Hayashi track

The nearly vertical path on the Hertzsprung–Russell diagram followed by a contracting pre-main-sequence star as it approaches the main sequence. It marks a phase when the star’s effective temperature changes little while its luminosity falls.

Pre-main-sequence star: A star that is contracting toward the main sequence before sustained hydrogen fusion begins in its core. Hayashi tracks describe the early contraction phase of these young stars.
Hydrostatic equilibrium: A balance in which pressure gradients counteract gravity within a fluid body. Hayashi-track models describe stars that remain close to this structural balance.
Henyey track: A pre-main-sequence evolutionary path that runs roughly horizontally across the Hertzsprung–Russell diagram. It follows the Hayashi phase for many stars as their interiors become more radiative.
Pre-main-sequence isochrone: A curve on a Hertzsprung–Russell diagram connecting stars of the same age during pre-main-sequence evolution. Isochrones incorporate Hayashi-track models to estimate ages from young-star positions.
Kelvin–Helmholtz mechanism: The release of gravitational potential energy as an object contracts, heating it and supplying radiated energy. Gravitational contraction powers a star’s luminosity along much of its Hayashi track.
Convection zone: A stellar region where bulk fluid motion transports energy more effectively than radiation alone. Young low-mass stars on Hayashi tracks are largely or entirely convective.
Main sequence: The band on the Hertzsprung–Russell diagram occupied by stars powered mainly by core hydrogen fusion. A star leaves its Hayashi track when it reaches stable main-sequence hydrogen burning.
Star-forming region: A region of interstellar gas and dust where new stars are forming. Its young stars can be compared with Hayashi tracks to infer their evolutionary stages.
Hertzsprung–Russell diagram: A plot relating stars’ luminosities to their surface temperatures or spectral types. The Hayashi track is defined as a path on this stellar diagram.
Opacity: A measure of how strongly matter absorbs or scatters radiation passing through it. High opacity helps make the outer layers of Hayashi-track stars convective.

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