I Chemical Elements produced inside the Sun

[mfb]

100 Gyr is 22 half-lives for U-238.

It means the number of atoms will drop by a factor of 5.5 million. The number of uranium atoms in the Sun will go down from about 10^42.5 to 10³⁶.
We expect the last atom to decay after about 140 half lives, or 630 billion years, neglecting that the Sun constantly gets new uranium atoms from the outside. Atoms are small, there are many of them.

When we speak of half-life we mean the atom left alone, don´t we?
Inside the Sun the mean durations of practically every chemical element tend to be far shorter than these estimates (half-lives), aren´t they?

The environment in the Sun doesn't change radioactive decays in any relevant way.

 

[snorkack]

Sun produces a lot of D.
Sun does not contain much of it, because it readily reacts. A major fate of d is
d+p→³He+γ
despite being an electromagnetic process.
This keeps the abundance of d low, so processes like
d+d→³He+n
have a low branching fraction
p is not the only common nucleus in Sun. But reaction
d+α→⁶Li+γ
is also electromagnetic, and faces a high Coulomb barrier.
How about reactions like
d+¹²C→¹³C+n?
It is a strong process, not electromagnetic.
And the proton does not need to get across the Coulomb barrier...
Is this a common fate for metals in Sun?

 

[mfb]

You mean d+¹²C→¹³C+p?
The Coulomb barrier is still there - the nuclei have to get close together to make the neutron transfer possible.
Didn't check the energy balance of the reaction. ¹³C is part of the CNO cycle. This reaction, if possible, mixes the two fusion chains.

 

[stefan r]

...So, given the size of the sun, many free neutrons are produced.

...The environment in the Sun doesn't change radioactive decays in any relevant way.

These two statements look contradictory. My understanding was that uranium 235 fissions when it is hit with a neutron. Uranium 238 can fission with fast neutrons and becomes plutonium 239 (via Np239) when exposed to slow neutrons. Plutonium 239 has a much shorter half life than U238. Pu239 will alpha decay to U235 or fission when exposed to neutrons.

"Fission reaction" is not the same as "radioactive decay". DaTario actually wrote "mean durations of chemical elements" so reactions should count too.

 

[snorkack]

These two statements look contradictory. My understanding was that uranium 235 fissions when it is hit with a neutron. Uranium 238 can fission with fast neutrons and becomes plutonium 239 (via Np239) when exposed to slow neutrons. Plutonium 239 has a much shorter half life than U238. Pu239 will alpha decay to U235 or fission when exposed to neutrons.

"Fission reaction" is not the same as "radioactive decay". DaTario actually wrote "mean durations of chemical elements" so reactions should count too.

Different posters, different beliefs as to facts.
Sun is big so Sun produces a large number of free neutrons.
However, these neutrons are a small fractions of all nuclei present in Sun, being so big. Induced fission is a small effect in the Sun.
The major free neutron producing reactions for s-process are
¹³C+α→¹⁶O+n
²²Ne+α→²⁵Mg+n
These reactions have a high Coulomb barrier, and Sun is not hot enough for them.
The likely reaction is
d+d→³He+n
but this is a minor side reaction because of competing reaction
d+p→³He+γ
Therefore the free neutron flux in Sun is modest at present.
How is the radial distribution of that neutron flux?

 

[mfb]

"Fission reaction" is not the same as "radioactive decay".

Exactly.
Not that the neutrons would matter - in absolute numbers there are a lot of them, but they get absorbed by other elements quickly.

 

[snorkack]

Not that the neutrons would matter - in absolute numbers there are a lot of them, but they get absorbed by other elements quickly.

Mainly protium.

 

[alantheastronomer]

There aren't many free neutrons in the solar core - d+d -> He-4 NOT He-3 + n, and the half-life of free neutrons is only fifteen minutes!

 

[mfb]

d + d -> He-4 + photon is a very rare reaction as it needs the electromagnetic interaction. d + d -> He-3 + n and d + d -> T + p are much more common (about 50% each).

and the half-life of free neutrons is only fifteen minutes!

Doesn't matter, neutrons in matter are nearly always caught (~microseconds, probably even less in the Sun) before they decay. You have to carefully keep them away from matter to observe their decays.

 

[eachus]

I was wrong about solar mass stars having convective cores - they're radiative. But this doesn't negate the fact that temperatures in the cores of solar mass stars aren't high enough to produce elements beyond carbon, oxygen, and nitrogen, and the C-N-O cycle doesn't occur in solar mass stars. The s-process and r-process elements are produced exclusively in stars of 8 solar masses and greater.

New information. The R-process seems to be an exclusive result of a kilonova, the merger of two neutron stars. We could discuss the latest on the maximum mass of a neutron star, but the kilonova, will be less than about 3 solar masses total. A kilonova was recently detected by LIGO then by lots of telescopes all across the electromagnetic spectrum. https://www.vox.com/science-and-hea...igo-gravitational-waves-neutron-star-kilonova The result was almost purely R-process elements, pretty much matching not only the expected abundances, but theoretical calculations. The result is that more than 90% and probably all of the R-process elements come from kilonovas.

