Question

second stage of fusion: medium/high mass stars:

what type of degeneracy do medium mass stars use to stop further collapse of the core?

What do we call the moment when the second stage of fusion begins for medium mass stars?

do other stars have or need degeneracy to ignite the second stage of fusion?

Answer 1

Medium mass stars are those ,which ends on the formation of Neutron star. The neutron degenracy pressure stops further collapse of core. As in case of small star the electron degenracy pressure stops further collapse and forms a White Dwarf while in case of medium mass star the graviational contraction passes the electron degenracy pressure and generates conditions to add proton and electron to form neutron . Neutron degeneracy pressure is the Hesinberg's uncertinty principle form of neutrons in which no two neutrons can have same energy state , therefore this generates a state of pressure which counter ballances the gravitational force. In case of large star gravitaional pull becomes larger than neutron degenracy pressure and core further collapes and forms black holes. At the state of degeneracy pressure the end of star becomes and there no thermonuclear reactions go on , like white dwarfs after long time they become cool .  The moment for begain of second stage fusion may be called The Expansion as the outer layer of star expands and next thermonuclear reaction begains in core.