Molecular Clouds & Gravitational Collapse: Structures & Vocabulary

Stars form inside the coldest, densest parts of molecular clouds, enormous complexes made mostly of molecular hydrogen with helium, dust, and trace molecules. A cloud does not collapse merely because gravity exists; collapse begins when self-gravity overcomes the combination of thermal pressure, turbulence, magnetic support, and rotation in a sufficiently dense region.

A dense core can become gravitationally unstable when its mass and density exceed the support available from internal motions and temperature. As the core contracts, gravitational potential energy is converted into heat and motion. Rotation causes collapsing material to flatten, while magnetic fields and turbulence can delay, redirect, or fragment the collapse.

Radio observations of molecules such as carbon monoxide trace cold gas that is otherwise difficult to detect, while infrared and submillimeter observations reveal dense dust cores and embedded young stars. Velocity measurements show infall, turbulence, and outflows, and surveys of regions such as Orion reveal a hierarchy from clouds to cores to newborn stars.

A useful hierarchy runs from giant molecular cloud to filament, clump, dense core, and protostellar envelope. Density and temperature rise inward, while dust shields molecules from destructive radiation. Carbon monoxide and other molecules trace gas that molecular hydrogen itself often does not reveal easily.