Unlike our Sun, which is a relatively low mass star and which will gradually fade before expanding into a red giant at the end of its life, larger mass stars undergo extreme explosive events when they die which outshine their entire galaxies. Scientists have been investigating the death of a high mass star and the dramatic events which occur in a hypernova.
When a very heavy star which is more than 25 times the mass of our Sun runs out of fuel, it explodes and produces a hypernova and sometimes a gamma-ray burst (GRB). “The first hypernova was detected in 1998 as a very energetic type of supernova that followed a gamma-ray burst. This was the first evidence of the connection between both phenomena,” Dr. Luca Izzo, researcher at the Institute of Astrophysics of Andalusia and leader of the study, said in a statement.
A hypernova, also known as a collapsar, is even more energetic than a supernova and occurs when a star collapses into a black hole or neutron star while emitting twin jets of energy. These jets are so powerful that they drill through the external layers of the star and shoot out into the space beyond. The jets produce gamma rays, which is what gives rise to the GRB. Eventually the material from the external layers of the star are ejected as well in a hypernova explosion which is brighter than a supernova by tens of times.
Scientists have long observed that GRBs are accompanied by a hypernova, but they did not understand the relationship between the two events as there were some hypernovas that did not produce GRBs.
The new research finds that there are two types of hypernova: one with a “hot cocoon” which is generated around the jet of energy as it pushes through the layers of the star and which produces a GRB, and one type without a cocoon that lacks the energy to pierce the external layers of the star and which does not produce a GRB. “This work has allowed us to find the missing link between these two types of hypernova through the detection of an additional component: A sort of hot cocoon generated around the jet, as it propagates through the outer layers of the progenitor star,” Izzo said. “The jet transfers a significant part of its energy to the cocoon and, if it manages to reach the surface of the star, will produce the gamma-ray emission that we know as a GRB.”
The study is published in Nature.