Spread a star's light into a spectrum and you see dark absorption lines. Around 1900, astronomers at Harvard, led by Annie Jump Cannon, sorted hundreds of thousands of stellar spectra by their patterns of lines. The classes they settled on, in order, are O, B, A, F, G, K, M, and they turned out to be a sequence of temperature, from the hottest (O) to the coolest (M).
Why it works
Most stars are made of nearly the same mix: about three-quarters hydrogen and a quarter helium by mass, with a sprinkle of everything else (as Cecilia Payne showed in 1925). Their spectra differ because temperature decides what state the atoms are in. In cool M stars, molecules such as titanium oxide survive and make broad bands. In K and G stars, metals like calcium and iron show strongly. Hydrogen's visible lines need electrons sitting in level 2; that needs heat, but too much heat strips hydrogen's electron off entirely, so these lines peak in A stars near $10{,}000$ K. Only the hottest O stars can ionize helium, so their spectra show ionized-helium lines. Each class is split into subclasses 0 to 9; the Sun is a G2 star.
| Class | Temperature (K) | Strongest lines |
|---|---|---|
| O (blue) | above 30,000 | ionized helium |
| B (blue-white) | 10,000–30,000 | neutral helium |
| A (white) | 7500–10,000 | hydrogen, strongest |
| F (yellow-white) | 6000–7500 | hydrogen, ionized calcium |
| G (yellow) | 5200–6000 | ionized calcium, metals |
| K (orange) | 3700–5200 | metals |
| M (red) | below 3700 | molecules (titanium oxide) |
Some familiar examples: the stars of Orion's Belt (O and B), Rigel (B), Sirius and Vega (A), Procyon (F), the Sun (G), Arcturus and Aldebaran (K), Betelgeuse and Proxima Centauri (M).
Key idea
The spectral classes O, B, A, F, G, K, M run from hottest to coolest. The lines a star shows depend mostly on its temperature, not on a different makeup.
Worked example
Worked example: Classify a star
A star's surface temperature is $9000$ K. What class is it, and which lines dominate its spectrum?
- $9000$ K lies between $7500$ and $10{,}000$ K: class A.
- A stars show the strongest hydrogen lines of any class.
Answer: Class A, with strong hydrogen lines.
Common mistake
The order O, B, A, F, G, K, M looks scrambled because it is not alphabetical and not by size. The letters first came from an older, alphabetical scheme based on hydrogen-line strength; when astronomers realized temperature was the real key, they kept the letters and reordered them. A traditional memory aid is "Oh Be A Fine Girl/Guy, Kiss Me." Check: the Sun (G) is cooler than Sirius (A) and hotter than Betelgeuse (M).