Protostars & Pre-Main-Sequence Evolution: Structures & Vocabulary

A protostar is a contracting stellar embryo that has not yet reached long-term hydrogen fusion in its core. During the pre-main-sequence stage, gravity supplies most of the luminosity while the object accretes material, interacts with a circumstellar disk, launches outflows, and moves toward the temperature and pressure conditions required for stable fusion.

Accreting gas releases gravitational potential energy as heat. Continued contraction raises the central temperature; convection and radiation transport energy outward while angular momentum is redistributed through the disk and winds. Once the core becomes hot enough for sustained hydrogen fusion, nuclear energy can balance radiative losses and the star settles onto the main sequence.

Infrared surveys detect deeply embedded young stellar objects, while jets produce Herbig-Haro shocks where fast outflows collide with ambient gas. Spectra reveal accretion signatures, disks show infrared excesses, and young clusters contain stars at different pre-main-sequence positions consistent with evolutionary calculations.

Young systems contain a central protostar, rotating disk, infalling envelope, bipolar cavities, and jets. As the envelope clears, T Tauri or analogous young stars become optically visible. Their positions above the main sequence on an H-R diagram reflect large radii and ongoing contraction.