Stellar Luminosity Calculator

About this calculator

A star's luminosity — its total power output — follows the Stefan-Boltzmann law: L = 4πR²σT⁴, where R is the star's radius, T is its surface (effective) temperature, and σ is the Stefan-Boltzmann constant. Since luminosity scales with the square of the radius but the fourth power of temperature, a modest increase in temperature has a much bigger effect on brightness than the same relative increase in size — this calculator typically expresses the result relative to the Sun's luminosity (L☉) for easy comparison.

Astronomers use this relationship to place stars on the Hertzsprung-Russell diagram, estimate distances to stars via the inverse-square law once luminosity is known, and classify stars by evolutionary stage — a red giant is luminous mainly because of its huge radius, while a blue main-sequence star is luminous mainly because of its high temperature. Enter a star's radius and surface temperature (relative to the Sun) to find its luminosity.