Starbirth. Computer illustration of a protostar accreting material before nuclear reactions begin. Stars form from clouds of gas which collapse due to gravitational attraction until the particles are hot and dense enough for nuclear reactions to begin. The energy from these reactions balances the collapse and the new solar wind blows away most of the remaining material. Denser material, such as the bodies in the disc around this protostar, may remain after the star is born and accrete over time to form a planetary system.