Protostars and Accretion
A protostar is the hot, growing core at the center of a collapsing cloud; it gains most of its mass by accretion before contracting along a pre-main-sequence track to ignite hydrogen fusion.
Definition
A protostar is a forming star that is still gaining most of its mass by accretion from its surrounding envelope and disk, before it settles onto the main sequence and begins steady hydrogen burning.
Scope
The topic covers the protostellar phase from the formation of a hydrostatic core through the main accretion phase, the observational classification of young stellar objects by their infrared spectral energy distributions, the accretion luminosity that powers embedded protostars, and the subsequent pre-main-sequence contraction along the Hayashi and Henyey tracks.
Core questions
- How does a protostar form and grow?
- Where does a young protostar get its luminosity?
- How are young stellar objects classified?
- What path does a pre-main-sequence star follow to the main sequence?
Key concepts
- protostellar core
- accretion luminosity
- young stellar object classes
- T Tauri star
- Hayashi track
- Henyey track
- deuterium burning
Key theories
- Accretion-driven protostellar growth
- After a collapsing core forms a small hydrostatic protostar, the object grows by accreting infalling material; the gravitational energy released as gas strikes the protostar provides its accretion luminosity, which dominates the output of deeply embedded young stars.
- Pre-main-sequence contraction
- Once accretion ends, the pre-main-sequence star contracts and descends a nearly vertical Hayashi track while fully convective, then moves along the Henyey track as a radiative core develops, until central temperatures rise enough to ignite hydrogen on the main sequence.
Mechanisms
Infalling cloud material accumulates onto a central hydrostatic core, releasing gravitational energy as accretion luminosity that heats the surrounding dust and emits in the infrared. As accretion wanes, the revealed pre-main-sequence star, supported by slow gravitational contraction and brief deuterium burning, contracts and heats until hydrogen fusion begins.
Clinical relevance
The protostellar and pre-main-sequence phases determine the final masses of stars, the timescales over which planet-forming disks exist, and the observable signatures, including infrared excesses and accretion activity, used to identify and date the youngest stars in star-forming regions.
History
Hayashi showed in 1961 that fully convective contracting stars follow nearly vertical tracks, the Henyey track described the later radiative phase, and the standard accretion picture of protostellar growth was developed and synthesized with observations of young stellar objects in the 1980s.
Key figures
- Chushiro Hayashi
- Frank Shu
- Louis Henyey
- Steven Stahler
Related topics
Seminal works
- shu1987
- hayashi1961
Frequently asked questions
- Why are protostars so hard to see directly?
- Young protostars are buried deep within dusty collapsing envelopes that absorb their visible and ultraviolet light and re-emit it in the infrared, so they are studied mainly at infrared and millimeter wavelengths that can penetrate the surrounding dust.
- Is a protostar already a real star?
- Not quite: a protostar is supported by gravitational contraction and accretion energy rather than by sustained hydrogen fusion in its core; it becomes a true main-sequence star only once it is hot enough inside to fuse hydrogen steadily.