Spectral Classification: OBAFGKM

## Spectral Classification: OBAFGKM

Every star's light carries a fingerprint — dark absorption lines created by elements in its atmosphere. The spectral classification system, developed at Harvard Observatory in the 1890s-1900s, sorts stars into temperature-based classes.

### The Harvard System

The classes O-B-A-F-G-K-M are ordered by decreasing surface temperature. The famous mnemonic: "Oh Be A Fine Girl/Guy, Kiss Me."

| Class | Temperature | Color | Example | Fraction of Stars |
|-------|-------------|-------|---------|-------------------|
| O | 30,000-50,000 K | Blue | 10 Lacertae | 0.00003% |
| B | 10,000-30,000 K | Blue-white | Rigel, Spica | 0.13% |
| A | 7,500-10,000 K | White | Sirius, Vega | 0.6% |
| F | 6,000-7,500 K | Yellow-white | Procyon, Polaris | 3% |
| G | 5,200-6,000 K | Yellow | Sun, Alpha Centauri A | 7.6% |
| K | 3,700-5,200 K | Orange | Arcturus, Aldebaran | 12.1% |
| M | 2,400-3,700 K | Red | Betelgeuse, Proxima | 76.5% |

Each class is subdivided 0-9 (e.g., G2 for the Sun, where 0 is hotter and 9 is cooler within the class).

### What Determines Spectral Lines

Temperature controls which absorption lines appear:

- **O stars**: Ionized helium (He II) lines. So hot that most elements are fully ionized
- **B stars**: Neutral helium (He I) dominates
- **A stars**: Hydrogen (Balmer series) lines strongest. Iconic example: Vega
- **F stars**: Hydrogen weakens, ionized metals (Ca II) appear
- **G stars**: Ca II H & K lines prominent, many metal lines. Solar spectrum
- **K stars**: Neutral metals dominate, molecular bands appear
- **M stars**: Titanium oxide (TiO) molecular bands dominate, giving a rich spectrum

### Extended Classes

| Class | Temperature | Description |
|-------|-------------|-------------|
| L | 1,300-2,400 K | Ultra-cool dwarfs, iron hydride bands |
| T | 550-1,300 K | Methane dwarfs (brown dwarfs) |
| Y | < 550 K | Coolest brown dwarfs, ammonia |

### Luminosity Classes (Yerkes/MKK System)

Spectral type alone doesn't distinguish a G2 dwarf (the Sun) from a G2 supergiant. The Morgan-Keenan-Kellman system adds luminosity classes:

| Class | Type | Example |
|-------|------|---------|
| Ia | Bright supergiant | Rigel (B8 Ia) |
| Ib | Supergiant | Deneb (A2 Ia) |
| II | Bright giant | Canopus (A9 II) |
| III | Giant | Arcturus (K1.5 III) |
| IV | Subgiant | Procyon (F5 IV-V) |
| V | Main sequence (dwarf) | Sun (G2 V) |

The full spectral classification of the Sun is **G2 V** — a yellow main-sequence star with a surface temperature of 5,778 K.

### The Women of Harvard

The classification system was largely created by women "computers" at Harvard: Williamina Fleming, Antonia Maury, and Annie Jump Cannon. Cannon personally classified over 350,000 stellar spectra and established the OBAFGKM sequence still used today.