The implosion of a star resulting from its own gravity; the result is a smaller and denser celestial object WordNet 3.0 & Open English WordNet

Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the center of gravity. Wikipedia

The stage in the evolution of a star in which the pressure of the star is insufficient to maintain it at a stable size, and its material falls inward under its own gravitational attraction, eventually forming a black hole or a neutron star, and sometimes accompanied by a supernova explosion. Wiktionary

Stage in the evolution of a star. Wiktionary (translation)