Spectral Classification of Stars

The temperature range of each class, along with the most prominent spectral lines which form the basis of the spectral identification, are described below. A common mnemonic for remembering the order of the spectral classes is Oh Be A Fine Girl (or guy) Kiss Me. In the original scheme there were a few additional classes (S, R, C, N) which turned out to represent stars that actually do have abnormal compositions. Today, these stars are usually in transient evolutionary phases, and are not included in the standard spectral classifications.

O (30,000 - 60,000 K, blue-white)-At such high temperatures, most of the hydrogen is ionized, and thus the hydrogen lines are less prominent than in the B and A classes (ionized hydrogen with no remaining electron has no spectral lines). Much of the helium is also ionized. Lines from ionized carbon, nitrogen, oxygen and silicon are also seen.

B (10,000 - 30,000 K, blue white)-In stars in this spectral class, the hydrogen lines are stronger than in O stars, while the lines of ionized helium are weaker. Ionized carbon, oxygen, and silicon are seen.

A (7500 - 10,000 K, blue white)-A stars have the strongest hydrogen lines (recall the ordering of the original Harvard classification). Other prominent lines are due to singly ionized magnesium, silicon and calcium.

F (6000 - 7500 K, yellow-white)-Lines from ionized calcium are prominent features in F stars.

G (5000 - 6000 K, yellow)-The ionized calcium lines are strongest in G stars. The sun is a G2 star.

K (3500 - 5000 K, orange)-The spectra of K stars contain many lines from neutral elements.

M (less than 3500 K, red)-Molecular lines seen in the spectra of M stars mean that the temperature is low enough that molecules have not been broken up into their constituent atoms. Titanium oxide (TiO) is particularly prominent.

The MKK luminosity classes are: I-Supergiants; II-Bright Giants; III-Normal Giants; IV-Subgiants; V-Main Sequence.