Star formation is the sequence of physical events that converts cold interstellar gas into luminous stars. It begins in dense, cold regions of the interstellar medium called molecular clouds, often nicknamed "stellar nurseries." Under the right conditions gravity overwhelms internal pressure and portions of these clouds condense into compact objects that heat as they contract, eventually becoming hot ionized plasma and igniting nuclear fusion to produce stars.
Typical stages
- Cloud and core formation: Turbulence and cooling let dense cores appear inside a larger molecular cloud.
- Gravitational collapse: A prestellar core contracts; central density and temperature rise.
- Protostar and accretion disk: Material falls inward, forming a rotating disk that feeds the growing protostar and can spawn planets.
- Outflows and jets: Young objects commonly eject collimated flows that remove angular momentum and shape their surroundings.
- Pre-main-sequence to main sequence: When core temperatures sustain hydrogen fusion the object reaches the main sequence and is a true star.
Timescales depend on mass: low-mass stars take a few million years to reach the main sequence, while massive stars form more rapidly. Not all the gas becomes stars; star formation efficiency in a cloud is typically low. External triggers such as shock waves from supernovae, collisions between clouds, or compression in spiral arms can accelerate collapse.
Astronomers study star formation using infrared and millimeter-wave observations that penetrate dust and reveal cold gas and disks. Instruments sensitive to molecular lines and continuum emission trace dense cores and protostellar envelopes; high-resolution arrays can image disks and jets. These observations inform models of how disks produce planets and how feedback from young stars regulates further formation.
Important topics and open questions include the origin of the stellar initial mass function (how many low- versus high-mass stars form), the formation pathways of massive stars and brown dwarfs, and the role of magnetic fields and turbulence. Understanding star formation links the life cycles of galaxies to the conditions for planet formation and habitability.
For more introductory material and detailed reviews see resources on molecular clouds and star-forming regions: molecular clouds, the physics of ionized gas such as plasma, and overviews of stars.