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The temperature of a star determines its [[spectral type]] via its effect on the physical properties of [[plasma (physics)|plasma]] in its [[photosphere]]. A star's energy emission as a function of wavelength is influenced by both its temperature and composition. A key indicator of this energy distribution is given by the [[color index]], ''B'' − ''V'', which measures the star's [[apparent magnitude|magnitude]] in blue (''B'') and green-yellow (''V'') light by means of filters.<ref group=note>By measuring the difference between these values, eliminates the need to correct the magnitudes for distance. However, this can be affected by [[Extinction (astronomy)|interstellar extinction]].</ref> This difference in magnitude provides a measure of a star's temperature.
=='''''Dwarf terminology'''''==
Main-sequence stars are called dwarf stars,<ref name=smith91/><ref name=powell06/> but this terminology is partly historical and can be somewhat confusing. For the cooler stars, dwarfs such as [[red dwarf]]s, [[orange dwarf]]s, and [[yellow dwarf star|yellow dwarf]]s are indeed much smaller and dimmer than other stars of those colors. However, for hotter blue and white stars, the difference in size and brightness between so-called "dwarf" stars that are on the main sequence and so-called "giant" stars that are not, becomes smaller. For the hottest stars the difference is not directly observable and for these stars, the terms "dwarf" and "giant" refer to differences in [[spectral line]]s which indicate whether a star is on or off the main sequence. Nevertheless, very hot main-sequence stars are still sometimes called dwarfs, even though they have roughly the same size and brightness as the "giant" stars of that temperature.<ref name=moore06/>
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