Lifecycle of Massive Stars

We've already talked about
the life cycle of stars

roughly the same mass as our
sun, give or take a little bit.

What I want to do
in this video is

talk about more massive stars.

And when I'm talking
about massive stars,

I'm talking about stars that
have masses greater than 9

times the sun.

So the general idea
is exactly the same.

You're going to start off
with this huge cloud of mainly

hydrogen.

And now, this cloud
is going to have

to be bigger than the clouds
that condensed to form stars

like our sun.

But you're going
to start with that,

and eventually gravity's
going to pull it together.

And the core of it is going
to get hot and dense enough

for hydrogen to ignite, for
hydrogen to start fusing.

So this is hydrogen,
and it is now fusing.

Let me write it.

It is now fusing.

Hydrogen fusion.

Let me write it like this.

You now have hydrogen
fusion in the middle.

So it's ignited,
and around it, you

have just the other
material of the cloud.

So the rest of the hydrogen.

And now, since it's so
heated, it's really a plasma.

It's really kind of a soup
of electrons and nucleuses

as opposed to well-formed atoms,
especially close to the core.

So now you have hydrogen fusion.

We saw this happens at
around 10 million Kelvin.

And I want to make
it very clear.

Since we're talking
about more massive stars,

even at this stage,
there's going

to be more gravitational
pressure, even at this stage,

during the main
sequence of the star,

because it is more massive.

And so this is going to
burn faster and hotter.

So this is going to be faster
and hotter than something

the mass of our sun.

And so even this stage
is going to happen over

a much shorter period of
time than for a star the mass

of our sun.

Our sun's life is going to be
10 or 11 billion total years.

Here, we're going to
be talking about things

in maybe the tens of
millions of years.

So a factor of 1,000
shorter life span.

But anyway, let's think
about what happens.

And so far, just the
pattern of what happens,

it's going to happen
faster because we

have more pressure, more
gravity, more temperature.

But it's going to happen
in pretty much the same way

as what we saw with a
star the mass of the sun.

Eventually that helium--
sorry, that hydrogen

is going to fuse into
a helium core that's

going to have a hydrogen
shell around it.

It's going to have a
hydrogen shell around it,

hydrogen fusion shell around it.

And then you have the rest
of the star around that.

So let me label it.

This right here is
our helium core.

And more and more
helium is going

to be built up as this
hydrogen in this shell fuses.

And in a star the size of our
sun or the mass of our sun,

this is when it starts
to become a red giant.

Because this core is getting
denser and denser and denser

as more and more
helium is produced.

And as it gets denser
and denser and denser,

there's more and more
gravitational pressure

being put on the hydrogen,
on this hydrogen shell

out here, where we have
fusion still happening.

And so that's going to release
more outward energy to push out

the radius of the actual star.

So the general
process, and we're

going to see this as the star
gets more and more massive,

is we're going to have heavier
and heavier elements forming

in the core.

Those heavier and
heavier elements,

as the star gets
denser and denser,

will eventually ignite,
kind of supporting the core.

But because the core itself
is getting denser and denser

and denser, material is getting
pushed further and further out

with more and more energy.

Although if the star
is massive enough,

it's not going to be
able to be pushed out

as far as you will have
in kind of a red giant,

with kind of a sun-like star.

But let's just think
about how this pattern is

going to continue.

So eventually, that helium,
once it gets dense enough,

it's going to ignite and it's
going to fuse into carbon.

And you're going to have
a carbon core forming.

So that is carbon core.

That's a carbon core.

Around that, you
have a helium core.

And near the center
of the helium core,

you have a shell
of helium fusion--

that's helium, not hydrogen--
turning into carbon, making

that carbon core
denser and hotter.

And then around that,
you have hydrogen fusion.

Have to be very careful.

You have hydrogen fusion.

And then around that, you
have the rest of the star.

And so this process is just
going to keep continuing.

Eventually that carbon
is going to start fusing.

And you're going to
have heavier and heavier

elements form the core.

And so this is a
depiction off of Wikipedia

of a fairly mature massive star.

And you keep
forming these shells

of heavier and heavier
elements, and cores

of heavier and heavier
elements until eventually, you

get to iron.

And in particular, we're
talking about iron 56.

Iron with an atomic mass of 56.

Here on this periodic table
that 26 is its atomic number.

It's how many protons it has.

56, you kind of view it
as a count of the protons

and neutrons, although
it's not exact.

But at this point, the reason
why you stop here is that you

cannot get energy
by fusing iron.

Fusing iron into heavier
elements beyond iron

actually requires energy.

So it would actually be
an endothermic process.

So to fuse iron actually
won't help support the core.

So what I want to do in this--
So just to be very clear,

this is how the heavy
elements actually formed.

We started with
hydrogen, hydrogen fusing

into helium, helium
fusing into carbon,

and then all of these things
in various combinations--

and I won't go into
all the details--

are fusing heavier
and heavier elements.

Neon, oxygen, and you
see it right over here.

Silicon.

And these aren't the only
elements that are forming,

but these are kind of the main
core elements that are forming.

But along the way, you have
all this other stuff, lithium,

beryllium, boron.

All of this other
stuff is also forming.

So this is how you form
elements up to iron 56.

And also, this is actually how
you can form up to nickel 56,

just to be exact.

There will also
be some nickel 56,

which has the same
mass as iron 56,

just has two fewer neutrons
and two more protons.

So nickel 56 will
also form, can also

be, it'll be like
a nickel-iron core.

But that's about
how far a star can

get, regardless of how massive
it is, at least by going

through traditional
fusion, through

the traditional
ignition mechanism.

What I want to do
is leave you there

just so you can think about
what might happen next,

now that we can't fuse
this star anymore.

And what we're actually going to
see is that it will supernova.