A white dwarf---a cooling remnant of a star, no longer fusing---is protected from collapse by the pauli exclusion principle which says that no two electrons can occupy the same state (position and momentum). So crowding electrons together forces them to compensate by having high momenta and a pressure arises called "electron degeneracy pressure" which would exist even if the temperature were zero. Chandrasekhar discovered a formula for the largest mass which can support itself against gravitational collapse by the pressure of its electrons. this formula has a term (Z/A)2 how to understand it? well for a given atom Z is the number of electrons! and A is the atomic weight. So Z/A gives a good idea of how electron-rich it is and how well it will resist collapse. In effect it is an "electrons per weight" number, giving a notion of "pressure per weight" as well. That's all I have to say about it for now. I tried to understand the formula better today. If anyone has some insight or wants to try to explain it go ahead. Its interesting. The formula is: 0.20 (Z/A)2 (hc/Gmprot2)3/2 which is a pure number expressing the mass as a multiple of the proton mass mprot In other words, if you want a complete expression for the mass you multiply the number times mprot and get 0.20 (Z/A)2 (hc/Gmprot2)3/2 mprot That's verbatim from Frank Shu, I use a version rewritten using hbar in place of h.