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Last week we gave you an update on the formation of elements from the Big Bang and in main sequence stars like the Sun. This week, we wrap up with a bang, talking about the death of the most massive stars and how they seed the Universe with heavier elements.

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Show Notes

• Stellar nucleosynthesis (Wikipedia)
• Stellar Nucleosynthesis: How Stars Make All of the Elements (thought.com)
• Gravitational collapse (Wikipedia)
• Supernova Nucleosynthesis (astro.princeton.edu)
• Supernova Nucleosynthesis in Massive Stars (Oxford)
• Electron degeneracy pressure (Swinburne)
• Chandrasekhar limit (Wikipedia)
• Photodisintegration (Cosmos)

Transcript

Transcriptions provided by GMR Transcription Services

Fraser:                         Astronomy
Cast, Episode 549. Stellar Nucleosynthesis Revisited, Part 2. Welcome to
Astronomy Cast, the facts-based journey through the cosmos where we help you
understand not only what we know but how we know what we know. I’m Fraser Cain,
publisher of Universe Today. With me as always, Dr. Pamela Gay, Senior
Scientist for the Planetary Science Institute and the Director of CosmoQuest.
Hey, Pamela. How’re you doing?

Pamela:                        I’m
doing well, how are you doing?

Fraser:                         Good,
good. It’s been so long. It’s been –

Pamela:                        I
know.

Fraser:                         Half
an hour since we talked. So, here we are again recording another episode.
Sometimes this just happens.

Pamela:                        It’s
true, it’s true. This is – we’re doing a double today.

Fraser:                         Yep.

Pamela:                        We’re
gone next week. We’re then doing a double next time, then we’re gone a week.

Fraser:                         Right.

Pamela:                        Then
we’re probably doing a –

Fraser:                         That’s
when I’m –

Pamela:                        That’s
when you’re gone.

Fraser:                         Right.

Pamela:                        And
then were probably gonna do a double again because of the holidays.

Fraser:                         Yep.
Yes. And then we’ll be in the same place.

Pamela:                        Yes.

Fraser:                         At
the American Astronomical Society.

Pamela:                        Yes.

Fraser:                         And
so, we’ll record one of those rare Fraser and Pamela located in the same
roughly location of the time space continuum, which is always fun. Because even
though it does sound like we’re in the same room hanging out, we rarely are.

Pamela:                        It’s
true.

Fraser:                         Mostly
we are time zones apart recording over the internets.

Pamela:                        We
have been recording for more years than the number of times that we’ve seen
each other face to face, I think.

Fraser:                         I…

Pamela:                        It’s
close.

Fraser:                         It’s
pretty close.

Pamela:                        Because
we see each other once or twice a year.

Fraser:                         Yeah,
yeah.

Pamela:                        But
then we’ve occasionally gone years without seeing each other early on because
we didn’t have as much…

Fraser:                         Yes.

Pamela:                        We
were younger and stupider.

Fraser:                         Yes,
and events to go to Yeah.

Pamela:                        Right.

Fraser:                         Yep.

Pamela:                        So,
yeah.

Fraser:                         Last
week, we gave you an update on the formation of elements from the Big Bang and
in main sequence stars like the Sun. This week, we wrap up with another bang.
Talking about the death of the most massive stars and how they seed the
universe with the heavier elements. And this is the part where hopefully, we’re
going to get to with all of the new news. All of the new research that has
changed dramatically since – I have been informed the last time we tackled this
was episode 109, nucleosynthesis.

Pamela:                        It’s
true.

Fraser:                         So,
it has been a good 400 plus episodes since we talked about nucleosynthesis and
there have been some dramatic events that have really helped us understand
where the stuff came from.

Pamela:                        So,
I think a good place to start is, let’s review where do things come from that
we’ve discussed so far. So, we have hydrogen, helium, largely from Big Bang
fusion. Little bit of lithium and beryllium in there as well. Then we have low
mass stars, when they puff off their outer atmosphere, here we’re getting
carbon and nitrogen.

We also have some other things that happen in the outer atmosphere of
stars where we end up with things like lithium and strontium and annoyingly
technetium and lead and cadmium. So, there’s a lot of elements that are
shockingly built in the atmospheres of stars and then just get puffed out in
planetary nebula. Now, where things start to sideways is when either bigger
stars go boom or little stars suck material cannibalistically off of their
neighbors, become big stars and go boom.

Fraser:                         Right.
Which version – let’s start with the first one.

Pamela:                        Okay.
So, giant stars, here I’m taking things are significantly larger than our Sun.
When they die, they have so much material left behind that instead of
collapsing down to form a white dwarf, they collapse down and try to form a
neutron star and, in the process, can expel matter. Or they collapse down and
try and form a black hole and expel matter. Or they just explode and leave
nothing left behind. And the reason I simply say significantly larger than our
Sun, is because we don’t fully understand mass loss rates in the largest stars.

And so, it’s reasonable to expect that it may be object that are 10 solar
masses or higher at their beginning that are still big enough to produce these
explosions when they end their lives. It’s all a matter of electron degeneracy
pressure giving up the ghost. Stars are supported with light pressure pushing
out and gravity pushing down. And when you don’t have enough light pressure
pushing out, the atoms attempt to get as compacted as they can and at certain
phases in giant star evolution, you can end up with a degenerate core that will
undergo a helium flash.

And in the death of smaller stars, as it collapses down, the electrons
all compress tightly together and go Pauli exclusion principal and make sure
that things align the correct spins and energy levels that it actually is able
to support the star from collapse. But electron degeneracy pressure can only
support so much mass before it’s overcome. And when that occurs, the atoms go,
electron and proton can save space by becoming a neutron and giving off a blast
of energy and particles. And it’s that blast of energy and particles, that is
one of the ways that we end up with a supernova occurring.

Fraser:                         And,
I mean, I was also sort of imagining it like in the cores of a star like the
Sun, as you mentioned, like you’ve got the hydrogen getting turned into helium.
When that runs out, you can turn the helium into carbon. You can turn the
carbon into silicon. But eventually, the star just runs out of enough pressure
and temperature to keep that process going and that’s when you get that thing
to cool down.

Pamela:                        And
–

Fraser:                         But
with the more massive stars, they just keep going. They just, you know.

Pamela:                        And
you end up with an end to nuclear fusion in two different routes. There’s the
one that you just hit on of, it’s not hot enough to go any further. So, our Sun
is never gonna be burning silicon in its core. It’s never going to be building neon
in its core. More massive stars can do that and for them, it’s not a matt