stars in the sky look pretty
flickering intense diamonds dotting the
velvety night but make no mistake they
are churning cauldrons of violence
barely constrained thermonuclear
generators creating enough energy to
vaporize the earth a thousand times over
their lives depend on it
heck our lives depend on it but in their
case how they live and how long they
live depends on that creation of energy
and in this universe nothing lasts
forever
very roughly speaking we can divide
stars into two groups low mass stars and
high mass stars that line lies around
eight times the mass of the Sun and the
reason for that is something we'll get
to shortly things get very exciting at
the high end but for now let's take a
look at the low mass end of the scale
things are simpler their stars make
energy in their cores by fusing hydrogen
into helium this is actually a pretty
complicated process with lots of steps
but in the end for protons plus some
other ingredients get turned into one
helium nucleus each time this happens a
little bit of energy is released but in
the core of a star it happens countless
times every second it adds up to a lot
of energy enough to power a star in fact
the rate at which hydrogen fusion occurs
depends on how much pressure the star
has in its core a higher mass star
squeezes hydrogen harder so it fuses far
more quickly the rate is so much higher
in fact that even though they're a lot
bigger than low mass stars high mass
stars run out of hydrogen fuel in their
cores more quickly the lower mass of
star is the longer it lives in the
lowest mass stars cool red dwarfs the
rate is so slow they last a long long
time but there's more to it than that
they fuse hydrogen only in their very
centers and their cores aren't very big
outside of this small region the gas is
convective which means the hot stuff
Rises all the way to the surface cools
then falls back to the core that's
important because it means the entire
star is available for fuel both hydrogen
and helium rise up through the star and
then fall back down the star can't fuse
the helium but the hydrogen that makes
it back to the center is just more grist
for the mill even after a long long time
these stars can still shine as the
hydrogen mixed throughout the star
eventually makes it down to the center
so how long can a really low mass red
dwarf last a trillion years a trillion
the universe itself is less than 14
billion years old so even the oldest red
dwarf stars are basically infants
they're just barely getting a start in
life and they look pretty much the same
for the next BOE 990 billion years or so
eventually though they do run out of
fuel when one does the star itself will
be nearly pure helium plus whatever
heavier elements it was born with
fusion will cease it will then cool
which will take many more billions of
years finally it'll be a cold black dead
star and for them that'll be pretty much
that or so we think the universe is far
too young for any of these red dwarfs to
have run out of fuel yet that's true
four stars at the lower end of the mass
scale up to roughly a third of the mass
of the Sun but stars with more mass than
that stars more like the Sun itself work
a little differently their cores are
bigger and hotter and denser the
different conditions inside them means
the material in them doesn't convect the
hot stuff doesn't rise away from the
core what's in the core stays in the
core outside the core there's a very
deep and active convection zone but that
doesn't interact with the material in
the core itself this means that not only
do stars near the sun's mass use up
their fuel more quickly than red dwarfs
they have relatively less fuel to work
with as well depending on their mass
their lives aren't nearly as long for
the Sun that total lifespan is about 12
billion years give or take the Sun is
nearly 5 billion years old now so it's
approaching middle age it's life is
similar to other stars of similar mass
so let's take a look into the far future
will our Sun go gently into that good
night
ever since the Sun was born it's been
fusing hydrogen into helium in its core
the helium is trapped there unable to be
used as fuel it's building up in the
core like ash in a fireplace as it does
the density in the Sun's core slowly
increases when you compress a gas it
heats up that means every day the Sun's
core gets a wee bit hotter that extra
heat works its way out through our star
heating up the outer layers to hotter
gas shines more brightly so the Sun has
been steadily increasing in brightness -
it's an incredibly slow but inexorable
process the sun's increased in
luminosity by about 40 percent since it
was born and it's not done helium is
still building up in its core and
eventually it'll run out of hydrogen to
fuse without fuel fusion in the core
will stop helium can be fused into
carbon and a little bit of oxygen and
neon but it has to be incredibly hot to
do so and the Sun's core won't be
anywhere near those conditions still
half the sun's mass is bearing down on
the core so even though fusion stops the
core will continue to contract and heat
up it'll get so hot that the temperature
outside the core will get two hydrogen
fusion temperatures like igniting a
match
by holding it near a flame the hydrogen
outside the core will start to fuse
that'll occur in a shell surrounding the
core adding its heat to the outer layers
of the Sun as well when you heat a gas
it expands the sun's outer layers will
do just that and our star will grow to
well over twice its current size
astronomers call a star like this a sub
giant not only will it grow but its
color will change the Sun will be giving
off more total energy than it does now
but its surface area will increase so
much that the amount of energy radiated
per square centimeter will go down there
will just be a lot more square
centimeters this means ironically that
the surface will actually cool the sun's
temperature will go down cooler stars
are red and so the Sun will be too but
the core still isn't done the details
are complicated but the core continues
to contract and heat up it gets so hot
that the outer layers swell even more
and the Sun can bloat up to a fantastic
10 to 150 times its present size it will
then be a red giant red giants are
