Molecular Clouds & Gravitational Collapse: Physical Mechanisms

Giant molecular clouds contain filaments, clumps, and compact cores rather than uniform gas. Temperatures are typically only tens of kelvins, allowing molecules to survive and reducing thermal pressure. Dust mixed with the gas absorbs visible light and reradiates in the infrared, making many star-forming regions appear dark optically but bright at longer wavelengths.

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.

Collapse accelerates when gravity overwhelms internal support. Rotation becomes dynamically important as radius shrinks, flattening infalling material toward a disk. Turbulence can both oppose collapse globally and create local compressions that seed cores, so it is not simply a stabilizing pressure term.