Star Spectra and Luminosity Classes

The hottest stars produce most light at the blue end of the spectrum (top image). The coolest stars produce most light at the red end (bottom image). In-between are stars that produce light spread evenly across the visible light spectrum. They appear white.

Astronomers have examined the spectra of hundreds of thousands of stars and have grouped them into a sequence of spectral classes.

Astronomers indicate the class to which a star belongs by a three part code made up of a letter, a digit and a Roman numeral:

The seven letters: O, B, A, F, G, K, M are used to divide stars into seven temperature classes ranging from the hottest: O type stars to the coolest: M type stars. A digit, 0 to 9, is used to precisely place a star within a class.

The Roman numeral is used to specify a star's luminosity or size. (The two are related because the amount of light radiated by a star is directly proportional to its surface area). The codes in order from largest stars to smallest are:

Super Giants GiantsSun-sized and DwarfsWhite Dwarfs

The a, ab, and b suffixes were added to class I stars after it was realised that their range of sizes was much bigger than first thought.

The letter, digit, Roman numeral system allows astronomers to compactly record a wealth of information about stars in catalogs and lists. The spectral luminosity codes for a selection of stars are shown in the table at the right.

O 50,000°K
B 20,000°K
A 10,000°K
F  7,500°K
G  6,000°K
K  4,000°K
M  3,500°K

Most stars fit into the standard spectra and luminosity classes. For a variety of reasons, some don't. They have unusal elements in their atmospheres. They have orbiting companions. They are surrounded by disks of dust and gas. They are too small to fully ignite their nuclear fuel. Astronomers are learning much about the evolution of stars by studying those that don't fit the normal pattern.

Spectral Luminosity Codes for a Selection of Stars

Star Code Meaning
Rigel B8Ia A very hot, bright, Blue Supergiant
Sirius A1V A hot, white, Giant
The Sun G2V A yellow medium sized star
Aldebaran K5III A cool, luminous Red Giant
M5V A very cool, dim, Red Dwarf

This page is based on a Stellar Spectrum Tutorial by Michael Lemke and Simon Jeffery