The normal route by which this happens involves a white dwarf drawing material off a main sequence or red giant star to form an accretion disc.
When they finally merge, if their combined mass approaches or exceeds the Chandrasekhar limit, carbon fusion is ignited, raising the temperature.
Since a white dwarf consists of degenerate matter, there is no safe equilibrium between thermal pressure and the weight of overlying layers of the star.
Because of this, runaway fusion reactions rapidly heat up the interior of the combined star and spread, causing a supernova explosion.
[5] Neutron star mergers occur in a fashion similar to the rare type Ia supernovae resulting from merging white dwarfs.
When they meet, their merger leads to the formation of either a heavier neutron star or a black hole, depending on whether the mass of the remnant exceeds the Tolman–Oppenheimer–Volkoff limit.
[14] An analysis of the eclipses of KIC 9832227 initially suggested that its orbital period was indeed shortening, and that the cores of the two stars would merge in 2022.
[16] [17][18] [19] However subsequent reanalysis found that one of the datasets used in the initial prediction contained a 12-hour timing error, leading to a spurious apparent shortening of the stars' orbital period.