stars come in a wide range of masses and
sizes some are stellar behemoths up to
300 times the mass of our Sun others are
similar in mass to our Sun and represent
our stellar cousins but the lowest mass
stars or the red dwarfs their small dim
and slow burning their lifespans are so
long that they will be the last stars
ever to shine in our universe and will
be the place where life will make its
final stand welcome back to launchpad
I'm Christian Reddy your friendly
neighborhood astronomer and in this
video we're going to learn about the
smallest dimmest stars in the universe
the red dwarfs as their name implies the
red dwarfs are smaller and cooler than
our sun shining brightest at infrared
wavelengths they're also much less
massive about 40% the mass of our Sun
all the way down to 8% in fact the
smallest and least massive red dwarfs
are fairly stars at all just massive
enough to sustain hydrogen fusion in
their cores when compared against other
stars luminosities and temperatures the
red dwarfs occupy the low dim end of the
main sequence of hydrogen burning stars
but red dwarfs are the most common type
of star in our galaxy making up to 85%
of the hundred billion stars in our
galaxy alone in fact the nearest star to
our Sun Proxima Centauri is a red dwarf
just 4.2 light years away but barely
noticeable in a time exposure image not
only that but most of the nearest stars
to our Sun are red dwarfs as well like
any main sequence star red dwarfs shine
by fusing hydrogen into helium in their
cores and releasing energy in the
process but the rate at which stars burn
their hydrogen fuel depends on their
mass and how much pressure they can
exert on their cores high mass stars
squeeze their cores harder so they fuse
their hydrogen fuel much more quickly
but low mass stars are lighter so their
cores are under less pressure and
therefore burn their hydrogen fuel much
longer but red dwarfs have the smallest
course so it might seem that they should
run out of their hydrogen fuel after a
relatively short amount of time but red
dwarfs particularly the
that are under 35% the sun's mass have
another tremendous advantage the core of
our Sun is surrounded by a radiative
zone which effectively isolates it from
the rest of the star as a result our Sun
only has about 10% of its hydrogen
supply to fuse over its lifetime but a
red dwarfs interior is fully convective
so it carries away the helium that's
produced in the core and mixes it with
the hydrogen and the rest of the star as
it cools it sinks back down to the core
bringing fresh hydrogen down with it
that means red dwarfs can use a hundred
percent of their hydrogen fuel so a red
dwarf with 10% the sun's mass has the
same amount of fuel to work with that
the Sun does only it burns at a thousand
times slower the result is a star that
can last anywhere from 1 to 12 trillion
years but don't let this picture of an
efficient slow burning star fool you
into thinking that red dwarfs are
somehow mild-mannered stellar citizens
on the contrary these little stars can
pack an enormous punch their convective
interiors create enormous star spots
that can cover as much as 40 to 50
percent of the entire star surface
giving the star a blotchy uneven
appearance and the younger a star is the
faster rotates this further intensifies
the magnetic activity on the star
resulting in enormous stellar flares
that are more than ten thousand times
those found on the Sun exactly how long
red dwarfs remain violent like this is
not well understood but suffice to say
these are not the sort of stars we'd
expect to find life around at least not
while they're young but maybe life has a
better chance later on as the red dwarf
evolves we're going to talk about how
red dwarfs evolve in a moment but first
I want to thank patty boussard Neil from
crypto cruising Eric from modern day
tech John cannon Erin Wilson Lesnick
score pesky Joe a Walters Jennifer
burrows ik niels christensen and i want
to welcome my two newest patrons chris
Conkel vlogs and Rasmus Moore thank you
so much for your support and if you want
to help support launchpad please head
over to my patreon page Red Dwarf spends
several trillion years on the main
sequence fusing hydrogen
to helium gradually that helium starts
to build up in the star increasing its
density the core shrinks and heats up
speeding up the fusion rate by a factor
of 10 this is why red dwarfs with 10%
the sun's mass can only last for seven
trillion years rather than ten trillion
years hey what are you gonna do as the
red dwarf ages its color changes from
red to white becoming as hot as the
present-day Sun if the red dwarf is a
little bit more massive its temperature
will continue to climb to become hotter
than the Sun becoming a blue dwarf in
fact blue dwarfs with about 16% the
sun's mass will maintain a steady
brightness for about five billion years
by that point the planets orbiting
farther away from the star will thaw out
allowing perhaps four atmospheres and
even running water on the surfaces by
billion years is enough time for at
least one intelligent civilization to
evolved in the universe so maybe around
this time trillions of years from now
there will be a renaissance of all kinds
of intelligent civilizations teeming
throughout the universe if we do not
destroy ourselves
maybe our descendants will witness this
new era but first they'll have to
survive the demise of our Sun seven
billion years from now maybe they'll
migrate to another sun-like star perhaps
one that will form 5 billion years from
now when the Andromeda and Milky Way
galaxies collide for the next several
billion years they'll watch as new stars
are formed and massive stars explode as
supernovae but this new era of star
formation will only last for so long as
our merged galaxy takes on a new
elliptical shape it will literally run
out of gas and star formation will
effectively cease by the time the first
red dwarf begins to evolve our Sun and
those that will come after it will have
long been dead their white dwarf cores
will have long faded away as black
dwarfs for hundreds of billions of years
the only stars shining in the universe
will be the red dwarfs albeit shining
very dimly that is until the first new
pinpoints of light appear in the sky
once again this will be the emergence of
blue dwarfs along perhaps
with their attendant civilizations maybe
our descendants will be able to welcome
these newcomers to the universe and
possibly tell them tales of their
ancient ancestors who trillions of years
ago lived under a sky full of bright
dazzling stars but eventually the last
of the hydrogen is burned and the star
becomes a solid ball of helium without
any radiation pressure to hold itself up
the star contracts under its own weight
squeezing down to the size of Earth in a
ball of degenerate helium it will spend
the next few billion years radiating
away its heat first as a white dwarf
then eventually slowly fading away as a
black dwarf perhaps our descendants will
witness this final act of the universe
as the last star fades to black
there's some interesting footnotes to
all this first we deferred our
discussion about life around a red dwarf
until far in their future when they
evolved
however red dwarfs are home to earth
sized planets many of which already live
in their stars
Goldilocks or habitable zones where
water can exist on their surfaces maybe
life can and already does exist around
such stars we're gonna find out more
about the challenges that life faces in
a future video footnote number 2 all red
dwarfs are rich in metals but those
metals had to be forged in the previous
generation of stars because the universe
was pure hydrogen and helium at the time
without metals to act as a kind of
braking mechanism against runaway cloud
collapse the first generation of stars
were very massive now obviously we can't
go back in time to see those first
massive stars forming but we can use one
of our satellite galaxies the Large
Magellanic Cloud as a kind of a natural
laboratory to understand how massive
stars form in low metal environments in
fact I just did a video about this very
recently and I'd like to invite you to
go check that out
and our final footnote our understanding
of red dwarf evolution is purely
theoretical the universe simply isn't
old enough to have ever seen such a star
evolve
however it's plenty old enough for stars
like our Sun to evolve and we're going
to take what we've learned about
sun-like star evolution and understand
how our own Sun will evolve and
eventually die so I invite you to
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new videos until next time stay curious
my friends
you