HYPERNOVA

'Hypernova' (pl. ''hypernovae'') refers to an exceptionally large star that collapses at the end of its lifespan—for example, a collapsar, or a large supernova. Up until the 1990s, it had a more specific meaning to refer to an explosion with an energy of over 100 supernovae (10⁴⁶ joules). Such explosions were proposed to explain the exceptional brightnesses of gamma ray bursts. An extensive sky search found several apparent hypernova remnants, but the frequency was too low to support the hypothesis.^[1] Today the term is used somewhat more prosaically to describe the supernovae of supermassive stars, the hypergiants, which have masses from 100 to 150 times that of the Sun. Hypernovae can pose serious threats to Earth in terms of radiation output, but no stars capable of creating hypernovae are located near Earth.
The word 'Collapsar,' short for ''collapsed star,'' is an early word for the end product of stellar gravitational collapse, a stellar-mass black hole. The word, used in this sense, is obsolete; but the term "collapsar" now sometimes refers to a specific model for the collapse of a fast-rotating star, as discussed below.

Contents

Collapsing star

Collapsars in science fiction

References

See also

References and further reading

Collapsing star

The core of the hypernova collapses directly into a black hole and two extremely energetic jets of plasma are emitted from its rotational poles at nearly the speed of light. These jets emit intense gamma rays, and are a candidate explanation for gamma ray bursts. In recent years a great deal of observational data on gamma ray bursts significantly increased our understanding of these events, and made clear that the collapse model produces explosions that differ only in detail from more or less ordinary supernovae. Nevertheless, they continue to sometimes be referred to in the literature as hypernovae. The word hypernova itself was coined by S.E. Woosley.
Since stars sufficiently large to collapse directly into a black hole are quite rare, hypernovae would likewise be rare, if they indeed occur. It has been estimated that a hypernova would occur in our galaxy every 200 million years.
'Collapsar' is currently used as the name of a hypothetical model where a fast-rotating Wolf-Rayet star with a massive (greater than 30 solar masses) core collapses to form a large, rotating black hole, drawing in the surrounding envelope of stellar matter at relativistic speeds with a Lorentz factor of around 150. These speeds would make collapsars the fastest known celestial objects. They may be considered to be "failed" Type Ib supernovae.
It is believed that collapsars are the cause of long (> 2 seconds) gamma-ray bursts, since powerful energy jets would be created along the rotation axis of the black hole, creating a burst of high-energy radiation to an observer whose line of sight is along the jet.
A possible example of a collapsar is the supernova Sn1998bw, which was associated with the gamma-ray burst GRB980425. This was classified as a type Ic supernova due to its unusual spectral properties in the radio spectrum, indicating the presence of relativistic matter. However, it should be noted that Sn1998bw was an unusual supernova, and that GRB980425 was an unusual gamma-ray burst.
Another type of hypernova is a Pair-instability supernova, of which SN 2006gy may have been the first observed example. This supernova event was observed in a galaxy about 240 million light years (72 million parsecs) from Earth. In a pair-instability supernova, the pair production effect causes a sudden pressure drop in the star's core, leading to a rapid partial collapse, which causes a rapid rise in temperature and pressure leading to an explosive thermonuclear burning and complete explosion of the star.

Collapsars in science fiction

Collapsars form Einstein-Rosen Bridges which enable the much faster-than-light movement of starships in Joe Haldeman's science fiction novel ''The Forever War.''
Larry Niven used a massive collapsar to create a time travel effect for the short story "Singularities Make Me Nervous."
The Disney movie The Black Hole featured a collapsar as a subject of scientific study by a deranged scientist determined to enter it.
Jerry Pournelle used a collapsar as a means of causing a ship to drop out of hyperspace unintentionally due to the gravitational effect in the story "He Fell Into a Dark Hole."

References

★ A. I. MacFadyen and S. E. Woosley "Collapsars: Gamma-Ray Bursts and Explosions in 'Failed Supernovae'" ''Astrophysical Journal'', Vol 524, Pages 262–289, October 1999.

★ Stanford E. Woosley "Gamma-ray bursts from stellar mass accretion disks around black holes" ''Astrophysical Journal'', Vol 405, Pages 273–277, March 1993.

★ Tsvi Piran "The Physics of Gamma-Ray Bursts" ''Reviews of Modern Physics'', Vol 76, October 2004

See also

★ Supernova

★ Nova

★ Dwarf nova

★ Gamma ray burst

★ Black hole

★ Wolf-Rayet star

★ Pair-instability supernova

References and further reading

★ Cosmological Gamma-Ray Bursts and Hypernovae Conclusively Linked European Southern Observatory (ESO)