How to Read Spectral Classification
TL;DR Summary
Understand the OBAFGKM spectral classification system and what it tells us about the temperature, color, and lifespan of stars.
Every star in the universe has a story written in its light. When astronomers pass starlight through a prism or diffraction grating, they see a spectrum — a rainbow of colors interrupted by dark absorption lines. These lines act as fingerprints, revealing a star's temperature, composition, and age.
The OBAFGKM Sequence
Astronomers classify stars using the Harvard spectral classification system, represented by the letters O, B, A, F, G, K, M (from hottest to coolest). A common mnemonic: "Oh Be A Fine Girl/Guy, Kiss Me."
| Class | Temperature | Color | Example |
|---|---|---|---|
| O | >30,000 K | Blue-violet | Alnitak |
| B | 10,000–30,000 K | Blue-white | Rigel |
| A | 7,500–10,000 K | White | Sirius, Vega |
| F | 6,000–7,500 K | Yellow-white | Procyon |
| G | 5,200–6,000 K | Yellow | Sun, Alpha Centauri |
| K | 3,700–5,200 K | Orange | Arcturus, Pollux |
| M | <3,700 K | Red | Betelgeuse, Proxima Centauri |
What the Numbers Mean
Each letter is followed by a number from 0 to 9, providing finer detail. G0 is hotter than G9. So our Sun (G2) is near the hot end of G-type stars.
Luminosity Classes
A Roman numeral follows the spectral type to indicate luminosity class:
- Ia/Ib: Supergiants (Rigel: B8Ia)
- II: Bright giants
- III: Giants (Arcturus: K1.5III)
- IV: Subgiants
- V: Main sequence / dwarfs (Sun: G2V)
So "M2Iab" (Betelgeuse) means: M-type (red), temperature subtype 2, luminosity class Iab (supergiant).
Why It Matters
Spectral class tells you almost everything about a star: its surface temperature, size, mass, likely age, and evolutionary stage. O and B stars burn hot and fast, living only millions of years. M dwarfs burn cool and slow, potentially lasting trillions of years.
Astroneo displays spectral class as a colored badge on every star's detail page, with colors matching the actual visual appearance of each stellar type.