Star formation

Star formation is the process by which dense regions of interstellar gas and dust collapse under gravity to form stars. It produces protostars that heat as they accrete material.

Giant molecular cloud: A large, cold interstellar cloud composed mainly of molecular hydrogen, where many stars form. These clouds provide the cold, dense gas from which stellar collapse begins.
Interstellar medium: The matter and radiation occupying the space between stars in a galaxy. Star formation transforms dense portions of this galactic material into stars.
Initial mass function: The distribution of stellar masses at the time a population of stars forms. It records the range of stellar masses produced by a star-forming population.
Population III star: A hypothetical first-generation star formed from nearly metal-free primordial gas. Its formation occurred in the early universe, before later generations enriched the gas with heavy elements.
Star formation efficiency: The fraction of a cloud's gas converted into stars over a specified region or period. Its low observed values challenge models of how rapidly gravity converts cloud gas into stars.
Jeans instability: The gravitational instability that causes a gas cloud to collapse when its mass exceeds a threshold set by pressure and gravity. It describes when a cloud region can overcome pressure and begin collapsing.
Molecular hydrogen: The molecule H₂, the most abundant molecular species in cold interstellar clouds. Its presence marks the cold molecular environments where collapse commonly occurs.
Stellar feedback: The influence of radiation, winds, and explosions from stars on their surrounding gas. Feedback from newly formed stars can disperse clouds or trigger further collapse.
First star: A star formed from primordial matter before any earlier stars had enriched the universe. The earliest stellar collapse happened in conditions unlike those in present-day molecular clouds.
Core accretion model: A model in which individual dense cloud cores collapse to produce individual stars or multiple systems. It offers a specific account of how cloud structure sets stellar masses.

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