10th Week: Supernovae and Nucleosynthesis B

Supernova

The final stage of the life of stars can be very different and depends strongly on the mass of the individual stars. Some star will just consume all their nuclear fuel and die quietly, but there are special cases in which the star ends its life with a violent explosion that can last for days. This process is called supernova.

Type Ia

Type 1a supernova, also called thermonuclear supernova, begins with a White Dwarf (WD) star with mass close to 1 solar mass. Very special conditions need to be satisfied in order for the WD to become a supernova. This is because the WD has to accrete matter, for example, if the WD is part of a binary system, the other star will be the source of matter for the WD. Other methods for the accretion of matter have been proposed but regardless of this, once the WD has reached Chandrasekhar limit (1.44 solar masses) the following reactions start taking place in the carbon-oxygen core of the WD

$^{12}C+\,^{16}O\rightarrow \,^{28}Si+\gamma$

$^{28}Si+\,^{28}Si\rightarrow \,^{56}Ni+\gamma$

After this the following $\beta$ decays take place

$^{56}Ni\rightarrow \,^{56}Co+e^{+}+\nu$

$^{56}Co\rightarrow \,^{56}Fe+e^{+}+\nu$

All these reactions occur in a very short time, of the order of seconds. Thus an enormous amount of nuclear energy is released violently in the form of an explosion.