Why the star alphabet runs O, B, A, F, G, K, M
Astronomy students memorise the star types with a cheeky line: Oh, Be A Fine Girl, Kiss Me. The jumbled letters run from the hottest stars to the coolest, but nobody who first assigned them knew they tracked temperature. The order is a fossil of a system repeatedly rebuilt as understanding grew.
Spreading a star's light through a prism or grating produces a rainbow crossed by dark lines, each tied to a particular element or molecule. Line strengths mostly reflect the temperature of the star's visible surface, so a star's spectral class is essentially a thermometer reading. In the 1860s and 1870s Angelo Secchi sorted spectra into classes, and later the Harvard astronomer Annie Jump Cannon reorganised an alphabetical scheme into a simpler one-dimensional sequence. Only in the 1920s did Meghnad Saha's theory of ionisation, and then Cecilia Payne's work, show that the sequence was really about temperature.
Each letter is split by a digit from 0, hottest, to 9, coolest, and fractions are allowed: Mu Normae is an O9.7. Because the subtypes were assigned by judging line strengths by eye, they do not mark even steps. Main-sequence stars range from roughly 2,000 to 50,000 kelvin, while freshly formed white dwarfs can top 100,000. Extra letters cover misfits, among them W, S and C for odd stars, a D reserved for white dwarfs, and the trio L, T and Y assigned to brown dwarfs.
In 1943 William Morgan, Philip Keenan and Edith Kellman at Yerkes Observatory added a second dimension, brightness, and the revised scheme, named Morgan-Keenan in 1953, is still in use. It relies on how pressure broadens spectral lines: a bloated giant has far weaker surface gravity than a compact dwarf of similar mass, so its lines are narrower. Roman numerals mark the result, from 0 for hypergiants and I for supergiants through III for giants and V for ordinary main-sequence stars. The Sun is G2V, a main-sequence star at about 5,800 kelvin.
The traditional colour names mislead. Stars shine across the spectrum, so they look paler than their labels. There are no green or violet stars in normal viewing; so-called yellow dwarfs like the Sun appear white, and red dwarfs look deep yellow-orange.
Source: Stellar classification
