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 $10^{44}$ joules of energy, roughly equivalent to the output of the Sun over its entire lifetime.

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.

Summary

Type I: No Balmer lines
Type II: Has Balmer lines

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 shock wave 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 explosion, 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 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. In the late 1990s, observations of type Ia supernovae produced the unexpected result that the universe seems to undergo an accelerating expansion.

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

Stars far more massive than our sun evolve in far more complex fashions. In the core of our sun, 665 million tons of hydrogen fuse into 660 million tons of helium every second, the extra 5 million tons of mass converted into pure energy which then radiates outwards. The helium, being heavier, sinks to the center of the star, where it builds up, since temperatures in the core are not yet high enough to cause it to fuse. Eventually, as the hydrogen at the core is exhausted (or really, progressively crowded out of the core by the ongoing build-up of helium), fusion begins to slow down and gravity begins to cause the core to contract. This contraction spikes the temperature high enough to initiate a comparatively brief phase of helium burning (less than 10% of the star's total life-time). When the helium begins to exhaust itself by the same process, and the star begins to contract again, the built-up carbon doesn't burn -- the total gravity of the star just doesn't cause temperatures to go high enough. This core of primarily unburned carbon is called a white dwarf.

A much larger star, however, has the kind of gravity needed to create temperatures and pressures sufficient to cause the carbon in the core to begin to fuse once the star contracts. The cores of these massive stars become layered like onions as progressively heavier atomic nuclei build up at the center, with an outermost layer of hydrogen gas, sinking down on a layer of hydrogen fusing into helium, with the helium sinking down into a layer of helium fusing into carbon, with the carbon sinking down to fuse into ever heavier elements. These stars go through progressive stages where the core will shrink, built-up atomic nuclei which were previously unfusable begin to fuse, and the core springs back into equilibrium with gravity.

The limiting factor in this process is in the binding energy of these atomic nuclei. These progressively heavier nuclei are also more and more tightly bound, releasing less energy per fusion reaction than their lighter counterparts. It happens that iron is the most tightly bound nucleus on the periodic table. Nuclei lighter than iron are progressively less tightly bound; nuclei heavier than iron are also, progressively, less tightly bound. This is why, to create energy from nuclei lighter than iron, you fuse them into a single nucleus with less total binding energy than the original two, which releases that energy outward; to create energy from nuclei heavier than iron, you split them, creating two nuclei with less total binding energy than the original, releasing that energy outward. So it is easy to see that, when it is iron building up in the star's core (as it eventually is), no further reactions in the core will release energy to prevent the star's collapse. What happens, then, when the iron core begins to build up?

The iron (Fe) core is under huge gravitational pressure, and since there is no fusion and cannot be supported by ordinary gas pressure, it is supported by electron degeneracy pressure, the electrons pushing against other electrons. When it builds up to the Chandrasekhar limit at which electron degeneracy pressure cannot sustain it, the iron core begins to collapse. The collapsing core produces high energy gamma rays, which decompose some iron nuclei into 13 He plus 4 neutrons, a process known as photodisassociation. However, no nuclear reaction of an iron nucleus can create energy; it can only absorb it. Thus, where reactions in the core have for millions of years been radiating energy outward, balancing the star against gravity, they suddenly begin sucking energy inwards, joining hands with gravity to cause the core, a massive structure the size of our sun, to collapse within a fraction of a second.

As the density in the collapsing core skyrockets, electrons and protons are pushed together until their electrical attraction overcomes their inherent nuclear repulsion from each other. This combination creates a neutron and releases a neutrino. This is called "electron capture". The neutrinos escape the core, carrying away energy and further accelerating the collapse, which proceeds in milliseconds as the core detaches from the outer layers of the star and reaches the density of nuclear matter, where the neutrons press against each other and the entire core is the density of an atomic nucleus. This is the core collapse. At this point the collapse is stopped and actually bounces a little, creating a shock wave which slams into the collapsing outer layers of the star. A "proto-neutron star" begins to form at the core, though if it is massive enough, it will continue collapsing to form a black hole.

Currently it is not understood how this shock wave completes the process of pressuring the outer layer of the star. This is the "supernova problem", and various models suggest different ways that the huge amounts of energy present are formed into a single explosion. One such model is the Convective overturn model, which suggests that convection, either from neutrinos from below, or infalling matter from above, completes the process of destroying the progenitor star. Heavier elements than iron are formed during this explosion by neutron capture, and from the pressure of the neutrinos pressing into the boundary of the "neutrinosphere", seeding the surrounding space with a cloud of gas and dust which is richer in heavy elements than the one the star originally formed from.

Neutrino physics, which is not well understood, is crucial to the understanding of this process, since half of the energy of the core collapse is radiated out in the form of neutrinos. The other crucial area of investigation is the "hydrology" of the plasma that makes up the dying star, how it behaves during the core collapse determines when and how the "shock wave" forms and when and how it "stalls" and is re-energized.

The remaining core of the star may become a neutron star or a black hole, depending on its mass, although because the processes of supernova collapse are poorly understood, 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 supernovae 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 a very massive 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 International Astronomical Union, 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

File:Crab.nebula.arp.750pix.jpg

The Crab Nebula is an expanding cloud of gas created by the 1054 supernova.

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.

External links

Supernova produces cosmic rays
List of recent supernovae
The SNEWS project (SuperNova Early Warning System) uses neutrino detectors to build a network that will (hopefully) provide advance notice of a supernova explosion
A review article on SNEWS