I guess you could say that each of the (two) stars involved were originally high enough mass (but not too high) to go supernova and leave a neutron star. Then the neutron stars have to spiral in for the kilonova--there are some pairs out there which won't merge for tens of billions of years. So kilonovas are pretty rare, and galaxies without one in their history are missing R-process elements. Oh, and you get a short gamma-ray burst. Lots of newly settled science from that one event.

To relate this to the original post, almost every R-process atom in the sun came from a kilonova.

 

[alantheastronomer]

The R-process seems to be an exclusive result of a kilonova, the merger of two neutron stars.

Yes it's true that the R-process elements observed in this event closely matched the predicted ratios - except for the higher mass end. Since we don't yet have a working model of a supernova explosion, it's impossible to estimate the R-process yields of such an event. It's worth noting that the frequency of supernova explosions is approximately one to two per century per galaxy while, as you have noted, the timescale for the merger of binary neutron stars is at least hundreds of thousands of years.

 

[eachus]

Yes it's true that the R-process elements observed in this event closely matched the predicted ratios - except for the higher mass end. Since we don't yet have a working model of a supernova explosion, it's impossible to estimate the R-process yields of such an event. It's worth noting that the frequency of supernova explosions is approximately one to two per century per galaxy while, as you have noted, the timescale for the merger of binary neutron stars is at least hundreds of thousands of years.

Good point. If some supernovas form degenerate cores even seconds before the explosion, and don't form a white dwarf, neutron star or black hole, they will probably form, and release into the interstellar medium some R-process elements. I thought I left enough wiggle room for that. However, I suspect that most of these elements if formed will get locked away inside whatever stellar remnant is formed. Looking at the spectra of white dwarfs would be the best way to look for this. (But white dwarf production is probably the signature of the mildest supernovas.) The data for white dwarf spectra that I have looked at pretty much has iron and nickel as the heaviest elements.

Hmm. That seems to say that at some point, in heavy stars core burning migrates outward. Think of Hydrogen or Helium burning when the core is nearly depleted. The burning would migrate to where the fuel was, which can happen much faster than atoms can move in the core. When burning produces iron the burning is still moving outward. If the core then cools, by neutrino emission if no other way, it will collapse and boom! The signature of iron production outside the core, would be iron in the outer layers of recently produced white dwarfs. I'm retired, and modelling that looks like a big job with a supercomputer needed for the calculations. So if anyone wants to take that idea and run with it? Feel free.

 

[alantheastronomer]

The signature of iron production outside the core, would be iron in the outer layers of recently produced white dwarfs.

You seem to be confusing white dwarfs with neutron stars. Neutron stars are the end products of supernova explosions, while white dwarfs are the cores of lower mass stars as they turn into red giant stars and ultimately planetary nebulae. To find a more thorough description look up "Stellar Evolution" and "Supernovae" in Wikipedia.

I suspect that most of these elements if formed will get locked away inside whatever stellar remnant is formed.

There is still nucleosynthesis going on in the layers of material outside the core, and r-process elements could conceivably be produced there during the explosion, if we only knew what that mechanism was.

 

[rootone]

I find it curious that the emissions of supernovae and similar contain so much heavy elements.
Obviously they do, otherwise rocky planets like Earth could never form.
Intuitively one would think that most of the heavy elements produced end up within the remnant of a core.
Typically that is a white dwarf.or a neutron star.

 

[eachus]

You seem to be confusing white dwarfs with neutron stars. Neutron stars are the end products of supernova explosions, while white dwarfs are the cores of lower mass stars as they turn into red giant stars and ultimately planetary nebulae.

I'm not confused. I'm trying to show that even at their hottest, white dwarfs do not produce significant amounts of iron. In the hottest white dwarfs helium burns to carbon and oxygen. Forming higher weight atoms by adding additional alpha particles doesn't occur, due to the higher and higher temperatures required. However, it is possible that higher weight atoms (up to Ni56 (which decays to Fe56.) could be produced by the s-process but the lack of heavier metals, especially iron, says that there are not enough neutrons floating around to produce them. (Some white dwarfs do have traces of elements up to silicon, but it is no clear if they are collected from nearby supernova or interstellar gas.

 

[alantheastronomer]

I'm not confused. I'm trying to show that even at their hottest, white dwarfs do not produce significant amounts of iron.

Oh, I see what you're saying! Sorry, never mind.

I suspect that most of these elements if formed will get locked away inside whatever stellar remnant is formed.

Intuitively one would think that most of the heavy elements produced end up within the remnant of a core.

In high mass stars that have formed iron cores that are ready to collapse, there is still shell burning of elements leading up to iron outside of the core. It is the mixing of these layers, and the rapid expulsion of neutrons from the surface of the newly formed neutron star by whatever mechanism that causes the supernova explosion, that presumably produces the rest of the heavier elements.