Main sequence

A main-sequence star is in a long-lived phase of core hydrogen burning.

An approximate relation between the mass and radius of main-sequence stars in a specified composition and mass range. It is not determined by hydrostatic equilibrium alone. For gas-pressure stellar homology with Kramers' opacity law, radiative diffusion gives $L\propto {\mu }^{15/2}{M}^{11/2}{R}_{\ast }^{-1/2}$. Substitution of $R_{\ast }\propto M^q$ gives $L\propto M^{(11-q)/2}$ at fixed composition, explaining precisely which extra radius assumption produces any claimed exponent.

The time a star spends with core hydrogen nuclear fusion as its principal energy source. If the usable fuel fraction $f$, conversion efficiency $ϵ$, and composition are comparable, the main-sequence lifetime scales as $M/L$. With a local mass-luminosity relation $L\propto M^{\alpha }$, the lifetime scales as $M^{1-\alpha }$. This is a scaling estimate: convective core size and changing luminosity affect detailed ages.

The main-sequence turnoff is the location in a coeval population's Hertzsprung-Russell diagram where stars are leaving the main sequence after exhausting their core hydrogen. More massive stars generally have larger luminosity per unit mass and shorter nuclear lifetimes, so the most massive stars still on the main sequence constrain the population's age. In a simple model, equating the stellar nuclear timescale $t\sim qM/L(M)$ to the common age determines a turnoff mass; detailed calibration requires composition and stellar models.

The terminal-age main sequence denotes the end of central hydrogen burning. It does not mark complete exhaustion of hydrogen throughout the star; a hydrogen-burning shell can subsequently supply its luminosity.

The lower main sequence consists of relatively cool, low-mass hydrogen-burning stars whose luminosity is supplied mainly by the proton–proton chain.

A red dwarf is a cool low-mass main-sequence star. The lowest-mass red dwarfs are fully convective.

The zero-age main sequence is the locus where stars begin stable core hydrogen burning with nearly their initial composition.