What happens to the electrons during nuclear fusion? So recently I watched a Khan Academy video regarding the birth of a star. Now in order for a star (or proto-star) to be born, there must be nuclear fusion that happens between hydrogen atoms. My question is what happens to the electrons?? Sal never mentions them, only isotopes of hydrogen, and basically only focuses on the nucleus of the atom. Do the electrons even matter?
 A: Yes, electrons do matter: they are needed to maintain electric neutrality of the matter. Without them repulsive electrostatic forces between protons would not allow formation of dense objects. But the temperatures at which the nuclear fusion occurs are too high for individual atoms, instead electrons and protons become unbound forming fully ionized plasma. On average, the numbers of electrons and protons in such medium are the same, maintaining net electric neutrality.
In the Sun, during the proton-proton fusion two protons fuse into deuteron (a nucleus of deuterium containing one proton and one neutron) also producing a positron ($e^{+}$, an antiparticle counterpart of electron) and an electron neutrino: 
$$
{}^1_1 \mathrm{H} +{}^1_1 \mathrm{H}\to{}^2_1\mathrm{H} + e^{+} +\nu_e+0.42\,\text{MeV}.
$$
Positron subsequently almost immediately annihilates with one of the plasma electrons producing two gamma rays:
$$
e^{+}+e^{-}\to 2\gamma+1.02\,\text{MeV}.
$$
A: The electrons are present in the star's core, but no longer bound to any individual nuclei. Fusion depends on how nuclei collide with each other. By the time the nuclei (also called ions) have gotten close to each other before a potential fusion event, the electrons have relatively little influence. The electrons also have about 2000 times less mass and are moving much faster, so even when an individual electron does get close to an ion, it doesn't stay close for very long and doesn't impart much momentum compared to an ion vs. ion collision. 
So,  electron vs. ion interactions do affect some plasma waves and behaviors, but they can be ignored when considering individual fusion relevant collisions between ions. 
A: Stars are made of plasma, a state of matter in which the electrons almost "detach" from their nuclei and float around like "soup". I'm sure that more knowledgeable physicists than myself can tell you the exact description of this state, but essentially, the electrons play little to no role in the fusion within a star, but it is interesting to note that electrons prevent the collapse of white dwarf stars.
A: Nuclear fusion for star formation from plasma means that higher Z nuclei form, as seen  in the periodic table . Z counts the number of protons in a nucleus, and protons are positively charged. The electrons in the plasma soup of the protostar will pair up with the highly ionized nuclei to turn the whole atom neutral, i.e. no longer attracting electrons. As equal numbers of positive and negative charges exist in the primordial soup, the end effect is that as many positive charges exist as negative, joined into nuclei,  and the star is neutral over all.
