Hypernova
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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 released energy of over 100 supernovae (1046 joules). Such explosions were proposed to explain the exceptional brightnesses of gamma ray bursts.
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[edit] 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 collapsar 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 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.
[edit] Collapsars in science fiction
Collapsars enable the much faster-than-light movement of starships in Joe Haldeman's science fiction novel The Forever War.
[edit] 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
