Spectral Type

From Slackerpedia Galactica

An excuse to make up stupid mnemonics. (This is a wiki, and that is a hint. You know what to do.)

This history of spectral types in astronomy is a long and complicated one, and since I don't feel like looking it up, please take my word for it. Ok, don't, fine. There are two primary classification schemes: the Morgan-Keenan or M-K, and the Yerkes. M-K deals primarily with temperature, while Yerkes deals with surface gravity and luminosity.

The classical M-K spectral types relate mostly to a star's surface temperature, which influences what spectral features appear (like absorption and emission lines) and at what wavelengths most of the light is emitted at. From hot to cool, the standard spectral types are: O B A F G K and M. Clearly, these were assigned with ease of rememberance in mind. *cough* Then, they're subdivided into numerical subclasses 0 through 9 (though some numbers get skipped). Increasing numbers mean cooler temperatures, so an A0 star is hotter than an A9 star, and an A9 star is a tiny bit hotter than an F0 star.

Finally, the Yerkes luminosity class often gets tacked onto the end, which indicates (... you won't believe this ...) how luminous it is. Way. Luminosity classes are indicated with Roman numerals I through V, with I reserved for "supergiants" (Betelgeuse is one) downwards through "bright giants" (II), "giants" (III), "subgiants" (I'm thinking of a Roman number...) and "dwarf" rounding out the top V. And I just mention VI and VII to satisfy any lurking pedants.

Our Sun is a G-type dwarf, whose classification is G2V.