protostars

Protostars are the early stages of star formation in which a dense region of a molecular cloud contracts under gravity. They are young stellar objects that have not yet begun sustained hydrogen fusion in their cores, so they are not main-sequence stars. During collapse, a central hydrostatic core forms and grows as material continues to accrete from the surrounding envelope of gas and dust. An accretion disk develops from angular momentum, and bipolar outflows or jets help remove excess angular momentum and clear surrounding material.

The energy output of a protostar is primarily gravitational, released as material falls inward and the object contracts. In many cases, deuterium fusion can briefly occur in the early stages, but sustained hydrogen burning has not yet started. The protostar remains enveloped in gas and dust, often making it visible mainly at infrared and submillimeter wavelengths. Its spectral energy distribution shifts as the surrounding envelope dissipates and the disk evolves.

Protostars are commonly classified into evolutionary stages based on their infrared properties and envelope content: Class 0 (deeply embedded, strong submillimeter emission), Class I (enveloped with substantial infall), Class II (disk-dominated, weaker envelope, typical T Tauri stars), and Class III (little or no disk remaining). Timescales are approximate and mass-dependent, with Class 0 lasting about 10^4–10^5 years, Class I up to ~10^5 years, and Class II–III spanning up to several million years.

Eventually accretion slows, the protostar contracts, and hydrogen fusion ignites to produce a pre-main-sequence star, marking the transition toward the main sequence. Protostars thus represent the early, accreting phase in the birth of stars.