# 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.

**Examples:** [vega](https://astropilot.space/object/vega.md), [capella](https://astropilot.space/object/capella.md), [arcturus](https://astropilot.space/object/arcturus.md), [betelgeuse](https://astropilot.space/object/betelgeuse.md)

**Related:** [Star](https://astropilot.space/glossary/star.md), [Star colour](https://astropilot.space/glossary/star-colour.md), [Magnitude](https://astropilot.space/glossary/magnitude.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/spectral-type