fantastically bright because of all the
energy percolating up from their
interiors coupled with their enormous
size when it turns into a red giant the
Sun will increase its luminosity
by an incredible 2,000 times that huge
increase in luminosity does more than
just make the Sun bright though as a red
giant the Sun will be so swollen that
the gravity at its surface will drop
substantially from what it is now the
force of gravity holding it down is weak
but the force from all that light
blasting out from below is strong easily
overpowering gravity material on the
surface will get blown off like a leaf
in a hurricane
think of it as a super solar wind that
means the Sun will shed mass a lot
faster than it does now while it's a red
giant it'll lose fully 1/3 of its mass
at some point the core contracts and
heats up so much that the conditions
will finally be primed for helium fusion
suddenly helium is converted into carbon
releasing a lot of energy in a weird
twist due to a lot of very complicated
physics the core itself winds up
expanding absorbing most of that energy
in the end less energy is pumped into
the outer layers so the sun's outer
layers contract the Sun shrinks the
outer layers get dense again which warms
them up a bit and the Sun turns from red
to orange it's still pretty big more
and ten times its current day size and
puts out twenty to fifty times as much
energy as it does now but then we see a
sort of reboot of what's happened in the
core before this time though helium is
fusing in the core and the carbon is
building up like ash once again the core
starts to contract and heat up this
dumps more energy into the outer layers
they expand and cool and the Sun turns
into a red giant again this time it's
even brighter and bigger and loses even
more mass shedding over half of what's
left in the meantime helium fusion isn't
very stable to say the least it swings
wildly increasing and decreasing its
rate by huge amounts on very short
timescales the Sun will probably undergo
a series of tremendous paroxysms epic
eruptions is the helium fusion spikes
creating huge surges in energy
production it'll lose even more mass
each time finally though we approach the
endgame the Sun doesn't have enough mass
to squeeze carbon atoms enough to fuse
them they build up in the core until
there's no more helium in fusion stops
on top of that well there's nothing on
top of that by this time the Sun will
have literally blown off while its outer
layers it's essentially a naked core a
hot intensely bright super compact ball
not much bigger than Earth we call this
a white dwarf its fate is sealed
it can't generate energy so it'll cool
and fade over the next few tens of
billions of years at that point we can
safely say the Sun is dead but that's
not always the case for all stars some
with more mass than the Sun still go
through another phase before dying they
form what are called planetary nebulae
and these are so amazing and beautiful
that they rate their own episode which
along with more fun stuff about white
dwarfs we'll get to next time so that's
it for the Sun but what about us
of course the earth won't fare very well
during all this it'll get cooked before
the Sun even becomes a subject and by
the time the Sun is a red giant we'll be
good and fried the average temperature
on our planet will be so high that it'll
melt rocks and in a sense it'll look
much like it once did so many billions
of years before when it first formed a
molten ball of rock but it gets worse
when the Sun expands it'll reach a
radius very close to the size of the
Earth's orbit now
does that mean our planet itself will be
consumed by the Sun maybe not as the Sun
expands remember it loses mass that
means its gravitational pull will weaken
and earth will move out from the Sun
into a bigger orbit if things work out
right for us the earth will back off
faster than the Sun expands and we'll
avoid finding out what it's like to be
inside a star due to a quirk of physics
if the Sun loses more than half its mass
its gravity will decrease so much the
planets will no longer be
gravitationally bound to it and they'll
be flung into interstellar space I'm not
sure that's any better for us either way
the earth will have been long dead and
hopefully humanity will have figured out
the trick to interstellar travel by that
time all those earth-like exoplanets
around younger stars will look pretty
nice in a billion years or two
mind you this isn't happening anytime
soon the Sun won't become a sub joint
until it's nearly 11 billion years old
over six billion years from now the
first red giant stage happens when it's
over eleven point five billion years old
and the second stage 1/2 billion years
later this is a long long time coming if
the Sun had more mass it could avoid
this fate a star with about eight times
the sun's mass can it has enough mass to
squeeze those carbon nuclei hard enough
to fuse them together those high mass
stars have a much different and far more
explosive fate than lower mass stars
like the Sun and trust me we'll cover
that in lots of fun detail soon and
while this all sounds very bleak not all
is lost this is all part of the natural
cycle of the universe and in fact we
wouldn't be here at all if it weren't
for the way stars live and die be
thankful for what you have and that we
can not only observe it but understand
it too today you learned that very
low-mass stars live a long time fusing
all their hydrogen into helium over a
trillion years more massive stars like
the Sun live shorter lives they fuse
hydrogen and helium and eventually
helium into carbon and also some oxygen
and Neil when this happens they expand
get brighter and cool off becoming red
giants they lose most of their mass
exposing their cores and then cool off
over many billions of years crash course
astronomy is produced in association
with PBS
yes Digital Studios really go over to
their YouTube channel check it out
lots of cool videos there this episode
was written by me Phil Plait the script
was edited by Blake de pastino and our
consultant is dr. Michelle thaller
it was directed by Nicholas Jenkins
edited by Nicole Sweeney the sound
designer is Michael Aranda and the
graphics team is thought cafe