Star Types Explained: O, B, A, F, G, K, M

Before you build a planet you need a star, and stars come in very different models. Astronomers sort them by surface temperature into seven main classes, written as the letters O, B, A, F, G, K, M — from hottest to coolest. Generations of students have remembered the order with the phrase “Oh Be A Fine Girl/Guy, Kiss Me”.

The seven classes at a glance

The figures below are typical for ordinary stars in the long, stable “main sequence” stage of life, where stars spend most of their time. Luminosity is compared with the Sun (Sun = 1).

Class	Colour	Surface temperature	Brightness	Lifespan	Share of stars
O	Blue	above 30,000 °C	tens of thousands to a million × Sun	a few million years	extremely rare
B	Blue-white	10,000–30,000 °C	roughly 25 to tens of thousands × Sun	tens to hundreds of millions of years	about 0.1%
A	White	7,500–10,000 °C	about 5–25 × Sun	about 1–3 billion years	under 1%
F	Yellow-white	6,000–7,500 °C	about 1.5–5 × Sun	about 3–7 billion years	about 3%
G	Yellow	5,200–6,000 °C	about 0.6–1.5 × Sun	about 10 billion years	about 7–8%
K	Orange	3,700–5,200 °C	about 0.08–0.6 × Sun	tens of billions of years	about 12%
M	Red	2,400–3,700 °C	less than 0.08 × Sun	hundreds of billions to trillions of years	about three-quarters

The Sun is a G-type star. The “colour” describes the light the star gives off most strongly; to the eye, the Sun looks white from space.

What the star decides for your planet

Where the habitable zone is

A brighter star pushes the habitable zone farther out; a dimmer star pulls it in close. Around a red dwarf, a planet with liquid water may need to orbit closer than Mercury does to the Sun — with a “year” of just days or weeks. Around a bright A star, the zone lies beyond the distance of Mars. Try it in the habitable zone calculator.

How much time you have

Massive stars burn their fuel furiously and die young. An O or B star may explode as a supernova after only a few million years — far too soon for a planet to cool, form oceans and develop life. Sun-like stars last about ten billion years. Red dwarfs last far longer than the current age of the universe (about 13.8 billion years); none has yet reached old age.

What the sky looks like

Under an orange K star, daylight would look warmer. Under a red dwarf it would be dimmer and redder, and plants — if any evolved — might use a different colour of light for photosynthesis. Under an F star, there would be more ultraviolet light.

How dangerous the neighbourhood is

Hot stars flood nearby planets with ultraviolet radiation. Small red dwarfs, especially young ones, often produce powerful flares. See planets around red dwarfs.

The best star for a living planet?

G stars like the Sun are the only type we know for sure can host life. Some astronomers argue that K-type orange dwarfs may be even better: they are calmer than red dwarfs, more common than Sun-like stars and live two to several times longer, giving life more time to develop. These are sometimes called “Goldilocks stars”.

Builder's choice

• O, B: spectacular but short-lived. Not recommended for long projects.
• A, F: bright and roomy habitable zones, shorter lifespans, more ultraviolet.
• G: the proven option.
• K: possibly the best long-term choice.
• M: the most common stars by far, with very long lives — but tricky for planets, as the next page explains.