A supernova is the explosive death of a star.
Supernovae are classified into 'types' according to their spectrum:
Type I
The observed spectrum has no evidence of hydrogen.
- Type Ia: Presence of Si II (615 nm) absorption line.
- Type Ib/c: No Si II line.
- Type Ib: Presence of He I (586.6 nm) absorption line.
- Type Ic: No He I line.
Type II
The observed spectrum has strong emission and absorption features due to hydrogen.
- Type II-P/L/n:
- Type IIP/L: No narrow spectral lines.
- Type IIP: Light curve reaches a plateau.
- Type IIL: Magnitude vs. time decreases linearly. -Type IIn: Presence of narrow spectral lines.
- Type IIP/L: No narrow spectral lines.
- Type IIb: Spectrum changes to later resemble Type Ib.
Origin
All types of supernova with the exception of Ia are thought to result from the collapse of the core of a massive star (more than ~8x the mass of the Sun), followed by its 'rebound', resulting in a violent explosion. These are broadly termed 'core collapse supernovae'. Which type of spectrum is observed then depends on the properties of the particular star in question. After the explosion, in most cases there is a stellar remnant left over, either a neutron star or black hole, with more massive progenitor stars typically resulting in the latter.
Type Ia are thought to be the runaway thermonuclear explosion of white dwarf stars. This occurs when the white dwarf exceeds the mass limit of ~1.44x the mass of the Sun. There are two proposed scenarios by which a white dwarf could exceed this limit. In the 'single degenerate' scenario, the white dwarf accretes material from a companion star, while in the 'double degenerate' scenario, it merges with a second white dwarf. Both are likely to occur in nature, but which scenario is most common, and how this depends on the properties of the local stellar population, is an area of active research.
Light curves
Along with the spectrum, a 'light curve', the luminosity of the object as a function of time, is often observed. The light curves of Type Ia events are of particular interest, since they have been shown to be 'standardizable candles', that is to say that based on the properties of the spectrum and light curve, the intrinsic luminosity can be inferred. This in turn allows for a measurement of the distance to the object. (The relationship has been calibrated using SN-Ia events with independently measured distances.) Because they are so bright, SN-Ia can be observed to cosmologically large distances. The combination of SN-Ia luminosity distances and redshift measurements of their host galaxies has notably been used to measure the acceleration of cosmological expansion.
