What is a hypernova?
A hypernova, also known as a collapsar, is an extremely energetic supernova. The two are not to be confused, even if their formation is very similar. In a supernova, a star shears off its outer matter but leaves a new star at its centre, often a neutron star. In a hypernova, the force of the explosion tears the inner star apart too. Hypernovas occur in stars with a mass greater than 30 times that of our Sun. Like in a supernova, as the star runs out of fuel it can no longer support itself under its own gravity. It collapses and subsequently explodes, sending out matter in all directions. This releases more energy in seconds than our Sun will in its entire 10 billion-year lifetime.
Hypernovas are incredibly rare. In fact, the rate of hypernovas occurring in the entire Milky Way is estimated to be one every million years, making the observation of the celestial explosions particularly difﬁcult. Twenty-ﬁve million light years from Earth in another galaxy astronomers have found what appear to be the remnants of a giant hypernova, providing new information about these huge explosions, but currently there are several theories as to what actually causes them. One school of thought is that a massive star rotating at a very high speed or encased in a powerful magnetic ﬁeld explodes, ripping apart the inner core. Alternatively, a hypernova could be the result of two stars in a binary system colliding with each other, merging into one gigantic mass and subsequently exploding.
The result is clear, however. A black hole is produced and a huge amount of energy is released in the form of a gamma-ray burst, one of the brightest known events in the universe. In fact, a hypernova releases several million times more light than all of the Milky Way’s stars put together.
In the image above a massive star (30+ solar masses) collapses to form a rotating black hole emitting twin energetic jets, surrounded by an accretion disc of debris. The star is subsequently torn apart by vigorous winds of newly formed isotope 56Ni blowing off the accretion disc, and shock waves produced as the jets plough through the stellar material. The hypernova, whose luminosity is powered by the radioactive decay of 56Ni, is the result of the explosion of the star.