Type Ia Supernovae

In a Type Ia supernova, matter accretes onto a white dwarf, normally through an accretion ring (adjacent right figure). As matter accumulates on the white dwarf its mass increases. Eventually it becomes unstable and essentially the entire star is consumed in a gigantic thermonuclear explosion.

The Type Ia Mechanism
A Type Ia supernova may be likened to the explosion of a thermonuclear bomb approximately the size of the Earth but containing the mass of the Sun. The fuel of the fusion bomb is carbon and oxygen, since the white dwarfs that explode as Type Ia supernovae are composed primarily of these elements. The preceding diagram and the following one illustrate the basic mechanism. The runaway fusion reaction is thought to start in carbon and produces typically about 0.6 to 0.8 solar masses of radioactive nickel-56. The nickel-56 produced in the explosion beta decays with a half-life of 5.5 days to cobalt-56, which then beta decays with a half-life of 77 days to iron-56.

Since the white dwarf at explosion is near the Chandrasekhar mass of about 1.4 solar masses (see below), this implies that half the mass of the original white dwarf is fused in the Type Ia explosion. The energy released in the radioactive decay of the nickel to cobalt and then to iron is primarily responsible for the luminosity of the Type Ia supernova after about 20-30 days, when the light curve becomes a decreasing almost straight line.

The Chandrasekhar Limiting Mass
The instability that triggers the explosion is caused by the Chandrasekhar limiting mass for white dwarfs that we discussed earlier in this chapter. Initially the white dwarf is below this mass and so is stable. However, as matter accumulates on its surface by accretion from the companion star, the white dwarf grows more massive. If the white dwarf approaches the Chandrasekhar mass, it becomes gravitationally unstable and sudden compression associated with this instability can trigger thermonuclear reactions in a local region. But because the white dwarf is so dense, the electrons are degenerate and the situation is similar to that discussed for the helium flash in red giant stars. The local fluctuation can trigger a thermonuclear runaway that races through the entire white dwarf, fusing the carbon and oxygen into iron-group nuclei and releasing enormous amounts of energy.