Supernova

A supernova is a type of 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 the explosion of a supernova compresses nearby clouds, it can induce their gravitational collapse to form new stars, and enrich those new stars in heavy elements.

Supernovae can release several times ergs of energy, roughly equivalent to the output of the Sun over its entire lifetime.

Table of contents

1 Classification

1.1 Type Ia
1.2 Type Ib and Ic
1.3 Type II

2 Naming of Supernovae
3 Notable supernovae
4 Role of supernovae on stellar evolution
5 See also

Classification

As part of the attempt to understand supernova explosions, astronomers have classified them according to the lines of different chemical elements that appear in their spectra. See 'Optical Spectra of Supernovae' by Filippenko (Annual Review of Astronomy and Astrophysics, Volume 35, 1997, pp. 309-355) for a good description of the classes.

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 and the shape of its light curve.

Type Ia

Type Ia supernovae lack helium and present a silicon absorption line in their spectra near peak light. The most commonly accepted theory of these type of supernovae is that they are the result of a carbon-oxygen white dwarf accreting matter from a nearby companion star, typically a red giant, until it reaches the Chandrasekhar limit. The increase in pressure from the resultant collapse of the star ignites carbon fusion in the star's core. This in turns causes the star to explode violently and to release a shockwave in which matter is typically ejected at speeds on the order of 10,000 km/s. The energy released in the explosion also causes an extreme increase in luminosity.

The theory of these type of supernovae is similar to that of novae, in which a white dwarf accretes matter more slowly and does not reach the Chandrasekhar limit. In the case of a nova, the infalling matter causes a fusion reaction of material near its surface but does not cause the star to collapse.

Type Ia supernovae have a characteristic light curve (graph of luminosity as a function of time after the explosion). Near the time of maximum luminosity, the spectrum contains lines of intermediate-mass elements from oxygen to calcium; these are the main constituents of the outer layers of the star. Months after the explostion, when the outer layers have expanded to the point of transparency, the spectrum is dominated by light emitted by material near the core of the star: heavy elements synthesized during the explosion, most prominently iron-group elements. The radioactive decay of Nickel-56 through Cobalt-56 to Iron-56 produces high-energy photons which dominate the energy output of the ejecta at intermediate to late times.

Unlike the other types of supernove, Type Ia supernovae are generally found in all types of galaxies, including ellipticalss. They show no preference for regions of current star formation.

The similarity in the shapes of the luminosity profiles of all known Type Ia supernovae has led to their use as a standard candle in extragalactic astronomy. The cause of this similarity in the luminosity curve is still an open question mark.

The Type Ia supernova releases the highest amounts of energy amongst all known classifications of supernovae. The farthest single object ever detected in the universe (galaxies or globular clusters don't count) was a Type Ia supernova located billions of light-years away.

Type Ib and Ic

The early spectra of Types Ib and Ic do not show lines of hydrogen, nor the strong silicon absorption feature near 6150 Angstroms. These events, like supernovae of Type II, are probably massive stars running out of fuel at their centers; however, the progenitors of Types Ib and Ic have lost most of their envelopes due to strong stellar winds or interaction with a companion. Type Ib supernovae are thought to be the result of a Wolf-Rayet star collapsing.

Type II

A supernovae of Type II results when the core of a very massive star, having exhausted all lighter elements, begins fusing iron, which absorbs 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.

Type II supernovae can be further classified based on the shape of their light curves into Type II-P and Type II-L. Type II-P reach a "plateau" in their light curve while II-L's have a "linear" decrease in their light curve ("linear" in magnitude versus time, or exponential in luminosity versus time). This is believed to result from differences in the envelope of the stars. II-P's have a large hydrogen envelope that traps energy released in the form of gamma rays and releases it slowly, while II-L's are believed to have much smaller envelopes converting less of the gamma ray energy into visible light.

One can also sub-divide supernovaed of Type II based on their spectra. While most Type II supernova show very broad emission lines which indicate expansion velocities of many thousands of km/s, some have relatively narrow features which may be produced by the interaction of the ejecta with circumstellar material; these are called Type IIn, where the "n" stands for "narrow". A few supernovae, such as SN 1987K and 1993J, appear to change types: they show lines of hydrogen at early times, but, over a period of weeks to months, become dominated by lines of helium. The term "Type IIb" is used to describe the combination of features normally associated with Types II and Ib. These are likely massive stars which have lost most, but not all, of their hydrogen envelopes. As the ejecta expand, the hydrogen layer quickly becomes optically thin and reveals the deeper layers.

Some exceptionally large stars may instead produce a "hypernova" when they die, a theoretical type of explosion. In the hypernova mechanism, 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 one of many candidate explanations for gamma ray bursts.

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
1885 - S Andromedae in the Andromeda Galaxy, discovered by Ernst Hartwig
1987 - Supernova 1987A in the Large Magellanic Cloud, 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.

Role of supernovae on stellar evolution

Supernovae tend to enrich the surrounding interstellar medium with metals (that for astronomers, are all the elements after helium). Thus, each stellar generation has a slightly different composition, going from an almost pure mixture of hydrogen and helium to a more metal-rich composition. The different chemical abundances have important influences on the star's life, and may decisively influence the possibility of having planets orbiting it.