Stars are formed through a process called stellar formation or star formation, which occurs within large clouds of gas and dust known as molecular clouds. The process involves gravitational collapse, condensation, and subsequent fusion of hydrogen atoms to form helium, releasing vast amounts of energy. Here’s a simplified overview of how stars are formed:
- Molecular Clouds: Star formation begins in regions of molecular clouds where dense regions of gas and dust accumulate. These clouds are primarily composed of molecular hydrogen (H2), along with other molecules and dust particles.
- Gravitational Collapse: Various factors, such as the influence of nearby supernovae explosions or the compression of the cloud due to gravitational disturbances, can trigger the gravitational collapse of a molecular cloud. As the cloud contracts under the influence of gravity, it fragments into smaller clumps or cores.
- Protostar Formation: Within these dense cores, the gravitational collapse continues, leading to the formation of a protostar. The protostar is a dense, hot object that is not yet generating energy through nuclear fusion. It is primarily powered by the release of gravitational potential energy as the material falls inward.
- Accretion Disk: As the protostar forms, a rotating disk of gas and dust, known as an accretion disk, develops around it. Material from the surrounding cloud continues to fall onto the protostar through the disk, increasing its mass.
- Nuclear Fusion: As the protostar grows in mass, its central core becomes denser and hotter. When the temperature and pressure at the core reach a critical threshold, nuclear fusion begins. Hydrogen nuclei (protons) fuse together, forming helium and releasing a tremendous amount of energy in the process. This marks the birth of a star.
- Main Sequence: Once nuclear fusion begins in the core, the protostar enters the main sequence phase. It becomes a stable star, where the inward gravitational force is balanced by the outward pressure generated from the energy released through fusion reactions. The star reaches a state of equilibrium and will continue to burn hydrogen fuel for millions or billions of years, depending on its mass.
The specific details of star formation can vary depending on factors such as the mass of the molecular cloud, the environment, and the initial conditions. Additionally, not all collapsing clouds may lead to the formation of stars, as some could result in the formation of brown dwarfs or failed stars.
Stellar formation is a fascinating and ongoing process in the universe, contributing to the creation of the diverse array of stars we observe.