Main sequence

The main sequence is the band on the Hertzsprung–Russell diagram occupied by stars that spend most of their lives fusing hydrogen into helium in their cores.

Proton–proton chain: A sequence of nuclear reactions that converts hydrogen into helium, releasing energy and neutrinos. It supplies most of the energy in the cores of lower-mass main-sequence stars.
Hertzsprung–Russell diagram: A plot that compares stars’ luminosities with their surface temperatures or spectral types. The main sequence is the diagram’s defining diagonal band.
Stellar classification: The system of grouping stars by properties such as spectral type and luminosity class. A main-sequence star receives luminosity class V, distinguishing it from evolved stars of similar temperature.
Protostar: A young, contracting stellar object that has not yet begun sustained hydrogen fusion in its core. A protostar precedes the stable core hydrogen burning that defines the main sequence.
CNO cycle: A set of nuclear reactions that uses carbon, nitrogen, and oxygen nuclei to convert hydrogen into helium. It dominates core hydrogen fusion in sufficiently massive main-sequence stars.
Stellar luminosity: The total energy emitted by a star per unit time. Luminosity is the vertical quantity used to locate stars on the Hertzsprung–Russell diagram.
Stellar evolution: The changes in a star’s structure and observable properties over its lifetime. A star’s arrival on and departure from the main sequence mark major phases of its evolution.
Red giant: A luminous, expanded star that has exhausted hydrogen in its core and burns hydrogen in a surrounding shell. A red giant has left the main sequence after central hydrogen depletion.
Hydrostatic equilibrium: A state in which inward gravity is balanced by outward pressure within a star or other fluid body. The balance between gravity and pressure lets core fusion sustain a stable main-sequence star.
Effective temperature: The temperature of a blackbody that would emit the same energy per unit area as a star’s surface. It sets the horizontal position and spectral appearance of a main-sequence star.

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