The categorization of stars according to the properties of their spectra. The first attempt to do so was the Secchi classification in the 1860s, but it was the Harvard classification scheme that led to the current system of spectral types. Stars were classified as type O, B, A, F, G, K, or M in order of decreasing surface temperature, and each type further subdivided into subclasses from 0 (hottest) to 9 (coolest). The prefixes d, g, and c were used to signify dwarf stars, giants, and supergiants. To these were added the R and N types (now known as carbon stars) and S type (heavy metal stars). Spectral types for stars in the Harvard system were published in the Henry Draper Catalogue (published 1918–24). Stars were originally thought to follow an evolutionary sequence from the ‘early’ O and B types to the ‘late’ K and M types. Although this is now known to be erroneous, the terms ‘early type’ and ‘late type’ are still in use. In the late 1990s, spectral types L, T, and Y were added to the sequence to accommodate the coolest stars and brown dwarfs.

Astronomers currently use the Morgan–Keenan classification system introduced in the 1940s. This is a revision of the Harvard system, the key point being the addition of a range of luminosity classes indicated by a Roman numeral from I (supergiants) to V (dwarfs, or main-sequence stars). Luminosity classes VI (subdwarfs) and VII (white dwarfs) were subsequently added, but are now rarely used. Luminosity subclasses a, b, and c are sometimes appended, especially for supergiants, while the most luminous hypergiants are assigned luminosity class Ia-0. The dominant spectral features and other properties of each spectral type are summarized in the table.

The effective temperatures given are for stars on the main sequence; giants and supergiants of the same spectral type have slightly different temperatures.

Each spectral type is notionally divided into subtypes 0–9, but not all subtypes are in common use and some finer divisions have proven necessary, such as types O9.5 and O9.7 for supergiants. At the extremes, the earliest recognized spectral type is O2, while the coolest brown dwarfs yet observed are classified T10. Additional suffixes indicate various unusual properties of the spectrum, as follows:

1. n nebulous (diffuse) lines

2. s sharp lines

3. v variable spectrum

4. e emission lines, when not expected (e.g. Be)

5. ev variable emission lines

6. f certain O-type emission-line stars

7. p peculiarity (e.g. anomalous line strengths)

8. eq P Cygni emission

9. k interstellar lines present

10. m metallic-line star