Understanding Spectral Classification: OBAFGKM Decoded

## Spectral Classification: OBAFGKM Decoded

Every star has a spectral type — a letter and number that encodes its surface temperature, color, and chemical composition. The system, developed at Harvard Observatory in the early 1900s by Annie Jump Cannon, classifies stars by the absorption lines in their spectra.

### The Sequence

From hottest to coolest: **O B A F G K M** (remembered by the mnemonic: 'Oh Be A Fine Girl/Guy, Kiss Me').

| Class | Temperature | Color | Example |
|-------|-------------|-------|---------|
| O | 30,000-50,000 K | Blue | Alnitak |
| B | 10,000-30,000 K | Blue-White | Rigel |
| A | 7,500-10,000 K | White | Sirius |
| F | 6,000-7,500 K | Yellow-White | Polaris |
| G | 5,200-6,000 K | Yellow | Sun |
| K | 3,700-5,200 K | Orange | Arcturus |
| M | 2,400-3,700 K | Red | Betelgeuse |

### Subclasses and Luminosity

Each letter is subdivided from 0 (hottest) to 9 (coolest). The Sun is G2 — a G-type star slightly hotter than mid-range.

A Roman numeral suffix indicates luminosity class:
- I = Supergiant
- II = Bright giant
- III = Giant
- IV = Subgiant
- V = Main sequence (dwarf)

So the Sun's full classification is **G2V** — a yellow main-sequence star.

### Why Temperature Determines Spectral Lines

The absorption lines in a star's spectrum depend on which atoms are ionized or excited, which in turn depends on temperature. A-type stars show the strongest hydrogen lines because hydrogen atoms are excited but not fully ionized at ~10,000 K. Hotter O/B stars ionize hydrogen completely, while cooler K/M stars don't excite hydrogen enough.