Hi! I'm a AJ from the Lawrence Hall of Science.
In our last week Astrophysics Friday video,
we talked about the HR diagram.
One of the most important parts
of this diagram is called the
"main-sequence."
Stars in their main-sequence
fuse hydrogen into helium at
their core.
But do stars have an infinite amount of this fuel
called hydrogen?
Well, the answer is no!
So what happens to these stars 
once they run out of hydrogen?
Well, like many things, the later stages of a star's life depend on
its initial mass.
When the start leaves the main sequence,
it enters was called the 
post main-sequence stages of its life,
where it will fuse heavier elements than hydrogen.
When we say heavier elements, what we mean is
an element that has more protons
and neutrons in its nucleus
than other elements.
Different things happen to
stars of different initial masses
on the post main-sequence stage.
Astronomers think of the masses of stars in units of solar masses
which has this symbol here.
The mass of our star, the Sun, is one solar mass,
which is equal to 2 times 10
to the 30 kilograms,
which is also equal to 400 trillion trillion elephants.
[elephant sounds]
There are three categories of initial masses to think about when talking about
the post main sequence stages of a star.
The three categories are 0.5 to 8 solar masses,
8 solar masses to 25 solar masses,
and finally, 25+ solar masses.
Let's talk about the 0.5 to 8 solar mass range first.
These stars become giants 
when they leave the main sequence.
When a low-mass
star runs out of hydrogen,
its core gets smaller,
contracting to get hotter in
order to fuse helium into heavier elements.
This reaction produces more
energy than fusing hydrogen.
The outer layers of the star 
expand with this increase in energy,
almost like they're being pushed out and they also cool down.
We can think of the core sort of like a campfire.
The campfire is very hot.
It'll be nice and warm if you're sitting right next to it
but the farther away you sit away from it,
the colder you are.
Our Sun's currently 4.6 billion years old.
when it's about 10 billion years old,
it'll expand into a Red Giant
eating up Mars, Venus, and maybe even Earth.
[Dun dun dun!]
This won't happen for another
4.5 billion years from now though.
So don't worry about getting gobbled up
by the Sun in your lifetime.
Stars in the lower end of this range, up to about two solar masses, experience something called
"Helium Flash" during the giant phase.
Compared to the star's entire lifespan,
the flash is very short.
It's a process that involves lots and lots of
helium being fused into carbon
through a
nuclear fusion process called the
"Triple Alpha Process."
After the Sun has its Helium Flash,
its radius will shrink back down
and it'll fuse helium into carbon and oxygen.
After this short period of fusing,
the outer layers will expand once
again
except this time, it'll definitely
reach at least the Earth's orbit.
The outer layers will continue to be blasted away.
For a low-mass star, something
called a "planetary nebula"
is often formed at this point.
All that stuff to the core will be a stellar remnant
about the size of the earth called a
"white dwarf."
What's happening here is a case of "hydrostatic equilibrium."
The red giant turning into the white dwarf is a case of the
inward gravity overcoming the outward pressure.
Now that we know what happens to low mass stars, such as our Sun,
let's think about stars with initial mass of 8 to 25 solar masses.
When these stars run out of 
hydrogen to fuse into helium,
they will soon become Super Giants.
Many familiar stars we see in the night sky
are actually Super Giants
like the red supergiant Betelgeuse
or the blue supergiant Rigel in the constellation Orion.
After helium, the star starts fusing
carbon, neon, oxygen, sulfur, and silicon.
After each element has fused, it leaves a shell of ash,
surrounding the core as the
next heavier element is fused.
It's kind of like Babushka dolls, where each time you open one of the dolls,
there's a smaller one inside
until you get to the very center, the core.
Throughout all these fusing stages, the
core contracts and expands
and the outer layers do the same.
This results in some pretty wacky
motions along the HR diagram.
Sometimes, temperature stays
constant and the luminosity increases
and sometimes vice-versa!
All of this is
to maintain the delicate balance of
hydrostatic equilibrium for the star.
The star would do anything to maintain its
internal pressure and gravity.
