Spectral (Stellar) Classification System

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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.