Star formation is the gravitational collapse and fragmentation of cold interstellar gas into protostars and eventually stars. Cooling, turbulence, magnetic fields, rotation, and feedback determine its efficiency and characteristic masses.
The Kennicutt–Schmidt law relates star-formation-rate surface density to gas surface density, commonly as . Disk-averaged total-gas measurements give near , while resolved molecular-gas relations are often closer to linear.
The gas-depletion time is the time required to consume a gas reservoir at its current star-formation rate, . It ignores replenishment, returned stellar mass, and changes in the rate.
The star-formation efficiency per free-fall time is the fraction of gas converted into stars during one gravitational free-fall time, . Molecular clouds commonly have values of order one per cent.
The Jeans mass is the characteristic gas mass above which self-gravity overcomes pressure support. Up to the convention used to define the collapsing region,for an ideal gas of fixed composition.
During isothermal collapse, is constant and the Jeans mass decreases as . Successively denser regions can therefore become independently unstable and fragment to smaller masses.
For adiabatic monatomic gas, and the Jeans mass increases as . Once cooling fails and collapse becomes adiabatic, further fragmentation is suppressed.
The initial mass function is the distribution of stellar birth masses. It records the mass scales selected by cloud fragmentation, accretion, feedback, and stellar multiplicity.
A Population II star is an old metal-poor star formed from gas enriched by earlier stellar generations. Metal and dust cooling let enriched gas fragment to lower masses than primordial gas under otherwise comparable conditions.
A Population III star is a first-generation star formed from essentially metal-free primordial gas. Limited cooling generally raises its characteristic fragmentation mass and favors a more top-heavy initial mass function than in enriched star formation.
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Star formation is the process by which dense regions within molecular clouds in space collapse under their own gravity to form new stars. This process involves several stages and is a fundamental aspect of astrophysics and cosmology. Here are the key steps involved in star formation: 1. **Molecular Clouds**: Star formation begins in molecular clouds, which are large regions of gas and dust that are cool enough for hydrogen atoms to combine into molecules. These clouds are often referred to as stellar nurseries.