Star Types and Colors: Spectral Classes O, B, A, F, G, K, M
Stars in the night sky shine in colors from blue to red. A star's color reveals its surface temperature and is the basis for sorting stars into seven spectral types: O, B, A, F, G, K and M.
| Order | O, B, A, F, G, K, M (hottest → coolest) |
|---|---|
| The Sun | G type, surface about 5,800 K |
| Most common star | M-type red dwarfs |
| What sets the color | Surface temperature (blackbody radiation) |
A star's color comes from its temperature
Heat a piece of iron and it first glows red, then turns yellow and white as it gets hotter. Stars work the same way. A hot, glowing object gives off light most strongly at a wavelength that depends on its temperature; this is called blackbody radiation. The hotter the object, the more its brightest wavelength shifts toward shorter wavelengths, that is, toward blue.
That's why stars with surfaces tens of thousands of degrees hot look blue, while stars around 3,000 degrees look red. The Sun shines brightest near green, but because its light is an even mix of many colors, it looks white to our eyes.
Spectral types at a glance
| Type | Surface temperature | Color | Examples |
|---|---|---|---|
| O | About 30,000 K or more | Blue | Alnitak's primary star (Orion) |
| B | About 10,000–30,000 K | Blue-white | Rigel, Spica |
| A | About 7,500–10,000 K | White | Sirius, Vega |
| F | About 6,000–7,500 K | Yellow-white | Procyon |
| G | About 5,200–6,000 K | Yellowish white | The Sun, Alpha Centauri A |
| K | About 3,700–5,200 K | Orange | Alpha Centauri B |
| M | About 2,400–3,700 K | Red | Proxima Centauri |
Temperature ranges vary slightly from source to source, so treat them as rough guides.
How to read a star's label
You may have seen the Sun's classification written as G2V. The G is the spectral type, and the number from 0 to 9 that follows splits each type into finer steps. Lower numbers are hotter, so G2 is on the hotter side of the G types. The Roman numeral at the end is the luminosity class, which shows how puffed up a star is. V means a main-sequence star (a dwarf) steadily burning hydrogen like the Sun, III means a giant, and I means a supergiant. For example, Rigel is classified as a B-type supergiant and Aldebaran as a K-type giant.
The order of these seven letters isn't alphabetical for a historical reason. At first, letters were assigned according to the strength of the hydrogen lines in a star's spectrum. Later, when the types were rearranged in order of temperature, the sequence became the O, B, A, F, G, K, M we use today.
Mass, brightness and lifespan
For main-sequence stars, which shine steadily by burning hydrogen, mass determines almost everything. The heavier the star, the hotter and brighter it is, but it burns through its fuel so fast that it doesn't live long.
- O type: More than about 15 times the Sun's mass and tens of thousands of times brighter or more. Lifespans are short, only a few million years.
- B type: About 2 to 16 times the Sun's mass, with lifespans of tens of millions to hundreds of millions of years.
- A and F types: A bit heavier and brighter than the Sun. Lifespans range from about a billion to several billion years.
- G type: About the Sun's mass, with a lifespan of about 10 billion years.
- K type: Lighter and dimmer than the Sun, with lifespans much longer than the Sun's.
- M type: About 8–45% of the Sun's mass and very dim. They use their fuel so slowly that their lifespans are calculated at hundreds of billions of years or more, far longer than the current age of the universe.
Which stars are most common?
Most stars that catch your eye in the night sky are bright B or A types, or swollen giants. But the most numerous stars in the Milky Way are M-type red dwarfs, which are almost invisible to the naked eye. Roughly three-quarters of the main-sequence stars near the Sun are M types, while Sun-like G types make up less than one in ten. O types are extremely rare, about one in several million. Even Proxima Centauri, the closest star to the Sun, is an M-type red dwarf.
Giants and supergiants
When a star uses up the hydrogen in its core, its outer layers swell up and it becomes a giant. As its surface expands, it cools and often turns orange or red. Aldebaran in Taurus and Arcturus in Boötes are K-type giants, while Betelgeuse in Orion and Antares in Scorpius are M-type red supergiants. Very massive stars like Rigel can also shine as hot blue-white supergiants.
How a star's type affects its planets
- Distance to the habitable zone: Bright stars have their habitable zones far out, while dim stars have them very close in. The habitable zone of an M-type star lies well inside the orbit of Mercury.
- Tidal locking: Planets orbiting close to their star easily become tidally locked, with one side always facing the star. Planets in the habitable zones of M-type stars are especially likely to be locked.
- Flares: M-type red dwarfs frequently unleash flares, sudden bursts of intense light and particles, which can strip away the atmospheres of nearby planets.
- Time: O- and B-type stars burn out so quickly that there may not be enough time for life to develop on their planets.
In Planetica
The central star of every system created in Planetica gets an O/B/A/F/G/K/M-style spectral type and brightness, and it's colored with the blackbody color that matches its surface temperature. The habitable zone and frost line are calculated from the star's brightness, and each planet's temperature is set by the star's brightness and its orbital distance. Check which type your system's star is and compare it with the table.
If your system has a red M-type star, its planets will huddle close to it; with a bright A- or F-type star, the habitable zone sits much farther out. It's also fun to head out to the Milky Way map, browse the colors of real stars and look for one that resembles yours.
Read more
- 9 Types of Planets
- Star Types and Colors: Spectral Classes O, B, A, F, G, K, M
- What Is the Habitable Zone (Goldilocks Zone)?
Create your own planetary system
Enter a name and a birthday to get a one-of-a-kind planetary system, placed next to a real star in your birthday constellation.