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Spectral (Stellar) 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 since then.

MK Spectral Classification System (Standard)

Class**	Color	Temperature (K)	Composition
O	Dark Blue	28,000-50,000	ionized atoms, especially helium
B	Blue	10,000-28,000	neutral helium, some hydrogen
A	Light Blue	7,500-10,000	strong hydrogen, some ionized metals
F	White	6,000-7,500	hydrogen and ionized metals; calcium and iron
G	Yellow	5,000-6,000	ionized calcium; both neutral and ionized metals
K	Orange	3,500-5,000	Neutral metals
M	Red	2,500-3,500	Strong molecules; titanium oxide, neutral calcium

**Three additional categories are also

in the scheme: R, N, and S types, which were later realized to represent stars with peculiar heavy-metal abundances. Other types (Q for novae, W for Wolf-Rayet stars, T for T Tauri stars, etc...) are encountered infrequently. Thus, the above listing is considered the standard classification scheme, and R, N, S, T, Q, W, etc...

are addendums in the rare cases they are observed.**

Each class is 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 the Sun is a G2.

Stars are then further classified

by their type into six luminosity categories by the Yerkes scheme, as seen in the Hertzsprung-Russell Diagram below (based on Luminosity and Temperature (K)):: Ia: Most Luminous Supergiants, Ib: Less Luminous Supergiants, II: Luminous Giants, III: Normal Giants, IV: Sub-Giants, V: Main Sequence (Dwarfs). Thus, Sol, is categorized a G2V classification. That is, a G-Type 2

Main Sequence star.