Pre-main-sequence star

A young star that radiates energy as it contracts before sustained hydrogen fusion becomes its primary energy source. Its structure and evolution depend strongly on its mass.

Hayashi track: The nearly vertical path taken by fully convective young stars on the Hertzsprung–Russell diagram. Low-mass pre-main-sequence stars contract along this track while their surface temperatures change little.
Molecular cloud: A cold, dense interstellar cloud composed mainly of molecular hydrogen and dust. Gravitationally collapsing regions within these clouds give rise to protostars and young stars.
T Tauri star: A low-mass, pre-main-sequence star, typically younger than about ten million years, showing strong activity or variability. This class provides a well-studied example of low-mass stars before the main sequence.
Main-sequence star: A star whose core sustains energy production primarily through hydrogen fusion. Its stable hydrogen-fusing state follows the contraction phase described here.
Henyey track: The nearly horizontal evolutionary path of pre-main-sequence stars with radiative interiors on the Hertzsprung–Russell diagram. More massive young stars follow this track as their luminosity falls and surface temperature rises.
Protostar: A forming star that gains mass from its surroundings before reaching hydrostatic equilibrium and sustained hydrogen fusion. It is the immediate earlier stage in the formation of a pre-main-sequence star.
Herbig Ae/Be star: A young, intermediate-mass star with emission lines and often a surrounding circumstellar disk. These stars show how pre-main-sequence evolution differs at higher stellar masses.
Brown dwarf: A substellar object too low in mass to sustain hydrogen fusion in its core. It can contract and resemble a young star but never reaches sustained hydrogen burning.
Kelvin–Helmholtz mechanism: The release of gravitational potential energy as an object contracts and radiates heat. It supplies most of a young star’s energy before sustained hydrogen fusion begins.
Hydrostatic equilibrium: A state in which inward gravity is balanced by outward pressure within a celestial body. A pre-main-sequence star is largely in this balance even while it continues to contract and radiate.

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