A sketch by British comedians The Two Ronnies explores the comic potential of a public library’s colour-based classification scheme. But the idea is not so absurd. Arrangement by colour is, in private libraries at least, an established way of shelving one’s books.

Stars, too, can be arranged by colour. Under the Morgan-Keenan system, the classification most commonly used by stellar astronomers, our sun is a G2V, with translates as “a ‘yellow’ two tenths towards ‘orange’ main-sequence star”. Library classifications are not arbitrary, and neither are stellar ones. As library classmarks provide a shorthand for bibliographical content, Morgan-Keenan colours designate spectral class, which describes the ionization of the photosphere, itself a measure of a star’s temperature.

Most stars are classified using the letters O, B, A, F, G, K, and M, with O stars the hottest and M the coolest. O stars are called ‘blue’, B stars ‘blue-white’, A stars ‘white’, F stars ‘yellow-white’, G stars ‘yellow’, K stars ‘orange’, and M stars ‘red’. A number indicates tenths of the range between two successive classes, and the Roman numerals I to VII the star’s luminosity, which is indicative of its size.

Library classification tends to evolve over time to reflect socio-political changes. Stellar classification remains fixed, though the objects themselves slide through the scheme, over billions of years, before falling out altogether. Stellar classification has hard boundaries, and at the end of a star’s life, the remnants (neutron stars, black holes, and exotic stars), though still astronomical objects, can no longer be schematically described.