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I was looking into neutron star creation and I read that when a star dies, it expells its outer atmosphere and leaves behind a really small really dense nucleus (a white dwarf if the first star was a red giant, or a neutron star if the first was a white dwarf). But why does some of the mass get ejected and some left behind? Which is the process that ejects the mass?

Also, why is electron degeneracy pressure bigger than neutron degeneracy pressure? Why is it that the electron pressure is the first "barrier" against gravitational collapse and only after it's the neutron pressure's turn?

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closed as too broad by Rob Jeffries, user36790, John Rennie, Kyle Kanos, Sebastian Riese Oct 28 '15 at 12:16

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    $\begingroup$ This is way too broad. You are asking about the final phases of a supernova; mass loss from AGB stars; and electron vs neutron degeneracy pressure. Much of this is unrelated. $\endgroup$ – Rob Jeffries Oct 27 '15 at 22:34
  • $\begingroup$ Note also that the more massive stars ($M\gtrsim8M_\odot$) typically form either a neutron star or black hole directly (i.e., skipping the white dwarf). But this certainly is a whole chapter's worth of material to answer these questions. $\endgroup$ – Kyle Kanos Oct 28 '15 at 10:35