Spectral Classes

Stars are assigned a spectral class based on the "rainbow" or spectrum produced when a star's light is passed through a prism or diffraction grating.  Dark absorbtion and/or bright emission lines found in these spectra reveal information about a star's composition, temperature, size, etc.  Stars do not have solid surfaces; the majority of a star's light is emitted from the top of a layer of hot gas which transports energy from the interior.  The temperature at the top of this layer determines the observed color of the star.  Cooler stars, around 3200 degrees Kelvin, appear reddish while very hot stars with "surface" temperatures above 11,000 degrees Kelvin appear bluish to the human eye.  Our Sun, has a "surface" temperature around 5800 degrees Kelvin and appears yellowish-white.

Spectral classes are represented by upper-case letters O, B, A, F, G, K and M from hottest to coolest.  A second numeric character (0-9) provides a more precise description and a third Roman numeral is broadly related to size.  The Sun is spectral class G2.