Spectral type

(astronomy) A label used to indicate the physical and chemical characteristics of a star as indicated by study of the star's spectra; for example, the stars in the spectral type known as class B are blue-white, and are referred to as helium stars because the dominant lines in their spectra are the lines in helium spectra.

An indicator of the physical and chemical characteristics of a star, based on study of the star's spectrum. Stars possess a remarkable variety of spectra, some simple, others complex. To understand the natures of the stars, it was first necessary to bring order to the subject and to classify the spectra.

The modern system of classification was initiated about 1890. The spectra were ordered by letter, A through O, largely on the basis of the strengths of the hydrogen lines. Several letters were found to be unnecessary or redundant, and on the basis of continuity of lines other than hydrogen, it was found that B preceded A and O preceded B. The result is the classical spectral sequence, OBAFGKM. The classes were decimalized, setting up the sequence O5, …, O9, B0, …, B9, A0, and so forth. (Not all the numbers are used.) The modern standard sequence, called the Harvard sequence after the observatory where it was formulated, runs from O3 to M9. Classes L and T were added to the sequence in 1999.

Class A has the strongest hydrogen lines, B is characterized principally by neutral helium (with weaker hydrogen), and O by ionized helium. Hydrogen weakens notably through F and G, but the metal lines, particularly those of ionized calcium, strengthen. In K, hydrogen becomes quite weak, while the neutral metals grow stronger. The M stars effectively exhibit no hydrogen lines at all but are dominated by molecules, particularly titanium oxide (TiO). L stars are dominated by metallic hydrides and neutral alkali metals, while T is defined as methane. At G, the sequence branches downward into R and N, whose stars are rich in carbon molecules. In class S, the titanium oxide molecular bands of class M are replaced by zirconium oxide (ZrO).

At first appearance, the different spectral types seem to reflect differences in stellar composition. However, within the sequence OBAFGKMLT the elemental abundances are roughly similar. The dramatic variations in spectra are strictly the result of changes in temperature. The different spectra of the R, N, and S stars, however, are caused by true and dramatic variations in the chemical composition, the result of internal thermonuclear processing and convection. See also Stellar evolution.

In the 1940s, W. W. Morgan, P. C. Keenan, and E. Kellman expanded the Harvard sequence to include luminosity. A system of roman numerals is appended to the Harvard class to indicate position on the Hertzsprung-Russell diagram: I for supergiant, II for bright giant, III for giant, IV for subgiant, and V for dwarf or main sequence. See also Astronomical catalogs; Hertzsprung-Russell diagram.