Type 1a supernovae

artist's conception of an accreting white dwarf, Credit: CXC/M.Weiss

Type Ia supernova are widely believed to be caused by the accretion of mass onto a white dwarf star (a star of carbon-ash left over from an active star) from a binary neighbor (a larger, active star that serves as a large reservoir of hydrogen). Hydrogen from the outer atmosphere of the companion star is gravitationally peeled off by the white-dwarf and slowly deposited on its surface. This may happen in other binary star systems with little or no effect. However, the white dwarf star is special in that it no longer generates energy (and therefore outward pressure) by fusion of light elements. Instead, it is ‘held up’ against gravitational collapse entirely by electron degeneracy pressure (electrons don’t like to occupy the same space at the same time).

There is a limit to how much mass this degeneracy pressure may hold up. This is the Chandrasehkar mass, which is 1.44 times the mass of the sun. Once the accreting hydrogen pushes the total mass of the white dwarf to this limit, carbon nuclei in the core begin to fuse. A flame-front of fusion burning moves explosively to the surface and rapidly destroys the white dwarf. Temperatures and densities present on this flame-front make electron-capture energetically favorable. Since electron-capture and explosive fusion burning occur simultaneously, understanding the dynamics of both is essential to understanding energy generation/use in the explosion. Since the electron-capture reaction has the effect of making the matter ejected from the explosion more neutron rich, understanding it is essential to predicting the abundances of heavy elements created in the explosion. For example, it is believed that as much as 50% of the iron found in the universe is due to fusion and electron-capture processes taking place in Type Ia supernovae.

More on core-collapse supernovae

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Carol J. Guess