwarfs Explained

One day, the last star will die,
and the universe will turn dark forever.

It will probably be a red dwarf,
a tiny kind of star,

that's also one of our best bets
to find alien life,

and might be the last home of Humanity
before the universe becomes uninhabitable,

so what do we know about them,
and why are they our last hope?

At least 70% of stars in the universe
are red dwarfs.

They are the tiniest stars out there,
with only about 7 to 50% of the mass of our sun,

not that much bigger than our planet, Jupiter,
which is still huge, though.

They are also very dim.

It's impossible to see them
with the naked eye.

You've never seen one in the night sky.

Even with all our technology,

we can only clearly observe red dwarfs
in our neighborhood.

Approximately 20 of the 30 stars
close to Earth are red dwarfs.

Like all stars,
red dwarfs fuse hydrogen into helium,

but while more massive stars
accumulate all the fused helium in their cores,

red dwarfs stay convective, meaning that
the helium and hydrogen constantly mix,

so they use up their fuel incredibly slowly
before they are extinguished.

Red dwarfs burn so slowly

that their average lifespan is between
one and ten trillion years;

by comparison, the Sun will survive
for another five billion years.

Because the universe is only
13.75 billion years old,

not a single red dwarf has reached
later development stages.

Every single one of the trillions that exist
is still a baby.

Speaking of babies,
the smallest star in the entire universe

is also a red dwarf because small red dwarfs
are right on the verge of being a star at all.

Just a tiny bit less hydrogen,
and they are mere brown dwarfs,

failed stars that cannot sustain
a fusion reaction for long,

so what about aliens
or a new home for Humanity?

Since our sun will die one day,
we'll eventually need to look for a new home,

and where there are habitable planets,
there might also be aliens.

The Kepler space observatory found that
at least half of all red dwarfs host

rock planets between half and four times
the mass of our Earth.

Many of them are in the habitable zone,
the area around a star where water can be liquid,

but since red dwarfs burn
at relatively cold temperatures,

a planet would need to be
really close to be hospitable,

probably as close as Mercury to our Sun
or even closer

which brings with it
all kinds of problems.

For example, a planet this close to a star
would probably be tidally locked,

meaning the same side would always face it.

This side would be incredibly hot,
while the shadow side would be frozen

which makes it hard for life to develop;

although, a planet with a big enough ocean
might be able to distribute the star's energy

and create some kind of stability.

All the gravitational forces of the red dwarf
could squeeze the planet

and heat it up so much
that it might lose all its water over time.

These planets could end up like Venus,
a hot burning hell.

Another problem is that many red dwarfs
vary in their energy output.

They can be covered in star spots that condemn
their emitted light by up to 40% for months

which would cause oceans on planets
to freeze over;

at other times,
they can emit powerful solar flares,

sudden outbursts of
energy incredibly powerful.

These red dwarfs could
double their brightness in minutes

which could strip away sizable portions of
a planet's atmosphere and burn it,

rendering it sterile;

on the other hand,
their extremely long life span is a big plus.

A red dwarf with just
moderate levels of activity

could be an amazing place for
a planet that hosts life.

Life on Earth has existed for about
four billion years,

and we have about a billion years left
before the Sun becomes so hot

that complex life on Earth
will become impossible.

We will either die out or leave Earth
and look for a new home.

We could build a civilization
for potentially trillions of years

around a red dwarf with the right conditions.

About 5% of the red dwarfs in the Milky Way
may host habitable, roughly Earth-sized planets.

That would be more than
four billion in total,

but life may not even need
a planet like Earth.

Candidates for life around a red dwarf
may be the moons of gas giants,

also called Super Earths,
really massive rocky planets.

All alone, there are an estimated
60 billion potentially habitable planets

around red dwarfs,
and that's in the Milky Way alone,

so red dwarfs might become really important
for our survival in the future,

but everything has to die at some point,
even red dwarfs.

When in trillions of years the life of the
last red dwarf in the universe is about to end,

it will not be a very spectacular event.

As its hydrogen runs out, it shrinks
becoming a blue dwarf, burning out completely.

After its fuel is spent, it's transformed
into a white dwarf,

an object about as small as Earth,
packed very densely,

and made of degenerate gasses,
mostly of Helium-4 nuclei.

Having no more source of energy,
it will cool extremely slowly

over trillions of years until it becomes
its final form: a cold black dwarf.

White and black dwarfs are so fascinating
that they deserve their own video;

anyway, it's going to be a long time
before the last stars in the universe vanish.

It's kind of uplifting to know that,
if Humanity succeeds in venturing into Space,

we have plenty of time
before the universe turns out the lights.

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