What is the OBAFGKM sequence?

It is the Harvard spectral classification, ordering stars by surface temperature from hottest to coolest: O, B, A, F, G, K and M. The mnemonic 'Oh Be A Fine Girl/Guy, Kiss Me' helps remember the order. The Sun is a G-type star.

How is spectral class linked to colour?

Hotter stars peak at shorter wavelengths and look blue or blue-white (O and B), while cooler stars peak in the red and look orange or red (K and M). The Sun, a G-type star at about 5,800 kelvin, appears yellow-white.

What spectral type is the Sun?

The Sun is class G2V. The G2 part places it in the G class near the hotter end, and the V is a luminosity class marking it as a main-sequence dwarf star. Its effective temperature is about 5,778 kelvin.

Why are M stars so common?

Low-mass red dwarfs of class M are by far the most numerous main-sequence stars, making up roughly three quarters of all stars. They are cool, faint and very long-lived, so most stars in the galaxy are M-type even though few are visible to the naked eye.

Are there classes beyond M?

Yes. Modern surveys add classes L, T and Y for very cool objects and brown dwarfs below the M range, plus rare classes like W for Wolf-Rayet stars. The classic OBAFGKM sequence still covers normal hydrogen-burning stars.

Stellar Classification (Spectral Types) Reference

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Sorting stars by temperature

Astronomers classify stars by the absorption lines in their spectra, which depend mostly on surface temperature. The Harvard system orders the main classes from hottest to coolest as O, B, A, F, G, K and M. This reference gives the effective-temperature range, colour, typical mass and example stars for each class, plus a lookup that maps any temperature to its spectral class.

How it works

Each class spans a band of effective temperature, in kelvin:

O  30,000 – 60,000 K   blue
B  10,000 – 30,000 K   blue-white
A   7,500 – 10,000 K   white
F   6,000 – 7,500 K    yellow-white
G   5,200 – 6,000 K    yellow   (the Sun)
K   3,700 – 5,200 K    orange
M   2,400 – 3,700 K    red

Each class is subdivided 0–9, with 0 hottest, so the Sun is G2. A luminosity class in Roman numerals is appended (V = main-sequence dwarf, III = giant, I = supergiant), giving the Sun’s full type G2V. The lookup returns the class whose temperature band contains the value you enter.

Tips and notes

Mnemonic: “Oh Be A Fine Girl/Guy, Kiss Me” gives the order O B A F G K M.
Hotter means bluer; cooler means redder. Colour tracks temperature.
The Sun is an ordinary G2V star, neither especially hot nor cool.
Cool M-type red dwarfs are the most common stars in the galaxy.
Classes L, T and Y extend the sequence to brown dwarfs below M.