The light produced by every single star is unique. The temperature of the star, its luminosity, and its chemical composition, will all have effects on the light that it produces, and these effects can be decoded by examining the star's spectrum. We see the Sun's spectrum (albeit in a very coarse form) whenever we look at a rainbow, or use a prism to split sunlight into its constituent colours. Starlight, too, is composed of these seven colours, but the relative intensities of light along the spectrum will vary considerably from star to star.

This fact, as well as providing a wealth of information about stars themselves, provides a basis for a systematic stellar classification. At its most basic, this system consists of sorting stars according to their surface temperature. Stars are divided into a number of basic types according to this temperature (and other corresponding factors), and each type is given an identifying letter. Within each type, stars can be further categorised by grading them from 0 to 9 (so for example, an M2 star is hotter than an M7).

The peculiar ordering of the identifying letters is a matter of historical accident, and the letters themselves have no particular significance. Most of the common types have associated colours, which are themselves a consequence of the temperature of the star (hotter stars are bluer, while cooler stars tend to be redder). In decreasing order of surface temperature, the main spectral types are shown below.

Two other rare classes are the C- and S-type stars. These are cool stars that overlap the K- and M-type classifications in terms of temperature, but are placed in separate categories due to unusual chemistry within the star. Very few of these stars are visible without optical aid, though the C-type U Hydrae, and the S-type Chi Cygni are unusual exceptions.

A star's full spectral classification often also includes a 'luminosity class', a Roman numeral from I to VII indicating the star's luminosity, which correlates with its mass. The luminosity class is simply appended to the spectral class. So, for example, the Sun's full spectral classification, including its luminosity class, is G2V. The seven luminosity classes are listed below.

In addition to the luminosity class, spectral classifications sometimes also carry commentary additions, usually lower case letters added before or after the main type. For example, the full spectral classification for Achernar is B3Vp, with 'p' indicating that it has a peculiar spectrum, while Castor in Gemini is classified as A2Vm, with 'm' demonstrating that the spectrum contains strong metal lines, and so on. These prefixes and suffixes lie rather outside the range of this brief article.