A hypernova is a supernova turned up by an order of magnitude. Its debris carries more than 10⁴⁵ joules of motion, about ten times as much as a typical core-collapse supernova, and parts of it can reach 99 percent of the speed of light. The star behind it must be enormous, more than about 30 times the mass of the Sun, and its core does not end as a neutron star. It becomes a black hole.
SN 1998bw
On 25 April 1998 a satellite detected a burst of gamma rays. A day later astronomers found a supernova in the same patch of sky, in the spiral galaxy ESO 184-G82, about 140 million light-years away. It was the first time a gamma-ray burst had been tied to a supernova.
SN 1998bw was no ordinary explosion. It was about 100 times as luminous as a standard supernova of its kind, and its spectrum showed no hydrogen and no clear helium. The star had lost its outer layers before it died. About ten solar masses of material were thrown out, among them some 0.4 solar masses of radioactive nickel, whose decay kept it shining for weeks. Simple, round models could not produce that much energy, which pointed to a lopsided blast driven by jets. The rest of the star, which had weighed perhaps 25 to 40 Suns, is thought to have fallen into a newborn black hole.
The collapsar
The leading explanation is called the collapsar model. A rapidly spinning, massive core collapses into a black hole while the surrounding star is still falling in. The infalling gas forms a disc, and the spin launches two jets along the axis. If the jets punch all the way out of the star and one points at Earth, we see a long gamma-ray burst. The rest of the star explodes as a hypernova.
This link was confirmed again in October 2022, when the brightest gamma-ray burst ever recorded, GRB 221009A, turned out to have a supernova much like SN 1998bw underneath it. Not every hypernova has to produce a gamma-ray burst we can see, and astronomers still debate exactly where the line between a hypernova and an ordinary supernova lies.