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.

What is the Stellar Classification (Spectral Types) Reference?

Reference table for the Harvard stellar spectral classification with effective-temperature ranges, star colour and example stars, plus a temperature-to-spectral-class lookup. It runs free in your browser on Gera Tools, with nothing uploaded.

Stellar Classification (Spectral Types) Reference

Name: Stellar Classification (Spectral Types) Reference
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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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.

Well-known example stars

Class	Example star	Temperature (K)	Notes
O	Theta1 Orionis C	~40,000	The brightest star in the Orion Nebula trapezium
B	Rigel	~12,000	The bright blue-white star in Orion’s foot
A	Sirius A	~9,940	The brightest star in the night sky
F	Procyon A	~6,530	One of the closest bright stars
G	The Sun	~5,778	Our star, an ordinary mid-life dwarf
K	Arcturus	~4,300	Prominent orange giant in Boötes
M	Betelgeuse	~3,500	A red supergiant in Orion, variable brightness

The luminosity class dimension

Spectral type alone describes surface temperature; the luminosity class (the Roman numeral suffix) describes how intrinsically bright the star is, which correlates with its size and evolutionary state:

Class I (Ia, Ib): Supergiants — rare, massive, very bright. Betelgeuse is M2Ib.
Class II: Bright giants
Class III: Giants — including Arcturus (K1.5III) and Pollux (K0III)
Class IV: Subgiants — stars evolving off the main sequence
Class V: Main-sequence dwarfs (the most common) — including the Sun (G2V) and Sirius (A1V)
Class VI / sd: Subdwarfs — underluminous for their temperature
Class VII / D: White dwarfs — the dense remnants of evolved Sun-like stars

A full type like G2V therefore tells you three things: temperature band (G), position within the band (2, near the hotter end), and evolutionary state (V, hydrogen-burning main-sequence dwarf).

Beyond OBAFGKM: extending the sequence

Modern surveys of very cool objects have added three more classes:

L dwarfs (1,300–2,400 K): very red objects straddling the boundary between the coolest true stars and brown dwarfs; characterised by methane and dust in their atmospheres.
T dwarfs (700–1,300 K): cool brown dwarfs dominated by methane absorption.
Y dwarfs (below ~700 K): the coolest known sub-stellar objects, some cool enough that water clouds may form in their upper atmospheres.

These extend the sequence to OBAFGKMLT Y, though the classic OBAFGKM mnemonic still covers all normal hydrogen-burning stars. The mnemonic “Oh Be A Fine Girl/Guy, Kiss Me” remains the standard way to remember the order.