Hypernova

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Eta Carinae, in the constellation of Carina, one of the nearer candidates for a future 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. Until the 1990s, it referred specifically to an explosion with an energy of over 100 supernovae (10⁴⁶ joules); such explosions were proposed to explain the origin of exceptionally bright gamma ray bursts. An extensive sky search found several apparent hypernova remnants, but too few to support the hypothesis.^[1]

After the 1990s, the term came to be used to describe the supernovae of the most massive stars, the hypergiants, which have masses from 100 to 150 times that of the Sun. Decaying ⁵⁶Ni, a short-lived isotope of nickel, is believed to provide much of a hypernova's light.^[2]

The radiation output of a nearby hypernova could cause serious harm to Earth, but no hypergiants have been found near Earth.^[3] Some have conjectured that a hypernova may have caused a mass extinction on Earth 440 million years ago, but there is no unambiguous evidence of it.^[1]

The word collapsar, short for collapsed star, is an early word for the end product of stellar gravitational collapse, a stellar-mass black hole. This word in this sense is obsolete, but it is sometimes used to refer to a specific model for the collapse of a fast-rotating star, as discussed below.

Collapsing star

The core of a 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 sometimes to be referred to in the literature as hypernovae. The word hypernova itself was coined by S.E. Woosley^[4].

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 unusual supernova Sn1998bw, which was associated with the gamma-ray burst GRB980425. This was classified as a type Ic supernova due to its distinctive spectral properties in the radio spectrum, indicating the presence of relativistic matter.

Another type of hypernova is a pair-instability supernova, of which SN 2006gy was possibly 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 sharp rise in temperature and pressure leading to an explosive thermonuclear burning and complete explosion of the star.

See also

• Gamma-ray burst progenitors
• Quark star

References and further reading