Spectral type
Also called: spectral class, stellar classification, OBAFGKM, luminosity class.
A star's class from its spectrum — O, B, A, F, G, K or M from hottest to coolest — with a Roman numeral for its size. The Sun is G2V.
Spread a star's light into a rainbow and dark lines appear: fingerprints of the elements in its atmosphere. Their pattern depends mostly on temperature, so stars fall into the classes O, B, A, F, G, K and M, hottest to coolest — remembered by generations of students as "Oh Be A Fine Girl (or Guy), Kiss Me".
Each class has ten steps (A0 to A9), and a Roman numeral tells the star's size: V for ordinary "dwarf" stars like the Sun, III for giants, I for supergiants. Vega is A0V, Betelgeuse M1–M2 Ia. The scheme took shape at Harvard around 1900, largely through Annie Jump Cannon, who sorted hundreds of thousands of stars by eye.
At the eyepiece
The class hints at the colour you'll see: O and B blue-white, A white, F yellow-white, G yellow, K orange, M orange-red. Carbon stars (class C) such as La Superba are the reddest of all.
In AstroPilot
Each star's page shows its spectral type with its colour, temperature and size.