The Universe started out with hydrogen and helium and a few other elements, but all around us, there are other, more proton-rich elements. We believe these heavier elements formed in stars, but which stars? And at what points in their lives? Today we’ll update our knowledge with the latest science.

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Show Notes

• Stellar nucleosynthesis (Wikipedia)
• Stellar Nucleosynthesis: How Stars Make All of the Elements (Thought.co)
• Stellar Nucleosynthesis (Astronomy Notes)
• Stellar nucleosynthesis (Science Daily)
• Stellar nucleosynthesis (Philosophy of Cosmology)
• Nucleosynthesis (NASA’s Cosmicopia)
• Nuclear fusion (Encyclopedia Britannica)
• Proton–proton chain reaction (Science Learning Hub)
• CNO cycle (Wikipedia)
• Deuterium fusion (Hyperphysics)
  
• Helium fusion (Hyperphysics)

Transcript

Transcriptions provided by GMR Transcription Services

Fraser:                         Astronomy
Cast, episode 548. Stellar Nucleosynthesis, Part 1. Welcome to Astronomy Cast.
A weekly 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?

Fraser:                         Good.
I read that intro in an excited way to wake you up. Wake up.

Pamela:                        It’s
the last Friday before a weeklong vacation and my body is like, tomorrow you
can sleep in. Can we start now?

Fraser:                         I
don’t – I think you need to check the calendar because, as always, Thanksgiving
happened a month ago –

Pamela:                        We’re
just always behind the ball here in the lower 48.

Fraser:                         Yeah,
yeah. We – In Canada, we already did it. So, try to catch up. Now you – we’re
actually recording two episodes today, but we will release them slowly and
carefully over the next two weeks. Where are you off to? Is it just for the
holiday or you got plans?

Pamela:                        I
am doing Friendsgiving out in California. I’m gonna see our good friend David
Joseph Wesley, who’s responsible for the music at the beginning of each
episode. And we’re gonna go ride the Millennium Falcon at Galaxy’s Edge because
that’s how you celebrate the holidays.

Fraser:                         That
is gonna be so great. I would love to go back to Disney Land and see the new
Star Wars stuff. Even though, I have a very low-key relationship with Star Wars
these days. I have – I think my childhood nostalgia is now – I finally just
wiped out all childhood nostalgia for Star Wars. Or maybe, the Star Wars – the
over, you know, monetization and release schedule of Star Wars has finally
destroyed my childhood love of Star Wars. But I would love to try all that
stuff, that sounds pretty great. So, you gotta let me know how it works.

Pamela:                        I
totally will, and you can follow along on Instagram, because I suspect there
will be pictures.

Fraser:                         Yeah,
The Expanse Season 4, though –

Pamela:                        Starts
December 12^th, which is my birthday.

Fraser:                         I
know. I’m so excited. The universe started out with hydrogen and helium and a
few other elements. But all around us, there are other, more proton rich
elements. We believe these heavier elements formed in stars, but which stars
and at what point in their lives? Today we’ll update our knowledge with the
latest science. Pamela, the fact that there are certain amounts of hydrogen and
helium with trace amounts of lithium, these are – this is one of the best
indications that the Big Bang is a thing.

Pamela:                        It
is.

Fraser:                         And
yet – and yet, I’m sitting on a chair surrounded by a house, living on a planet
and there’s not as much hydrogen, helium and trace amounts of lithium in all of
these things. And so, these heavier elements had to come from somewhere.

Pamela:                        Exactly,
exactly. And this is actually a story that usually gets simplified far more
than it should. And over the past few years, different things keep coming up
during our show that require us to realize that nucleosynthesis isn’t just
something that happens in the cores of stars. It isn’t just something that
occurs when stars explode. It’s something that occurs in all sorts of weird and
awesome places that lead to, well hydrogen – sorry. That lead to helium-3 on
the Moon and while two weird and esoteric elements like technetium in the
atmosphere of stars. And I thought well, it was time to come back and fill in
some of the gaps that we left when we tried to cram all of nucleosynthesis into
one episode in the past.

Fraser:                         And
do you remember how long ago it was that we covered nucleosynthesis? It was a
long time ago.

Pamela:                        I
want to say it was somewhere around year one or two, so – yeah.

Fraser:                         Yeah.
Yeah, it would’ve been one of the topics that we would’ve gone after early on
and there have been events and there has been new knowledge that has updated
the whole process. And you think so much that it’s a two parter, so – So, let’s
dig in. So, where do you want to – which parts of it do you want to cover
today?

Pamela:                        So,
I think that we start with the Big Bang and we end with the death of a
non-exploding star and get all of the nucleosynthesis that occurs in those
places.

Fraser:                         Okay.
All right. So, let’s start with the Big Bang, then.

Pamela:                        So,
initially as we’ve talked about a bunch in these episodes, our universe was
just a big old ball of energy. And as that energy expanded and cooled and
expanded and cooled – Less than three minutes after it’s start, our entire
universe was similar in conditions to the inside of a star. Now, it wasn’t identical,
we didn’t have the same kinds of reactions that happen in stars.

But, the energy, the density, they temperature of everything that was
present was such that hydrogen atom cores, so protons, were able to come into
existence and these protons collided with one another. We were able to get
helium, we were able to get some lithium, we were able to get some beryllium.
Electrons weren’t really bothered at this point because it was just a soup of
hot ionized everything. But it got us somewhere.

Fraser:                         Now,
I want to make just like one quick, sort of distinction here. So, I mean you
said that it’s a ball of energy and that’s a bit of a misnomer just because it
could have gone on forever. It could have been infinite in all directions, just
highly dense, right?

Pamela:                        Right.

Fraser:                         Just
this –

Pamela:                        Yes,
that’s true. I – as a slip of the tongue because it was convenient –

Fraser:                         Yeah.

Pamela:                        Gave
our universe shape.

Fraser:                         Yes.

Pamela:                        We
don’t know if our universe has a finite to it. If it does, we’re a four-dimensional
hypertoroid, which is a donut, not a ball. But we were basically a big old
something.

Fraser:                         Right,
right. Something, that could’ve been finite, could’ve gone on forever. And when
we think about a star, right? We imagine the gravity is pulling in on the star,
the light pressure is pushing out on the star and that balance creates the
shape of the star. And yet, at the very core of the star, you’ve got the place
where the magic happens, where the fusion is going on. And it is because these
atoms of hydrogen are mashed together so tightly and at high temperature, that
you get this fusion.

And so, how was – you said it wasn’t exactly the same as what’s going on
in a star, and I kind of imagine it like it was rushing through this phase from
whatever came before when it was just li