Stars form inside a nebula, a huge cloud of dust and gas that is mostly hydrogen.1. Collapse. Slightly denser regions of the nebula pull in more material by gravitational attraction. The region collapses inwards and breaks into clumps, each of which may become a star.2. Heating. As a clump collapses, gravitational potential energy is converted into kinetic energy of the particles, so the temperature rises. The clump becomes a hot, dense ball of gas called a protostar. It glows (mainly in the infrared) because it is hot, but no fusion is happening yet.3. Fusion begins. The protostar keeps contracting and heating. When the core reaches a temperature of about 107 K and a high enough density, hydrogen nuclei are moving fast enough to overcome their electrostatic repulsion. Nuclear fusion of hydrogen into helium begins, releasing large amounts of energy.4. Main sequence star. The energy released creates an outward pressure (gas pressure and radiation pressure). When this balances the inward pull of gravity, the star stops collapsing and is in stable equilibrium. It is now a main sequence star, and will stay there for most of its life while it fuses hydrogen in its core.Material left over in the disc around the new star may clump together to form planets.