Protostar: the formative stage of a star

Structure and evolutionary stages

Typical protostellar structure includes a dense central object, an accretion disk fed by an infalling envelope, and bipolar jets or outflows that carry angular momentum away. Observationally protostars are classified by their spectral energy distribution into phases often labeled Class 0, I, II and III; Class 0 are deeply embedded and youngest, while later classes show clearer disks and reduced envelopes. If the core mass is below the threshold for sustained hydrogen burning, the object may become a brown dwarf instead of a main‑sequence star.

Key characteristics

• Primary energy source: gravitational (Kelvin–Helmholtz) contraction.
• Early nuclear reactions: fusion of light isotopes such as deuterium can occur briefly in more massive protostars.
• Typical duration: formation spans a few hundred thousand to a few million years, depending on mass and environment.
• Associated phenomena: circumstellar disks, jets, Herbig–Haro objects and strong infrared emission.

Physical processes and observables

As a protostar contracts it follows tracks in temperature–luminosity space (for low‑mass objects often described by the Hayashi track) until central temperatures and densities permit stable hydrogen fusion. During the protostellar phase accretion shocks, magnetic fields and rotation shape the flow of matter; excess energy escapes mainly at infrared and submillimeter wavelengths, so young objects are often detected as bright IR sources embedded in the interstellar medium. Outflows and jets produce emission-line features and shock‑excited nebulae that mark active accretion.

Importance, outcomes and historical note

Protostellar evolution sets the initial mass, rotation, magnetic properties and disk mass that determine whether a system will form planets and what kind of star it will become. Understanding protostars is therefore central to theories of star and planet formation and to interpreting observations from infrared and radio facilities. The modern theoretical framework for low‑mass protostars was developed in the mid‑20th century, with influential work such as Chushiro Hayashi's models in the 1960s that described contraction tracks and pre‑main‑sequence behavior.