A hypernova (plural: hypernovae) is a very powerful and extremely luminous stellar explosion. They are thought to be caused by the collapse of a massive star (> 20 solar masses) into a black hole or neutron star. Hypernovae are incredibly rare events, with only a handful of confirmed cases in our galaxy.
Hypernovae are characterized by their extremely high luminosity (typically 10^44 erg/s), large ejecta mass (10-50 solar masses), and high expansion velocities (up to 10% the speed of light). These characteristics make them some of the most energetic events in the universe.
The first hypernova was discovered in 1998, when astronomers observed the supernova SN 1998bw. This event was associated with the gamma-ray burst GRB 980425, making it the first known case of a “long” gamma-ray burst (GRBs are typically shorter lived). Subsequent observations have shown that many other long GRBs are likely associated with hypernovae.
The mechanism by which hypernovae occur is still not fully understood, but it is thought to be related to the way in which massive stars evolve and die. Massive stars burn through their fuel quickly and end their lives in spectacular supernova explosions. The most massive stars (> 40 solar masses) are thought to collapse directly into black holes, while slightly less massive stars (< 30 solar masses) form neutron stars. It is believed that intermediate-mass stars (20-40 solar masses) may produce either type of object depending on their rotation rate; faster rotatingstars are more likely to form black holes while slower rotators will become neutron stars. It is thought that hypernovae occur when a rotating star collapses into a black hole. As the star falls inward, it spins up due to conservation of angular momentum; this increased rotation rate amplifies the magnetic fields within the star, leading to an influx of energy that eventually causes the star to explode. This scenario has been supported by observations of magnetars (a type of neutron star with an exceptionally strong magnetic field), which are often found near sites of pasthypernova explosions.