Field guide · Stars
StarSpectral class
A star’s temperature, written as a letter: O and B are blue-white and fierce, then A, F, G (the Sun), K, and cool red M. The colour you see is the thermometer.
- Hottest
- O — over 30,000 °C, blue
- The Sun
- G2 V — about 5,500 °C, yellow-white
- Coolest
- M — under 3,500 °C, orange-red
- Mnemonic
- Oh Be A Fine Girl/Guy, Kiss Me
Split a star’s light through a prism and it is crossed by dark lines, the fingerprints of the elements in its atmosphere. In the 1890s Williamina Fleming, Antonia Maury and Annie Jump Cannon at Harvard sorted hundreds of thousands of these spectra by hand. Cannon alone classified around 350,000 stars.
Her order — O, B, A, F, G, K, M — turned out to be a temperature sequence, hottest first. Cecilia Payne showed in 1925 what the lines meant: stars are made almost entirely of hydrogen and helium, a result so unexpected she was talked into calling it “spurious”. She was right.
A number from 0 to 9 splits each letter finer, and a Roman numeral gives the size: V is an ordinary star like the Sun, III a giant, I a supergiant. The Sun is G2 V. Betelgeuse is M1–2 Ia: cool, red and enormous.
See it in the sky
Each one opens the sky map pointed at it, for tonight, from where you are.