Supernova

A supernova is a stellar explosion which appears to result in the creation of a new star, upon the celestial sphere. ("Nova" is Latin for "new"). The "super" prefix distinguishes this from a nova, which also involves a star increasing in brightness, though to a lesser extent and through a different mechanism. Supernovae involve the expulsion of a star's outer layers; filling the surrounding space with hydrogen and helium (along with other elements); the debris eventually forms clouds of dust and gas. When a supernovic explosion compresses nearby clouds (the results of nearby explosions), such compression can form a solar nebula.

Astronomers have classified supernovae in several classes, according to the lines of different elements that appear in their spectra.

The first element for your mum 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.

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A Slow Explosion

Why would a gamma ray burst fade so slowly? This behavior, recorded last October 2002, is considered a new clue into the cause of gamma-ray bursts, the most powerful explosions known in the universe. The burst, first detected by the orbiting HETE satellite and later tracked by numerous ground-based telescopes, showed an unusually slow and tumultuous decay in visible light. Speculations on the cause of the unusual light curve include a blast wave striking a windy circumburst medium, a blast wave energetically refreshed by a faster outgoing shock, and non-uniformity in a fast moving jet. Pictured above is the massive Wolf-Rayet star WR124, a star itself undergoing a slow explosion by producing a very powerful but tumultuous wind. Popular candidate progenitor sources for GRBs include supernova or hypernova explosions from massive stars, possibly ones with similarities to Wolf-Rayet stars. (Larger Version)

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 supernovae 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.

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Colliding Supernova Remnants

When a massive star exhausts its nuclear fuel it explodes. This stellar detonation, a supernova, propels vast amounts of starstuff outwards, initially at millions of miles per hour. For another 100,000 years or so the expanding supernova remnant gradually slows as it sweeps up material and ultimately merges with the gas and dust of interstellar space. Short lived by cosmic standards, these stellar debris clouds are relatively rare and valuable objects for astronomers exploring the life cycles of stars. Yet this double bubble-shaped nebula 160,000 light-years away in the Large Magellanic Cloud may represent something rarer still - the collision of two supernova remnants. This image in the light of excited Hydrogen atoms along with images at X-Ray, radio and other optical wavelengths, suggests that the bubbles are indeed two separate regions of hot gas surrounded by cooler dense shells begining to interact as they expand and make contact. (Larger Version)

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

Supernova discoveries are reported to the IAU, which 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.

Notable supernovae

1054 - the formation of the Crab Nebula, recorded by Chinese astronomers and possibly by Native Americans
1572 - Supernova in Cassiopeia, observed by Tycho Brahe, whose book De Nova Stella on the subject gives us the word "nova"
1604 - Supernova (Kepler's Star) in Ophiuchus, observed by Johannes Kepler; last supernova to be observed in the Milky Way
1987 - Supernova 1987A observed within hours of its start, it was the first opportunity for modern theories of supernova formation to be tested against observations.

The 1604 supernova was used by Galileo as evidence against the Aristotelian dogma of his period, that the heavens never changed.

Supernovae often leave behind supernova remnants; the study of these objects has helped to increase our knowledge of supernovae.