Stars look like identical points of light, but they differ enormously in temperature, size, brightness and lifespan. Astronomers sort them with two simple tools: a letter for temperature and a Roman numeral for size.
Spectral classes: sorting by temperature
The Harvard system, developed in the early 1900s by Annie Jump Cannon and colleagues, groups stars by the lines in their spectrum, which depend mostly on surface temperature:
- O: blue, hotter than about 30,000 K, rare and short-lived
- B: blue-white, very luminous (Rigel, Spica)
- A: white (Sirius, Vega)
- F: yellow-white (Procyon)
- G: yellow, around 5,300 to 6,000 K (the Sun)
- K: orange (Arcturus, Aldebaran)
- M: red, cooler than about 3,900 K (Proxima Centauri, Betelgeuse)
Each class is divided from 0 to 9. The Sun is a G2 star.
Luminosity classes: sorting by size
The same temperature can belong to a small or a huge star. A Roman numeral adds the size: I for supergiants, III for giants, V for main-sequence (dwarf) stars. The Sun is a G2V star: an ordinary yellow dwarf in the long, stable hydrogen-burning stage of its life.
So which stars are most common?
Red dwarfs (class M main-sequence stars) dominate. They make up roughly three quarters of the stars in the Milky Way, including our nearest neighbor, Proxima Centauri. None is visible to the naked eye because they are so dim. Bright naked-eye stars are a biased sample: mostly rare, luminous stars that can be seen from far away.
Other common types:
- K dwarfs, slightly smaller and cooler than the Sun
- G dwarfs like the Sun, only a few percent of all stars
- White dwarfs, the leftover cores of dead Sun-like stars, see the different types of white dwarfs
Giants, supergiants and hot O and B stars are rare, partly because massive stars burn out in a few million years while red dwarfs can shine for trillions.
The Hertzsprung-Russell diagram
Plotting temperature against luminosity gives the famous H-R diagram. Most stars fall on a diagonal band, the main sequence. Giants sit above it and white dwarfs below. A star’s position changes as it ages, see how stars form and evolve.
Many stars also have companions, see the different types of binary stars.
