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Journal 3/1/17
Why do you think God made other stars besides the sun?
Objective
Tonight’s Homework
To learn about how stars die
and explode
p 514: review 1, 3, 4, 5, 6, 8,
11
Extra Notes on Stellar Evolution
When a star fuses elements, this energy creates
a pressure. This pressure is what pushes
outwards, keeping everything from simply
collapsing down and crushing the core of the
star.
As long as we have nuclear fusion pushing out,
our star can fight off gravity.
Extra Notes on Stellar Evolution
So when a star like the sun runs out of
hydrogen to fuse, the core will temporarily shut
down. For a little while, the entire star starts to
slowly crush down, putting more and more
pressure on the core.
The good news is that this makes the core
hotter. Hot enough, in fact, to start fusing
helium into lithium. This new fusion suddenly
gives the core pressure to push out again. At
least, until the star runs out of helium…
This process can keep going until we reach iron.
Extra Notes on Stellar Evolution
But iron is special. In the graph below, we’re
seeing something called “binding energy”. This
is a measure of how much energy is stored in
the nucleus of an atom. On the left is fusion.
This is where we smash atoms together and
they release some of this “binding energy” stuff
as they merge. On the right is fission. This is
where we
rip an atom
apart and
binding
energy
comes out
as our atom
rips.
Extra Notes on Stellar Evolution
Right at the top of the graph is iron. Iron is
special. It doesn’t release any binding energy
when smashed together with other atoms, and
it doesn’t release binding energy when torn
apart. In fact, there’s nothing you can do to iron
that will give you more energy back than you
had to use in the first place.
This is
where stars
get in
trouble.
Extra Notes on Stellar Evolution
When a star’s core reaches iron and stops
fusing, we start getting everything collapsing
down like before. But no amount of new heat or
pressure will let it do any more fusion.
This is when the star will explode. But we’ll get
to that in a second.
The sun is only going to have enough pressure
to get to the point where it fuses carbon. After it
stops, the rest of the sun collapsing down just
isn’t hot enough to let it do more fusion. So
what happens next? This is where it gets a bit
odd.
Extra Notes on Stellar Evolution
While the sun is still trying to fuse stuff a few
steps below carbon, the outer layers start
getting hotter (due to some complex stuff). As
they get hotter, they start to puff up, making
the sun way bigger but way less dense.
Expanding gasses cool, so this hotter stuff cools
down pretty quickly.
This leaves us with a star that’s now way bigger
but also way cooler (on the outside). At this
point, our star has become a Red Giant.
Extra Notes on Stellar Evolution
For reference, here’s how big the sun will get.
Extra Notes on Stellar Evolution
If a star has less than 9 times the mass of the
sun, it’s going to stop before iron and die like
ours, with something we call a “nova”.
When the middle reaches carbon and can’t fuse
any more, it shuts down. All those puffed up
layers drift off into space and we’re left with a
tiny, white-hot core of carbon just sitting there.
This is a “white dwarf”.
All the stuff that puffs off into space forms a
planetary nebula.
But what about the big stars?
Notes on Novae and Supernovae
Supernovae are violent. This happens in the last
dying stage for a giant star.
As this star dies, it has a number of shells, each
one fusing a different
element. The problem
is in the core, where
we’ve reached iron.
Since iron can’t fuse,
the pressure holding
up the star in the
center is about to
collapse.
Notes on Novae and Supernovae
When the core collapses, it’s fast. Really fast.
We’ve seen in astronomy that most things seem
to take millions of years. The actual collapse of
the core takes 1 second.
As the core is collapsing, the outer layers stop
expanding and get pulled back in.
The inner layers
bounce off the
iron core in a
series of collisions,
and the whole
thing explodes out
in the most violent
way possible.
Extra Notes on Stellar Evolution
As the star explodes, it makes unimaginable
heat. This creates all the elements heavier than
iron. And this is the only way they’re made.
To summarize this – the only way to get
elements heavier than iron is through a
supernova. Gold comes from exploded stars!
Extra Notes on Stellar Evolution
This supernova is so bright and violent that for
about an hour the star outshines the rest of the
entire galaxy it’s part of.
Notes on Novae and Supernovae
Here’s a few facts about this kind of explosion:
- It releases 1046 Joules of energy over 10
seconds. This could completely disintegrate the
sun 100,000 times over. This is as much
energy as 400 billion suns can put out in 5
billion years! Or in other words, its as much
energy as our entire galaxy has made for 1/3
of the life of the entire universe!
- The outer core implodes at about 23% the
speed of light. So fast that time and space
start to bend.
- The iron core can get as hot as 100 billion K.
This is 10,000 times hotter than the core of
the sun!
Exit Question
When the sun dies, it will leave behind a core of…
Hydrogen
Helium
Calcium
Iron
Creamy nougat
None of the above