Collapse
Interstellar molecular clouds of dust and gas exist in a quasi-stable balance between self-gravity and internal pressure. If the mass and density are large enough, and the temperature and pressure low enough, they can collapse to form stars, usually in clusters of a few dozen to a few thousand. Both gaseous and magnetic pressure can be factors. Various factors can trigger collapse, including shocks from collisions with other interstellar clouds, passing through one of the spiral density waves that give spiral galaxies their names, or the shockwave of a nearby supernova. Once a collapse starts, it can become irreversible, as gravity gets stronger as the cloud shrinks. The cloud will heat up as a result of the collapse, but if it contains plenty of dust, it can radiate that heat away in the infrared. Actual stellar formation seems to take place in localized regions of overdensity called cloud cores. Because the molecular clouds are turbulent, individual pieces are likely to have a...