A star’s spectral type is a classification code that tells you its surface temperature, color, and size at a glance. Every star visible in the night sky falls into one of seven main categories, labeled O, B, A, F, G, K, or M, running from the hottest blue stars to the coolest red ones. The Sun, for example, is classified as G2V, a code that packs three pieces of information into just three characters.
The Seven Main Spectral Classes
Stars are sorted into classes based on the patterns of light they emit, specifically which wavelengths get absorbed by gases in their outer layers. The sequence runs O, B, A, F, G, K, M. The letters look random because they’re left over from an older alphabetical system that was later rearranged by temperature. Annie Jump Cannon, working at Harvard Observatory in the early 1900s, reorganized and refined an earlier scheme developed by Williamina Fleming, reordering the groups and dropping some letters entirely to reflect what was actually happening physically: hotter stars produce different absorption patterns than cooler ones.
Each class corresponds to a temperature range and a characteristic color:
- O: 37,000 to 54,000+ K. Blue. These are the rarest, most massive stars, pumping out hundreds of thousands of times the Sun’s energy.
- B: 11,000 to 29,000 K. Blue-white. Still extremely hot and luminous.
- A: 7,900 to 9,600 K. White. Sirius, the brightest star in the night sky, is an A-type star.
- F: 6,300 to 7,350 K. Yellow-white. Slightly hotter than the Sun.
- G: 5,400 to 6,050 K. Yellow-white. The Sun sits here.
- K: 4,000 to 5,200 K. Orange.
- M: 2,700 to 3,750 K. Red. The coolest and most common stars in the galaxy.
What Each Letter Actually Measures
The classification isn’t based on color alone. When you spread a star’s light into a spectrum (a rainbow of wavelengths), dark lines appear where specific elements in the star’s atmosphere absorb particular wavelengths. The pattern of those lines changes with temperature, and that pattern is what defines the spectral type.
O-type stars are so hot that helium atoms in their atmospheres lose electrons, producing absorption lines from ionized helium. B-type stars are slightly cooler, showing lines from neutral (non-ionized) helium instead. A-type stars have the strongest hydrogen absorption lines, which peak at the A0 subclass and weaken in cooler stars. By the time you reach F and G types, lines from metals like calcium and iron become prominent. G-type stars like the Sun show strong lines from ionized calcium. K and M stars are cool enough for simple molecules to survive in their atmospheres, and M-type stars display bands from titanium oxide, a molecule that would be torn apart at higher temperatures.
The 0 to 9 Subclasses
Each letter class is divided into 10 numerical subclasses, from 0 (hottest within that class) to 9 (coolest). A B0 star sits right at the hot edge of the B class, while a B9 star is just slightly warmer than an A0. This gives astronomers a finer temperature scale. The Sun is a G2, meaning it falls near the hotter end of the G class, with a surface temperature of about 6,000 K. A G8 star, by contrast, would be closer to 5,400 K.
Luminosity Class: The Roman Numeral
Temperature alone doesn’t tell the whole story. Two stars can have the same surface temperature but wildly different sizes and brightnesses. A red supergiant and a red dwarf might both be M-type, but one is thousands of times larger than the other. To capture this, astronomers add a Roman numeral that indicates the star’s luminosity class:
- Ia, Ib: Supergiants (luminous and less luminous)
- II: Bright giants
- III: Normal giants
- IV: Subgiants
- V: Main sequence (dwarf) stars
This system, called the Morgan-Keenan classification, combines the spectral letter, the numerical subclass, and the luminosity class into a compact label. The Sun’s full designation, G2V, means it is a G-class star in subclass 2, sitting on the main sequence. A star labeled K3III would be an orange giant. The luminosity class reflects a star’s evolutionary stage: main sequence stars (V) are still fusing hydrogen in their cores, while giants and supergiants have moved past that phase and expanded enormously.
How Spectral Type Relates to Size and Mass
For main sequence stars, spectral type correlates tightly with mass and physical size. O-type stars on the main sequence can be 40 times the Sun’s radius and dozens of times its mass. These stars burn through their fuel fast and live only a few million years. At the other end, M-type main sequence stars are as small as one-tenth the Sun’s radius and far less massive, but they burn so slowly they can last tens of billions of years.
The progression is smooth. An A0 main sequence star is roughly 3.2 times the Sun’s radius. An F0 is about 1.7 times. A G0 is close to 1.1 times, nearly identical to the Sun. K-type stars shrink to about 0.7 to 0.8 solar radii, and M stars continue the trend downward. This is one reason spectral type is so useful: from a single classification code, you can estimate a main sequence star’s temperature, color, size, mass, and expected lifespan.
Color and What You Actually See
Stars don’t look as vividly colored to the naked eye as their classifications suggest. An M-type star appears more orange-red than deep red, and an O-type star looks more blue-white than pure blue. Astronomers quantify color with a measurement called the B-V color index, which compares a star’s brightness through blue and visual (yellow-green) filters. A negative value means the star is bluer; a positive value means it’s redder.
O and B stars have B-V values around -0.3, meaning they emit significantly more blue light. A0 stars sit near zero, roughly balanced. G2 main sequence stars like the Sun come in around +0.63, which corresponds to a warm yellow-white. By M0, the index reaches about +1.4, firmly in orange-red territory. These numbers shift slightly depending on whether the star is a dwarf, giant, or supergiant, since larger stars of the same temperature can appear somewhat redder.
Reading a Spectral Type in Practice
When you encounter a stellar classification like B8V or M2III, you can decode it in three steps. The letter tells you the temperature range: B is hot and blue-white, M is cool and red. The number refines that temperature within the class: 8 is near the cooler end, 2 is near the hotter end. The Roman numeral tells you the star’s size category: V is a normal main sequence star, III is a giant. Put together, B8V is a hot blue-white star on the main sequence, and M2III is a cool red giant.
This system has been in use for over a century with only minor additions. A few extra letters have been introduced for unusual objects (L and T for very cool brown dwarfs, for instance), but the core O through M sequence remains the standard framework for describing nearly every star you’ll encounter in astronomy.