Spectral Classification System

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 periodically since then.
Spectral Class
Color
Temperature (Kelvin)
Composition
Class O
Dark Blue
28,000-50,000
 Ionized atoms, especially helium
Class B
Blue
10,000-28,000
 Neutral helium, some hydrogen
Class A
Light Blue
7,500-10,000
 Strong hydrogen, some ionized metals
Class F
White
6,000-7,500
 Hydrogen and ionized metals; calcium and iron
Class G
Yellow
5,000-6,000
 Ionized calcium; both neutral and ionized metals
Class K
Orange
3,500-5,000
 Neutral metals
Class M
Red
2,500-3,500
 Strong molecules; titanium oxide, neutral calcium
Class R
-
-
 Abundance of heavy metals - makeup unknown
Class N
-
-
 Abundance of heavy metals - makeup unknown
Class S
-
-
 Abundance of heavy metals - makeup unknown
Class Q
-
-
 Novae / Supernovae
Class W
-
-
 Wolf-Rayet stars
Class T
-
-
 T Tauri stars

The O-M classes are subdivided into 10 categories by decreasing temperature within each class.
This scale ranges from 0 (the hottest) to 9 (the coolest), i.e., A0, A1, A2, ... A8, A9. So, an A4 star is hotter then an A7.
For instance, Vega is an A0 and SOL is a G2.

Stars are then further classified by their type into six luminosity categories as seen in the Hertzsprung-Russell Diagram below based on Luminosity and Temperature in Kelvins.

Ia: Most Luminous Supergiants
Ib: Less Luminous Supergiants
II: Luminous Giants
III: Normal Giants
IV: Sub-Giants
V: Main Sequence
VI: Dwarf / Subdwarf
Sol, is categorized a G2V classification.
That means Class G, Type 2 Main Sequence star.
Alcyone is B7III meaning a Class B, Type 7 Giant star.		 Spectral - luminosity chart

The information contained on this page is not fictitious, this is the actual classification system in use today by astronomers worldwide. If there are any known errors please contact webmaster for a correction.

Original document from the
Daystrom Institute Main Page
Used with thier kind permission.

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