Using the Spectrum

Precise analysis of the absorption lines in a star's spectrum gives us information not only about the star's temperature, but also about its chemical make-up (see Chapter 7). Using spectral analysis to gauge surface temperatures with precision, astronomers have developed a system of spectral classification, based on the system worked out at the Harvard College Observatory. The presence or absence of certain spectral lines is tied to the temperatures at which we would expect those lines to exist. The stellar spectral classes and the rough temperature associated with the class are given in the following table.

Spectral Class

Surface Temperature

O (violet)

>28,000 K

B (blue)

10,000-28,000 K

A (blue)

~10,000 K

F (blue/white)

~7,000 K

G (yellow/white)

~6,000 K

K (orange)

~4,000 K

M (red)

<3,500 K

The most massive stars are the hottest, so astronomers refer to the most massive stars they study as "0 and B" stars. The least massive stars are the coolest. The letter classifications have been further refined by ten subdivisions, with 0 (zero) the hottest in the range and 9 the coolest. Thus a B5 star is hotter than a B8, and both are hotter than any variety of A star. The sun is a spectral type G2 star. Our Galaxy and others are chock full of type G2 stars.

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