Supernova — A supernova is a star that increases its brightness drastically within a matter of days, making it appear as if a “new” star was born (hence “nova”).
The “super” prefix distinguishes it from a mere nova, which also involves a star increasing in brightness, though to a lesser extent and through a much different mechanism.
Astronomers have classified supernovae in several classes, according to the lines of different elements that appear in their spectra.
The first element for division is the presence or absence of a line from hydrogen. If a supernova’s spectrum does not contain a hydrogen line, it is classified type I, otherwise type II.
Among those groups, there are subdivisions according to the presence of other lines.
Type Ia supernovae don’t have helium, and present a silicon line. They are generally thought to be caused by the explosion of a white dwarf, at or close to the Chandrasekhar limit.
One possibility is that the white dwarf was orbiting a moderately massive star. The dwarf pulls matter from its companion to the point that it reaches the Chandrasekhar limit. The dwarf collapses into a neutron star or black hole, and the collapse causes the remaining carbon and oxygen atoms in it to fuse.
This fusion produces a shockwave, and the dwarf is blown to bits. This is different from the mechanism of a nova in which the white dwarf doesn’t reach the Chandrasekhar limit and collapse, but merely ignites nuclear fusion in the matter it has accreted on its surface.
The increase in luminosity is given by energy liberated by the explosion, and the rather long time it takes to decline is fueled by radioactive cobalt decaying into iron.
Type Ib and Ic do not have the silicon line and are even less understood. They are believed to correspond to stars ending their lives (as type II), but they would have lost their hydrogen before, thus the H lines don’t appear on their spectra. Type Ib supernovas are thought to be the result of a Wolf-Rayet star collapsing.
Type II results when a very massive star’s core begins fusing iron, which uses energy instead of liberating it. When the mass of the iron core reaches the Chandrasekhar limit (this takes only a matter of days), it decays spontaneously into neutrons and collapses.
A tremendous burst of neutrinos is produced, removing energy from the star. Through a process that is not well understood some of the energy liberated in the neutrino burst is transferred to the outer layers of the star.
When the shock wave reaches the surface of the star several hours later, there is a massive increase in brightness. The core of the star may become a neutron star or a black hole, depending on its mass, although because of the lack of understanding of the processes of supernova collapse, it is unknown what the cutoff mass is.
There are also other slight variants of these types with designations such as II-P and II-L, but these just describe the behavior of the light curves of the events (II-P show a temporary plateau in the light curves, whereas II-L do not) and not fundamentally different causes.
Some exceptionally large stars may instead produce a “hypernova” when they die, a relatively new and largely theoretical type of explosion. In a hypernova, the core of the star collapses directly into a black hole and two extremely energetic jets of plasma are emitted from its rotational poles at nearly light speed. These jets emit intense gamma rays, and are a candidate explanation for gamma ray bursts.
Type I supernovae are considerably brighter than Type IIs, all other things equal.
Naming of Supernovae
Discoverings of supernovae are reported to the IAU, that sends out a circular with the name it assigns to it. The name is formed by the year of discovery, and a one or two letter designation. The first 26 supernovae of the year get a letter from A to Z, after Z, they start with aa, ab, and so on.