A hypernova is an exceptionally massive star that collapses at the end of its life. Until the 1990s, it was used to refer to an explosion with energy equivalent to over 100 supernovae (greater than 10^48 joules). Such an explosion was believed to be the source of long-duration gamma-ray bursts.
Since the 1990s, the term has been used to describe superluminous supernovae of the largest stars, supergiant stars with masses ranging from 100 to over 300 times that of the Sun. The decay of 56Ni, a short-lived nickel isotope, is believed to provide most of the light from a hypernova.
Hypernova is a special large star that collapses at the end of its lifespan.
The radiation from a hypernova at close range could cause serious damage to Earth, although no supergiant stars are known to be close enough to Earth to pose such a threat. A group led by Brian Thomas, an astrophysicist at Washburn University in Kansas, speculated that a hypernova might have caused a mass extinction during the Ordovician-Silurian period on Earth 440 million years ago, but there is no clear evidence to support this.
The term “collapsar,” short for collapsing star, was previously used to refer to the ultimate result of the gravitational collapse of a star, a black hole with the mass of a star. Nowadays, this term is sometimes used to refer to a specific model of the collapse of a rapidly rotating star.
The collapse of the core of a hypernova directly forms a black hole, and two ultra-relativistic plasma jets are ejected at nearly the speed of light from its two polar regions. These plasma jets emit powerful gamma rays and may provide an explanation for long-duration gamma-ray bursts. However, hypernovae cannot account for short-duration gamma-ray bursts, which appear to have no direct connection to massive stars because some short bursts occur in regions devoid of new star formation – a necessary condition since massive stars have short lifetimes and must be freshly formed.
The energy generated by a hypernova from a massive star, along with the dazzling effects, is necessary to explain the extreme brightness and great distance (over 10 billion light-years, as measured by the redshift of the ionization afterglow) of long-duration gamma-ray bursts (>2 seconds), which require gamma-ray emissions over 10^44 joules. The main mechanism is the creation of powerful energy streams along the rotational axis of the black hole, which produce high-energy radiations visible to observers along the path of these plasma streams, even at great distances on the outskirts of the observable universe. To date, hypernovae and long gamma-ray bursts have been observed in other galaxies but have not yet been observed in our galaxy.
Because stars massive enough to collapse directly into a black hole are relatively rare, hypernovae would be quite rare if they indeed occur. It is estimated that a hypernova occurs approximately once every 200 million years in our galaxy.
In recent years, a large amount of observational data on long-duration gamma-ray bursts has significantly increased our understanding of these events. It has become clearer that the collapse models result in different specific explosions with varying amounts of brightness. However, these are sometimes referred to in the literature as hypernovae. The term “hypernova” was coined by S.E. Woosley.
Unstable hypernova 2006gy.
Collapsars are now used to refer to a model where a rapidly rotating Wolf-Rayet star with a large core (over 30 times the mass of the Sun) collapses to form a rapidly rotating massive black hole that swallows up all matter around it at relativistic speeds with a Lorentz factor of around 150. The speeds make collapsars the fastest-moving objects known. They can be considered a subtype of Type Ib and Ic supernovae, “failed” supernovae.
An example could be the unusual supernova SN 1998bw, along with gamma-ray burst GRB 980425. This supernova was classified as a Type Ic supernova due to its characteristic spectral properties in the spectrum. It showed the presence of material relativistically.
Another type of hypernova is an unstable binary supernova, which includes SN 2006gy, possibly the first example ever observed. It was observed in a galaxy located about 240 million light-years (72,000,000 parsecs) away from Earth. In an unstable binary supernova, the production of pairs creates a sudden drop in pressure in the star’s core, leading to a rapid core collapse, a sharp increase in temperature, and pressure, leading to a thermonuclear explosion and the complete detonation of the star.