gravitational collapse

Gravitational collapse is the actual falling-together: once gravity has won its tug-of-war with pressure, the gas in a dense core stops resisting and rushes inward toward its own centre. It is the moment a cold, dark blob of cloud begins, in earnest, to become a star. Think of it as the cloud falling in on itself under its own weight.

When a core crosses its critical mass, every part of it begins to fall toward the centre, and as the gas pulls together it gets denser, which makes gravity locally stronger, which makes it fall faster still, a runaway in which collapse accelerates as it proceeds. The pull-in releases gravitational energy that heats the gas. At first the gas stays cool because it radiates that heat away through its dust in the infrared, but as the centre grows dense enough to trap its own radiation, it warms and pressure builds back up, slowing the fall and forming a small, hot, still-growing seed at the core: a protostar. A core of about a solar mass can complete its main collapse in roughly a hundred thousand years, brief by cosmic standards.

Gravitational collapse is the central event of star formation, the hinge between inert gas and a living star, and the same physics, scaled up or down, governs how galaxies assemble and how massive stars eventually die. A key subtlety is that gas almost never falls straight in: the cloud is slowly rotating, and as it shrinks it must spin faster (the way a skater speeds up by pulling in their arms), so the infalling gas cannot all reach the centre and instead flattens into a spinning disk around the new protostar. Collapse and spin together are what make stars come with disks, and disks are where planets are later born.