BBC: Type Ia supernovae have long been thought to be caused when a white dwarf gains enough mass to push it over the Chandrasekhar limit. White dwarfs are the remains of stars that have burned through most of their light elements and have a core of carbon and oxygen that is no longer undergoing fusion. If they steal enough material from a companion star, or merge with another white dwarf, the extra mass compresses the core and causes the heavier elements there to fuse. The sudden release of energy causes the star to explode into a supernova. Although the theory has been widely accepted for decades, a supernova detected on 21 January 2014 has provided the first confirmation that a white dwarf is the source of type Ia supernovae. When a white dwarf’s carbon and oxygen fuse, they create a radioactive isotope of nickel that decays into radioactive cobalt that then decays to stable iron. Observations of the supernova using the European Space Agency’s INTEGRAL spacecraft revealed the signature gamma-ray emissions associated with the cobalt isotope’s decay. The emissions also closely matched the model of a white dwarf explosion. However, there is still not enough information to determine whether it was a white dwarf drawing material from a companion or a collision of two white dwarfs that caused the supernova.