How Stars Are Classified
Astronomers sort stars in two ways:
- Spectral class, based on surface temperature (and therefore colour): O, B, A, F, G, K, M. A common way to remember the order is "Oh Be A Fine Girl/Guy, Kiss Me".
- Luminosity class, based on size and brightness: from I (supergiants) to V (main-sequence or "dwarf" stars).
Our Sun is a G2V star: spectral class G, subclass 2, luminosity class V (main sequence).
Spectral Classes
| Class | Surface temperature | Colour | Example |
|---|---|---|---|
| O | above 30,000 K | Blue | Zeta Puppis |
| B | 10,000 – 30,000 K | Blue-white | Rigel |
| A | 7,500 – 10,000 K | White | Sirius A, Vega |
| F | 6,000 – 7,500 K | Yellow-white | Procyon A |
| G | 5,200 – 6,000 K | Yellow | The Sun (about 5,770 K) |
| K | 3,700 – 5,200 K | Orange | Arcturus |
| M | 2,400 – 3,700 K | Red | Proxima Centauri, Betelgeuse |
Hotter stars are bluer and much rarer. Cool M stars are by far the most common type.
Luminosity Classes
| Class | Name |
|---|---|
| 0 / Ia+ | Hypergiants |
| Ia, Ib | Supergiants |
| II | Bright giants |
| III | Giants |
| IV | Subgiants |
| V | Main-sequence stars (dwarfs) |
| D | White dwarfs |
The Main Types of Stars
Main-Sequence Stars
About 90 % of the stars we see are on the main sequence. They are in the stable, longest phase of their lives, fusing hydrogen into helium in their cores. Their mass decides everything else: heavier stars are hotter and brighter and burn out much faster.
| Mass (Sun = 1) | Typical lifetime |
|---|---|
| 0.1 (red dwarf) | trillions of years |
| 1 (the Sun) | about 10 billion years |
| 10 | about 20–30 million years |
| 40+ | a few million years |
Red Dwarfs
These are small, cool M-type stars with roughly 0.08 to 0.6 solar masses. They make up about three quarters of all stars in the Milky Way, but none of them is visible to the naked eye. Our nearest neighbour, Proxima Centauri, is a red dwarf.
Giants and Supergiants
When a star runs out of hydrogen in its core, it swells up:
- Red giants are what Sun-like stars become. Their surface cools while they grow 100 or more times larger. In about 5 billion years, the Sun will become one.
- Supergiants form from stars with more than about 8–10 solar masses. Betelgeuse (a red supergiant) and Rigel (a blue supergiant) are well-known examples. Placed at the centre of our Solar System, Betelgeuse would extend roughly to the orbit of Jupiter.
Brown Dwarfs
These "failed stars" have about 13 to 80 times the mass of Jupiter. That is not enough to fuse ordinary hydrogen in a stable way. They glow dimly from leftover heat.
Variable Stars
Some stars change their brightness regularly. Cepheid variables are especially important: their pulsation period tells you how bright they really are, which makes them one of the key tools for measuring distances to other galaxies.
Wolf-Rayet Stars
These are very hot, massive stars that have blown away their outer hydrogen layers with extremely strong stellar winds. They are often a late stage before a supernova.
Stellar Remnants
When a star dies, what remains depends mainly on its starting mass:
| Starting mass | End of life | Remnant |
|---|---|---|
| below about 8 Suns | Sheds its outer layers as a planetary nebula | White dwarf: about the size of Earth, extremely dense, at most about 1.4 solar masses (the Chandrasekhar limit) |
| about 8 – 20 Suns | Core-collapse supernova | Neutron star: about 20 km across. One teaspoon would weigh billions of tonnes. Rotating neutron stars that send out beams are called pulsars |
| above about 20 Suns | Supernova or direct collapse | Black hole, see Black Holes |
These mass limits are approximate. They also depend on the star's composition and on whether it has a companion.
Stellar Populations
Astronomers also group stars by age and chemical make-up:
- Population I: young stars with lots of heavy elements, such as the Sun.
- Population II: old stars with few heavy elements, often in globular clusters.
- Population III: the very first stars, made only of hydrogen and helium. None has been observed directly yet.
The Hertzsprung–Russell Diagram
The most important chart in stellar astronomy plots temperature (x-axis, hot on the left) against brightness (y-axis). Stars group into clear regions on it: the diagonal band of the main sequence, giants and supergiants at the upper right, and white dwarfs at the lower left. How a star moves across the diagram tells its life story.
See also: The Laws of Thermodynamics for why stars shine stably for billions of years.