Credit: Hyper Physics
Not all supernovae are the same, so they can be divided into several groups or types. When supernovae were first classified, it was done by looking at their spectra. If there was no hydrogen, it was a Type I supernova. If it contained hydrogen, it was a Type II.
We now know it's a bit more complicated and there are several more groups. Today we use the shape of a supernova's light curve to work out which group it belongs to. A light curve is a graph that shows the brightness of a supernova over time.
Type II Supernovae
We'll start with type II. This type of explosion is also known as a core-collapse supernova. This kind of event takes place in the final stage of a massive star's life. Stars that end in a type II supernova, may contain 8 to 200 times the mass of the Sun. They use up their nuclear fuel much more quickly than low-mass stars and may only shine for a few million years.
There are several sub-types within the Type II group. These include Types II-L, II-P, II-b, and II-n. The L and P types refer to the shape of the light curve and how it fades (P stands for 'plateau', L for 'linear'). The b and n represent spectral features involving the hydrogen spectra (b is 'broad', and n is 'narrow').
Credit: NASA/CXC/M. Weiss
Type I Supernovae
Type I supernovae initially confounded astronomers. They thought 70 - 80% of a star's mass was hydrogen. So how could a star explode and leave no trace of that element behind? Since the original classification, type I have been further divided into types Ia, Ib, and Ic.
Type Ia Supernovae
Type Ia are also known as thermonuclear supernovae. These take place in a binary star system - a star system that contains 2 stars. For a Type Ia explosion to take place, one of the stars must be a white dwarf. The other star is a less evolved star, like our Sun, on the main sequence, or a red giant star. However, a new discovery has shown evidence of 2 white dwarf stars colliding to create a Type Ia supernova.
The gravitational force of the white dwarf pulls material from the less-evolved star onto itself. The white dwarf gradually becomes more massive. This process is known as accretion. If the white dwarf reaches 1.44 solar masses (a point known as the Chandrasekhar limit), it becomes unstable and explodes as a type Ia supernova.
(top to bottom: Type II, Ib, Ic)
Credit: M. Modjaz
Type Ib and Ic Supernovae
Some massive stars are so extreme that they start to lose their outer layers. They evolve into Wolf-Rayet stars or Luminous Blue Variables. These stars end their lives like any other massive star in a supernova explosion. However, because they have lost their outer layers, which is where most of the hydrogen is, they are classed as type I. This type of explosion is also known as a stripped-envelope core-collapse supernova.
These supernovae are further divided into type Ib and type Ic based on the amount of helium present in their spectra. Type Ib supernovae have lost their outer layer of hydrogen but held onto some helium. Type Ic has lost its helium as well as the hydrogen layers.