| 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) |
