From nebula to protostar
Stars form within large clouds of gas and dust called nebulae. Gravitational attraction draws material together into denser regions, causing the cloud to undergo gravitational collapse.
As a collapsing region becomes smaller, collisions between particles increase their kinetic energy, so the gas becomes hotter. The resulting hot, glowing object is a protostar.
Nebula
Gas and dust gather into denser clumps under gravitational attraction.
Protostar
Continued gravitational collapse raises the temperature and pressure of the central region.
Fusion begins
When the core is sufficiently hot and dense, hydrogen nuclei begin to fuse into helium nuclei.
Conditions for fusion
Positively charged nuclei repel one another. For nuclear fusion to occur, the nuclei need sufficiently high kinetic energy to overcome this electrostatic repulsion and approach closely enough to fuse.
- A very high core temperature gives nuclei large kinetic energies.
- High pressure and density make collisions between nuclei more frequent.
- Fusion of hydrogen nuclei into helium releases large amounts of energy.
- The released energy contributes to the outward radiation pressure within the star.
Once sustained hydrogen fusion begins in the core and the inward and outward effects become balanced, the star enters its main-sequence stage.