|
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 the explosion of a supernova
compresses nearby clouds (the results of nearby explosions), such compression can form a solar nebula.
Supernovae can release several times 1044 joules of energy. This has resulted in the
foe (1044 joules) being the standard unit of energy
in supernova research.
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.
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
Type Ia supernovae lack helium and present a silicon line in the emission spectrum. The most prevalent theory of these type of supernovae is that they are the
result of a carbon-oxygen white dwarf which accretes 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 luminosity profile. Near the time of maximum luminosity, the spectrum contains lines
of intermediate-mass elements from oxygen to calcium. At the tail end of the curve, the spectrum is dominated by ionized iron which is largely the result of the decay of radioactive cobalt.
Unlike the other types of supernove, Type Ia supernovae are generally found in all types of galaxies, including ellipticals. 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
Type Ib and Ic do not have the silicon line and 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
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.
Type II supernovae can further be divided into type II-P and II-L. Type II-P reach a "plateau" in their light curve while II-L's have a "linear" decrease in their light curve. 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.
The decay of the light curve in Type-II supernova follows the decay of nickel-58.
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
- 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.
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.
See also
|