gravitational collapse

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The implosion of a star or other celestial body under the influence of its own gravity, resulting in a body that is many times smaller and denser than the original body.
• noun The process by which stars, star clusters, and galaxies form from interstellar gas under the influence of gravity.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the implosion of a star resulting from its own gravity; the result is a smaller and denser celestial object