The original version of the current MK system was defined by Morgan, Keenan, and Kellman (1943), and a revised list of standards was published in 1953 by Morgan and Johnson. Additional refinements have been added since then.

MK Spectral Classification System (Standard)

Class O - Dark Blue

Temperature: 28,000 - 50,000° K
Composition: Ionized atoms, especially helium
Example: Mintaka (O1-3III)

Class B - Blue

Temperature: 10,000 - 28,000° K
Composition: Neutral helium, some hydrogen
Example: Alpha Eridani A (B3V-IV)

Class A - Light Blue

Temperature: 7,500 - 10,000° K
Composition: Strong hydrogen, some ionized metals
Example: Sirius A (A0-1V)

Class F - White

Temperature: 6,000 - 7,500° K
Composition: Hydrogen and ionized metals, calcium and iron
Example: Procyon A (F5V-IV)

Class G - Yellow

Temperature: 5,000 - 6,000° K
Composition: Ionized calcium, both neutral and ionized metals
Example: Sol (G2V)

Class K - Orange

Temperature: 3,500 - 5,000° K
Composition: Neutral metals
Example: Alpha Centauri B (K0-3V)

Class M - Red

Temperature: 2,500 - 3,500° K
Composition: Ionized atoms, especially helium
Example: Wolf 359 (M5-8V)