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I understand that binary star systems may cause type Ia supernovae.

I have a question regarding supernova explosion in a binary system. What is the last nuclear reaction before massive explosion?

I am thinking of two 2 close stars of 3M and 6M.

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    $\begingroup$ Why a binary system in particular? $\endgroup$ – SE - stop firing the good guys Apr 19 '16 at 12:34
  • $\begingroup$ it is the context. but i guess it does not matter $\endgroup$ – Mary.Lou Apr 19 '16 at 15:10
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    $\begingroup$ I don't understand the question. Stars of 6 and 3 solar masses do not explode as supernovae. Nuclear reactions go on throughout the supernova explosion and afterwards, so there is no "last nuclear reaction". $\endgroup$ – ProfRob Apr 19 '16 at 15:54
  • $\begingroup$ why they will not explode as supernova? $\endgroup$ – Mary.Lou Apr 19 '16 at 16:43
  • $\begingroup$ Dear Rob, then i am confused. I thought that in binary stars even with the same mass the process is different. (cause i realize that 3M and 6M are not considered as massive stars to cause supernova). What evolutionary path then this binary system will follow? $\endgroup$ – Mary.Lou Apr 19 '16 at 16:55
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I guess it depends on the type of Supernovae.

Type 1a supernovae are produced by a white dwarf star orbiting a companion close enough that matter transfers from the companion to the surface of the white dwarf.

Assuming only your star system of a 3M star and 6M star, the 6M star would become a white dwarf star first. At the end of it's life, it would have been furiously converting hydrogen to helium, and helium to carbon via the C-N-O cycle and the triple alpha process (C-N-O being dominant in stars above 1.3M sol). The highest reaction in this stars main sequence lifetime would have been the creation of oxygen, but oxygen is a catalyst in the C-N-O cycle, and therefore the last probable reaction in the star would be that of Helium in some form of shell burning in the outer core (more on that next).

The white dwarf, while orbting the companion, may siphon off hydrogen from the surface of the donor star. This hydrogen will accumulate evenly over the surface of the dwarf star, slowly growing the mass of the dwarf. Also, the hydrogen will be shell-burned into helium during this time increasing the density.

If sufficient mass can be accumulated, then the carbon within the white dwarf can ignite in a runaway reaction - literally blowing the entire star apart. These supernovae produce every element up to Nickel (Ni-56).

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