The apparent brightnesses of stars are often expressed in terms of magnitudes, which is an old system based on how human vision interprets relative light intensity A color index of a star is the difference in the magnitudes measured at any two wavelengths and is one way that astronomers measure and express the temperature of stars

The apparent brightness of a star depends on both its luminosity and its distance from Earth. Thus, the determination of apparent brightness and measurement of the distance to a star provide enough information to calculate its luminosity.

apparent brightness — a measure of the amount of light received by Earth from a star or other object—that is, how bright an object appears in the sky, as contrasted with its luminosity. apparent brightnesses of stars — often expressed in terms of magnitudes, which is an old system based on how human vision interprets relative light intensity. color index of a star — the difference in the magnitudes measured at any two wavelengths and is one way that astronomers measure and express the temperature of stars. color index — difference between the magnitudes of a star or other object measured in light of two different spectral regions—for example, blue minus visual (B-V) magnitudes.