Across the vast expanse of our galaxy, stars shine in a dazzling array of colors, sizes, and luminosities. The Hertzsprung-Russell system provides a framework for categorizing this stellar diversity, based on a star's temperature and absolute magnitude.
| Class | Temperature | Mass | Radius | Luminosity | Examples |
|---|---|---|---|---|---|
| O | >30,000 K | >16 M☉ | 6-8 R☉ | 30,000-1,000,000 L☉ | R136a1 |
| B | 10,000-30,000 K | 2-16 M☉ | 1.5-6 R☉ | 25-30,000 L☉ | Rigel |
| A | 7,500-10,000 K | 1.5-2 M☉ | 1.5-3 R☉ | 5-25 L☉ | Sirius |
| F | 6,000-7,500 K | 1-1.5 M☉ | 1-1.5 R☉ | 1.5-5 L☉ | Procyon |
| G | 5,200-6,000 K | 0.8-1 M☉ | 0.8-1.1 R☉ | 0.6-1.5 L☉ | Sol |
| K | 3,700-5,200 K | 0.45-0.8 M☉ | 0.7-0.96 R☉ | 0.08-0.6 L☉ | Alpha Centauri B |
| M | 2,400-3,700 K | 0.08-0.45 M☉ | 0.1-0.7 R☉ | 0.0001-0.08 L☉ | Proxima Centauri |
In addition to the main sequence classes, there are also giant, supergiant, and hypergiant stars that represent later evolutionary stages. As a star exhausts its core hydrogen fuel, it expands and cools, moving off the main sequence into these categories.