A Short History of Stellar Typing

Chinese radio signal fallout 3. William Wollaston discovered in 1802 that the solar spectrum (the rainbow) produced by a prism possessed a series of dark lines superimposed opon it, which he attributed to natural boundaries between colors. In 1814, Joseph Fraunhofer made careful measurements of the solar spectrum and found not fewer than 600 dark lines. Capitalizing on the 1859 work of Gustav Kirchhoff and Robert Bunsen (of burner fame), Sir William Huggins, in 1864, matched some of these so-called 'Fraunhofer lines' in spectra from the Sun and other stars with the colors emitted by known gaseous substances energized to emit their line-spectra. With this simple experiment, Huggins conclusively showed that stars are made of the same materials found on Earth, rather than mysterious or exotic substances.

Despite these important discoveries, the systematic classification of stars by their spectral features wasn't undertaken until early in the 20th century. The current spectral classification scheme was developed by Henry Draper at Harvard Observatory in 1872. After his death, the majority of the classification work was done by Annie Jump Cannon from 1918 to 1924 (with important contributions from Henrietta Swan Leavitt, Antonia Maury -- who, in 1943, was given the Annie J. Cannon Award in Astronomy, and Williamina Fleming ) under the direction of Harvard College Observatory director Edward Pickering. In her 40 year carrier, Annie Cannon classified and cataloged nearly 500,000 stars. Her original scheme used capital letters running alphabetically but in subsequent revisions, as stellar evolution and typing has become better understood, the basic classification scheme has been reorganized and reduced to eight letters - O, B, A, F, G, K, M, and C.

The modern stellar classification scheme is based on spectral absorption or emission lines, which are sensitive mostly to the star's surface temperatures, rather than differences in gravity, chemical composition, or luminosity. The important spectral lines are 1) the so-called hydrogen Balmer (visible light) lines, 2) lines of neutral and singly ionized helium, 3) iron lines, 4) the doublet of ionized calcium, 5) the molecular absorption band due to the CH molecule, 6) the neutral calcium line, 7) assorted metal lines, and 8) the absorption bands of titanium oxide.

Standard Stellar Types (O, B, A, F, G, K, M)

Figure 1. Relationship of predominate spectral lines visible and stellar class and stellar surface temperature. (Adapted from Abell, Morrison, Wolf Realm of the Universe, 4th Ed. Saunders, 1988.)

Table 1. Characteristics of the 8 Basic Star Classifications

Subtypes

Figure 2. Stellar spectra with absorption line spectra characteristic of elements in the stellar atmospheres.
(From R. Bell, University of Maryland, and M. Briley, University of Wisconsin, Oshkosh.)

Luminosity Classes

Luminosity
Class

Characteristic

Examples

Other Classification Nomenclature

Characteristic

Example

Asymptotic Giant Branch Stars (R, N, and S)

Stars in the AGB phase of stellar evolution have proven difficult to model. A significant problem under intense scrutiny is one of heavy element dredge-up from the carbon-rich core. One current theory is that convection cells in the fusion layers form, which reach from the core to the hydrogen fusion layer and provides the necessary mechanism for material deep within the star to be dredged up to the surface. This may explain stellar types which have cooler surface temperatures (<3200 K) than M stars but have spectral features as if their outer atmospheres had been enriched with heavier elements. These are the R, N, and S types.

R and N type stars

S type stars

For the s-process to operate -- the predominant process in AGB stars -- a source of neutrons is required. Sources of neutrons include ²²Ne (via²²Ne(a,n)²⁵Mg) and during the production of ¹⁶O in the CNO-cycle (¹³C(a,n)¹⁶O). However, these two mechanisms will not provide a sufficient number of neutrons to sustain the s-process but other process during dredge-up can provide the additional neutrons.

In addition to the usual lines of titanium, scandium, and vanadium oxides characteristic of M-type giants, S-type stars show heavier elements such as zirconium, yttrium, and barium. Virtually all S-type stars are variable.