Why is it said that it takes a supernova to make elements heavier than iron? You're not going to get iron-iron fusion, but what about proton-iron fusion or similar? Also, we can make reactors here on earth that convert Thorium into Uranium, and we can also make plutonium in a proper reactor. We mustn't confuse reactions useful for power production with reactions for element production right? Why can't a regular star produce some heavy elements?
“Elements heavier than iron, up to bismuth, are primarily produced via the s-process (slow neutron capture) in low to medium-mass stars during their later evolutionary stages.
The remaining and heaviest elements (beyond iron and bismuth) are formed through explosive events: core-collapse supernovae generate elements between neon and nickel, while the r-process (rapid neutron capture) in supernovae and, predominantly, neutron star mergers creates elements like uranium and thorium, dispersing them into the interstellar medium for planetary formation.”
From https://www.astronomy.com/science/the-universes-guide-to-cre...
I think you're right that heavier elements can be made, it's just energy negative to do so. But without a nova they would never leave the inside of the star to find their way into a new planet.
But they do leave. Stars not large enough to go supernova do still form planetary nebulas when the more gradually lose their outer layers to space. Only the core is left behind to form a white dwarf. This will be the Sun's eventual fate.
Wouldn't the heavier elements generally sink to the core and the outer layers be composed of the lighter ones?
No, gravitational segregation like that is a very slow process and would be overwhelmed by any convection. In Earth's atmosphere, for example, it doesn't occur until very high altitude (80 km or so) where diffusion is fast enough to overcome mixing.
See also "dredge-up".
https://en.wikipedia.org/wiki/Dredge-up
"By definition, during a dredge-up, a convection zone extends all the way from the star's surface down to the layers of material that have undergone fusion."
That seems to cover elements up to carbon. Not sure heavier elements would be convected?
"The third dredge-up brings helium, carbon, and the s-process products to the surface," (emphasis added)
In the early universe, stars had so little in the way of "seeds" for the s-process to act on that the few seeds that were there absorbed large numbers of neutrons, eventually producing weird stars highly enriched in lead (the end point of the s-process). These stars have been detected from lead (and bismuth) in their spectra.
https://en.wikipedia.org/wiki/Lead_star
I mean it's not hard to do spectrometry on said nebula, and I don't think there is near enough heavier matter detected there.
s-process elements (including radioactive ones like technetium) are detected in the spectra of the stars where the process occurs, which means they are right out at the "surface".
>> But without a nova they would never leave the inside of the star to find their way into a new planet.
Sure dispersion takes a supernova, but production is a different word ;-)
Because of the Iron Peak: https://en.wikipedia.org/wiki/Iron_peak
In a star, a huge number of reactions take simultaneously due to collisions between nuclei. Some collisions result in the fusion of lighter nuclei into a heavier nucleus, other collisions result in the fission of a heavy nucleus into lighter nuclei.
At iron 56 there is a peak in binding energy, both for lighter and heavier nuclei the binding energy is lower.
It is possible for nuclei with lower binding energy to form after a collision, but the probability for this to happen becomes lower and lower with decreasing binding energy.
Thus if one computes the probabilities of the reactions that happen during collisions one can compute the abundances of chemical elements that are reached when there is an equilibrium between the rates at which a certain chemical element is created and destroyed.
At this equilibrium, there is a maximum abundance for iron 56 and the heavier nuclei have abundances that decrease very quickly with the atomic number. For example, zinc may be 600 to 700 times less abundant than iron and germanium may be 7000 to 8000 times less abundant than iron.
Therefore, in an old star, which reaches equilibrium concentrations of elements, there are elements heavier than iron, but in extremely small concentrations, which become negligible for the elements much heavier than germanium.
Significant quantities of heavy elements cannot be produced by collisions between nuclei in a star, because they are destroyed in later collisions faster than they are produced.
So most of the elements heavier than germanium are produced by a different mechanism, i.e. by neutron capture, followed by beta decay. A small number of the heavy nuclei produced by neutron capture also capture protons after their formation, producing thus also some isotopes that are richer in protons.
In normal stars, the number of neutrons is negligible so neutron capture reactions do not happen often. On the other hand, some catastrophic events, like a supernova explosion or the collision between two neutron stars, can produce huge amounts of neutrons. In this case a lot of neutron capture reactions happens, exactly like on Earth during the explosion of a nuclear fission or fusion bomb.
These neutron capture reactions can produce all the chemical elements until fermium (Z=100), i.e. well beyond uranium. Heavier elements than that are not produced, because they fission spontaneously too quickly, before being able to capture other neutrons.
Of the trans-uranium elements, most decay very quickly, but plutonium 244 has a half-life long enough to reach other stellar systems, together with uranium, thorium, bismuth and all elements lighter than bismuth, except technetium and promethium (the latter 2 elements decay quickly, but technetium can survive for a few tens of millions of years, so small quantities of it may reach a nearby star, but they will disappear very soon after that; the elements between bismuth and thorium, and also protactinium, decay quickly and those that exist on Earth are recently created, through the decay of Th and U). The other primordial elements can survive many billions of years, but the amount of primordial plutonium becomes negligible after a few billions of years.