However, the star runs out of luck once it gets to iron.
Fusing iron actually uses up
energy
rather than create it,
stopping the process of fusion.
As a result, the star can no longer maintain the high pressures that it needs in its core
to stay in hydrostatic equilibrium.
So, gravity wins and the core begins to collapse,
creating what we call a Type 2 Supernova.
As the core collapses, a rebound
shockwave is created
and it propagates throughout all the layers of the star,
giving them energy and breaking them apart.
Let's do a little activity to show how the 
star makes energy blow up!
For this activity, you are going to need two bouncy balls.
I chose a tennis ball and a basket ball.
Make sure that one's bigger than the other.
Next, you want to go ahead and bounce each one individually and see what happens.
[bounces the tennis (smaller) ball]
Tennis ball.
Now the basketball.
[bounces the basket (bigger) ball]
All right, now let's
recreate that shockwave with them.
To see this, we're going to go ahead and combine the balls by placing one above another like so.
Make sure the smaller one is on top of the bigger one.
Next we're going to go ahead and
drop both at the same time and see what
happens.
Here we go.
Whoa!
[Let's take a closer look]
Notice that the big ball barely bounced back up
while the smaller ball rocketed up much higher than you would expect.
So how does this work?
Well, the energy that the bigger ball would have to bounce back up
is now transferred to the smaller ball
and it has extra energy
making it bounce really really high.
Now you can imagine that dense energetic core of a massive collapsing star
sending a huge shockwave to the outer layers of the star,
giving the outer layers lots and lots of extra energy.
What's left behind after an 8 to 25 solar mass star goes supernova
is a very dense object called a "neutron star."
A neutron star has a very small radius.
It's only about the size of a city.
Imagine the mass of a big star smushed
into the volume of an object the size of
San Francisco.
Wild!
Finally, let's talk about post main sequence stars that have initial masses
greater than 25 solar masses.
Like 8 to 25 solar mass stars, these stars will turn into Super Giants after they have left the main sequence.
There are a few different ways
for them to be Super Giants.
Some are red supergiant's for
the rest of the time they're fusing.
Others start as red supergiants, and then get hotter and turn blue for a while
and others become blue supergiant soon after leaving the main sequence,
skipping the red supergiant phase altogether.
All of these super
massive stars will fuse heavier and
heavier elements until they get to iron
and experience a Type 2 Supernova.
But when their core collapses, they are often so massive that they collapse to a singularity,
a point of infinitely small
space but infinitely large mass.
This is what we call a "black hole."
Astrophysicists have yet to determine
exactly why supermassive stars
collapse into neutron stars
while others collapsed the black holes.
They may initially think
that it has to do with mass
but there is some evidence suggesting that it has to do more with the internal structure of neutron stars.
Alas, another great mystery of the
universe that you may help solve.
If you've heard of black holes, you probably
lots and lots of questions about them
We'll talk about all the stellar
remnants we mentioned today:
white dwarfs, neutron stars, and of course, black holes,
in a later video.
We can compare the timelines of the post main sequence phases
for all three initial mass ranges all at once.
We see that the higher a
star's initial mass is,
the less time it spends on the main sequence.
All post-main sequence phases are much shorter than the main sequence phase.
So the more massive a star is, the shorter its lifespan is.
Astronomers like to say that:
"stars live fast and die hard."
All stars start out as
big balls of mostly hydrogen
that spend most of their lives
fusing hydrogen into helium.
Many heavier elements are fused in the cores of the stars
in the post-main sequence stage
such as carbon, nitrogen, oxygen, sodium, magnesium, sulfur, and silicon.
No star can fuse the element iron
but very heavy elements like gold
are formed in supernova.
Nearly every element on the
periodic table gets made in stars.
The elements of the periodic table make up all the stuff in the universe, including you and me.
So we can even say that we're made of star stuff.
Alright well, that's all for today.
If you enjoyed learning about the later lives of stars, make sure to like this video
and follow the Lawrence Hall of Science for more science content.
See you later and keep asking those big questions
about the universe!
