Star formation is the process by which gas in molecular clouds gets transformed into stars.

In the current paradigm of star formation, cores of molecular clouds (regions of especially high density) become gravitationally unstable, and start to collapse. Part of the gravitational energy lost in this collapse is radiated in the infrared, with the remainder increasing the temperature of the core. The accretion of material happens partially though a circumstellar disc. When the density and temperature are high enough, deuterium ignites, undergoing fusion, and the outward pressure of the resultant radiation slows (but does not stop) the collapse. After it is exhausted, material from the cloud continues to "rain" onto the protostar. In this stage bipolar flows are produced, probably to eliminate part of the angular momentum of the falling material. Finally, hydrogen ignites in the core of the star, and the rest of the enveloping material is cleared away.

The stages of the process are well defined stars with masses around one solar mass or less. In high mass stars, the length of the star formation process is comparable to the other timescales of their evolution and the process is not so well defined. The later evolution of stars are studied in stellar evolution.

Observations

Key elements of star formation are only available by observing in wavelengths other than the optical. The structure of the molecular cloud and the effects of the protostar are best observed in rotational transitions of CO and other molecules; these are observed in the millimeter and submillimeter range. The radiation from the protostar and early star has to be observed in infrared astronomy wavelengths, the extinction caused by the rest of the cloud where it is being formed is usually too big to allow us to observe it in the visual part of the spectrum.

The formation of individual stars, can only be directly observed in our Galaxy, but in distant galaxies star formation has been detected through its unique spectral signature.