Supernovae are exploding stars, which are so bright that the light produced can briefly outshine an entire galaxy.
There are a number of different types of supernova, each produced by a different mechanism. These different types of supernova are distinguishable from Earth by their distinctive ‘light curves’, i.e. the graph of the intensity of light produced by the supernova plotted against time.
The main different types of supernova are categorised as follows:
Type Ia Supernovae:
If a star in a sufficiently close binary orbit with a white dwarf star enters the red giant phase of its life cycle, the expansion of this star can lead to material from its outer layers being captured by the gravitational pull of the white dwarf. This captured material can gradually increase the mass of the white dwarf until it passes the Chandrasekhar limit (the maximum mass for a white dwarf, of around 1.44 times the mass of the Sun) and collapses under its own gravity, causing it to explode with enough energy to completely destroy both stars.
A non-standard Type Ia supernova can also occur when two white dwarf stars combine.
Type II Supernovae:
When a star has burnt the majority of its nuclear fuel, the energy produced in the centre will eventually become insufficient to stop the star from collapsing under its own gravity. If the star is of sufficient mass, it will then explode as a type II supernova.
Type II supernovae leave a neutron star remnant surrounded by a nebula formed from the outer layers of the star ejected during the supernova explosion.
If the dying star is sufficiently massive (greater than around 15 solar masses), however, it is possible that the star will collapse directly to a black hole, without producing a visible supernova explosion.
There are several subtypes of type II supernovae. These are:
- II-P supernove, which are produced by a supergiant star
- II-L produced by a supergiant with a depleted hydrogen shell
- IIn produced by a supergiant in a dense cloud of expelled material
- IIb produced by a supergiant with highly depleted hydrogen
- IIpec which are possibly produced by blue supergiants
Type Ib and Ic Supernovae:
Type Ib and Ic supernovae are produced by a similar mechanism to type II supernovae, as described above. However, if the star develops a particularly strong stellar wind in the final stages of its life, it can eject its outer layers into space prior to exploding as a supernova. Alternatively the star can loose it’s outer layers through interaction with a binary companion star.
These stars that have stripped themselves of their outer layers are known as Wolf-Rayet stars. Wolf-Rayet stars are classified either as type WO, which have Nitrogen absorption lines dominating their spectrum, or WC, with their spectrum dominated by carbon and oxygen. WC Wolf-Rayet stars are the progenitor of type Ib supernova and WN Wolf-Rayet stars produce type Ic supernova.