Stellar Classification - Harvard Spectral Classification

Harvard Spectral Classification

The Harvard classification system is a one-dimensional classification scheme. Stars vary in surface temperature from about 2,000 to 40,000 kelvin. Physically, the classes indicate the temperature of the star's atmosphere and are normally listed from hottest to coldest, as is done in the following table:

The mass, radius, and luminosity listed for each class are appropriate only for stars on the main sequence portion of their lives and so are not appropriate for red giants. The spectral classes O through M are subdivided by Arabic numerals (0–9). For example, A0 denotes the hottest stars in the A class and A9 denotes the coolest ones. The Sun is classified as G2.

The reason for the odd arrangement of letters is historical. An early classification of spectra by Angelo Secchi in the 1860s divided stars into those with prominent lines from the hydrogen Balmer series (group I, with a subtype representing many of the stars in Orion); those with spectra which, like the Sun, showed calcium and sodium lines (group II); colored stars whose spectra showed wide bands (group III); and carbon stars (group IV). In the 1880s, the astronomer Edward C. Pickering began to make a survey of stellar spectra at the Harvard College Observatory, using the objective-prism method. A first result of this work was the Draper Catalogue of Stellar Spectra, published in 1890. Williamina Fleming classified most of the spectra in this catalogue. It used a scheme in which the previously used Secchi classes (I to IV) were divided into more specific classes, given letters from A to N. Also, the letters O, P and Q were used, O for stars whose spectra consisted mainly of bright lines, P for planetary nebulae, and Q for stars not fitting into any other class.

In 1897, another worker at Harvard, Antonia Maury, placed the Orion subtype of Secchi class I ahead of the remainder of Secchi class I, thus placing the modern type B ahead of the modern type A. She was the first to do so, although she did not use lettered spectral types, but rather a series of 22 types numbered from I to XXII. In 1901, Annie Jump Cannon returned to the lettered types, but dropped all letters except O, B, A, F, G, K, and M, used in that order, as well as P for planetary nebulae and Q for some peculiar spectra. She also used types such as B5A for stars halfway between types B and A, F2G for stars one-fifth of the way from F to G, and so forth. Finally, by 1912, Cannon had changed the types B, A, B5A, F2G, etc. to B0, A0, B5, F2, etc. This is essentially the modern form of the Harvard classification system.

The fact that the Harvard classification of a star indicated its surface temperature was not fully understood until after its development. In the 1920s, the Indian physicist Meghnad Saha derived a theory of ionization by extending well-known ideas in physical chemistry pertaining to the dissociation of molecules to the ionization of atoms. First applied to the solar chromosphere, he then applied it to stellar spectra. The Harvard astronomer Cecilia Helena Payne (later to become Cecilia Payne-Gaposchkin) then demonstrated that the OBAFGKM spectral sequence is actually a sequence in temperature. Because the classification sequence predates our understanding that it is a temperature sequence, the placement of a spectrum into a given subtype, such as B3 or A7, depends upon (largely subjective) estimates of the strengths of absorption features in stellar spectra. As a result, these subtypes are not evenly divided into any sort of mathematically representable intervals.

O, B, and A stars are sometimes called "early type", while K and M stars are said to be "late type". This stems from an early 20th century model of stellar evolution in which stars were powered by gravitational contraction via the Kelvin–Helmholtz mechanism in which stars start their lives as very hot "early-type" stars, and then gradually cool down, thereby evolving into "late-type" stars. This mechanism provided ages of the Sun that were much smaller than what is observed, and was rendered obsolete by the discovery that stars are powered by nuclear fusion. However, brown dwarfs, whose energy comes from gravitational attraction alone, cool as they age and so progress to later spectral types. The highest-mass brown dwarfs start their lives with M-type spectra and will cool through the L, T, and Y spectral classes.