Nuclear fusion within stars combine atom nuclei, so the atoms are ionized and electrons run for free. My question is, when or how these electrons recombine with the nuclei to form neutral atoms? inside the star? outside?
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3$\begingroup$ There are no neutral atoms in stars. It is too hot for them. $\endgroup$– Ivan NeretinCommented Sep 24, 2020 at 10:48
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$\begingroup$ so do these neutral atoms form in the interstellar medium? in the planet formation? $\endgroup$– josepCommented Sep 24, 2020 at 10:58
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$\begingroup$ Interstellar medium, I guess. $\endgroup$– Ivan NeretinCommented Sep 24, 2020 at 11:08
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Nuclear fusion in stars produces huge amounts of energy that raise the temperature of the star (e.g. our sun has a surface temperature of about 6000K). At such high temperatures, a lot of atoms possess enough energy to become ionized. However, what percentage of atoms at a particular temperature has what amount of energy is determined by the Boltzmann distribution. So, there will always be some atoms which are not completely ionized (i.e. maybe the electrons are excited from the ground energy level, but they have not completely 'left' the nucleus).
The upshot of all of this is that as the temperature rises, more and more atoms get ionized. Atoms with lower ionization energies like Na, Mg and other metals lose their electrons first, whereas atoms with high ionization energies like He hold onto their electrons until the temperature reaches ~30,000K. Elements like C, N, O, H come somewhere between them. Also, the Boltzmann distribution means that as the temperature rises, more number of a certain atom become ionized. (This page contains a nice diagram showing which ions form at which temperature)
Conversely, if you lower the temperature, you start to form more neutral atoms, and even molecules after a point. However, that happens only below ~3000K, where it starts to look less and less like a star, and more and more like a huge gas giant planet (see Brown dwarf).
As you lower the temperature even more, you can get big complicated molecules like the ones we have on Earth. All of this happens outside of stars, of course*. (*It requires elements like C,N,O,P all of which were synthesized in a pre-existing star, see how planets form )
As an aside, there is another way electron and nuclei can 'recombine' in stars. At the end of the life of a heavy star (> 8 times the mass of sun), the nuclear fusion (that provides an outwards radiation force) suddenly stops. The star then collapses inward on itself due to the heavy gravitational force. This force is so massive that it crushes the electrons and protons together to form neutrons and neutrinos and a huge amount of energy (a type of nuclear reaction). That type of rapid nuclear reaction causes the star to detonate in a supernova.
answered Sep 24, 2020 at 14:18