Hypernova

A hypernova is a supernova with an electromagnetically radiated energy of more than 1045 joules assuming a spatially isotropic radiation. A Hypernova represents the upper end of the super luminous or ultra-bright supernovae

Properties

Hypernovae are divided according to their light curves and spectral characteristics into three classes:

• In type I, no traces of hydrogen show in their spectra.
• In type II, however, hydrogen can be detected in the spectra during the explosion.
• In type R the tail of the light curve can be described by the radioactive decay by an unusually large amount of 56Ni. The amount needed is in the order of five solar masses.

Compared to core-collapse supernovae hypernovae are very rare, with 1,000 to 10,000 core-collapse supernovae to each Hypernova. They are almost exclusively observed in small galaxies with high star formation rates similar to the Magellanic Clouds.

Pair instability supernova

The term hypernova has been used for the first time by Woosley & Weaver to describe known today as a pair instability supernova phenomenon. Here achieve very massive stars with masses greater than 100 or - depending on the source - also 150 solar masses, at its core, a temperature of more than 1010 Kelvin. After the central carbon burning is here a process of pair instability when convert extremely high-energy photons into electron- positron pairs and thus a gravitational instability occurs. Cause of this instability is that the mass and gravity in the conversion of photons into electron- positron pairs does not change, the radiation pressure as a counteraction to gravity but eliminated. Depending on the mass of the star is thus either completely torn or into a black hole. This can result in up to 50 solar masses of 56Ni, whose radioactive decay is the main energy source for the energy shown in the light curve, emitted by the supernova. It can amount of energy of up to 1046 Joule can be released.

The pair instability supernovae are particularly frequent in population III. These are the first stars that have formed from the three elements ( hydrogen, helium and lithium) of the primordial nucleosynthesis or from the first next generation. In contrast to today's population I the vanishingly low metallicity limited the intensity of the radiation pressure caused by the stellar wind and thus the upper mass limit of Blue giant is not around 150 solar masses. Therefore hypernovae in the form of pair instability supernovae have occurred in the early universe much more common. Today, as massive stars form mainly through the merger of two stars in a close binary system.

CSM model

A normal core-collapse supernova can release additional energy when the progenitor star was a supergiant or luminous Strong Blue variables. These stars have lost significant amounts of matter in stellar winds before their gravitational collapse and accelerated in the supernova explosion of matter collides with the circumstellar matter. This type of HyperNova shows a broader light curve because the additional energy is carried out by the conversion of kinetic energy into electromagnetic radiation after the explosion process. It also shows the spectral properties of supernovae of type IIn.

The collapsar model

The collapsar model describes a core-collapse supernovae, from which a black hole. This is first formed in the supernova explosion a proto neutron star and expanding matter. However, the liberated kinetic energy is not enough to break out of the star's surface, and the matter falls over an accretion disk back to the neutron star, which then exceeds its stable mass limit and collapses into a black hole. Rotates the progenitor star fast enough, so you can form relativistic jets and escape from the star along the rotational axis. Are the jets aligned in the direction of the earth, so they appear as gamma-ray bursts. Even more energy can be released when the proto- neutron star has a magnetic field with a magnetic flux density of more than 1015 gauss. The decay of the magnetic field can release energies of up to several 1053 erg. Also to the collapsar models include a variant, according to which a massive star collapses directly into a black hole and the additional energy of the supernova from the rapid accretion of matter is generated in the black hole. In this scenario, the forerunner of the Hypernova is a blue supergiant, preventing its gravitational potential, the shock wave of the supernova speeds up the largest part of the atmosphere above the escape velocity addition.

Core-collapse models

The observed luminosities of hypernovae can also be simulated with traditional gravitational collapse models. The luminosities would be incurred if it is possible to generate more than 3.5 solar masses of 56Ni and it is an asymmetric supernova explosion in the direction of the observer Stripped - Envelope Supernovae. According to computer simulations stars can to avoid a pair instability event, bring forth this amount of radioactive nuclides with an initial mass of more than 100 solar masses and a metallicity that is just sufficient. However, this is highly dependent on the little-known mass-loss rate to explode as a supernova.

Threat

Some scientists have proposed as a candidate Eta Carinae in the Milky Way, which could end within the next 20,000 years as a hypernova. Extremely massive stars burn their fuel reserves much faster than lower mass stars like the sun. Therefore, they have a lifetime of only a few million instead of a few billion years.

A principle existing threat of gamma-ray bursts is illustrated by the theory that the mass extinction on Earth 440 million years, to have been of such a gamma-ray burst during the transition from Ordovician to Silurian era, the result.