Stars form from interstellar clouds of dust and gas, mostly hydrogen. These clouds are called nebulae.If a region of the cloud is dense enough, the gravitational attraction between its particles makes it begin to collapse. As the cloud collapses, gravitational potential energy is transferred to kinetic energy of the particles, so the temperature of the gas rises. The hot, dense, collapsing core is called a protostar. It glows because it is hot, but no fusion is happening yet.The collapse continues and the core becomes hotter and denser. When the core temperature reaches about 107 K, hydrogen nuclei are moving fast enough to overcome the electrostatic repulsion between them, and nuclear fusion begins: hydrogen nuclei fuse to form helium nuclei, releasing energy.The energy released produces an outward radiation pressure, which acts together with the gas pressure of the hot core. The star becomes stable when this outward pressure balances the inward pull of gravity. The star is then in equilibrium and is called a main sequence star.A star stays on the main sequence for most of its life, fusing hydrogen into helium in its core. The Sun has been a main sequence star for about 4.6 billion years. More massive stars have hotter cores, fuse their hydrogen much faster and so spend less time on the main sequence.