The Spectra of Stars of Secchi's Fourth Type (Classic Reprint) - Hardcover

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Excerpt from The Spectra of Stars of Secchi's Fourth Type

The possibility of basing a systematic scheme of stellar evolution on spectroscopic observa tions is foreshadowed in the work of Fraunhofer, who in 1823 observed for the first time the spectra of a few of the brightest stars. Though wholly ignorant of the origin of the dark lines in these spectra, Fraunhofer recognized that their number, appearance, and grouping differed greatly from star to star, and that in certain cases the solar spectrum seemed to be exactly duplicated. But it required such a general survey as that of Secchi, who examined the spectra of more than four thousand stars, to afford any basis for a scheme of classification. The purely empirical classification which he adopted includes a very large percentage of the stars among its five principal types, and subsequent systems have done little more than to add subgroups to provide for the comparatively few peculiar spectra which do not fall within Secchi's divisions.

Secchi's classification, as we have said, was a purely empirical one, intended to serve only as a convenient means of grouping similar spectra. But the researches of Huggins and Vogel soon intro duced the idea of development, and the changes of spectra from type to type came to be regarded as synonymous with progressive changes in the stars themselves. Spectroscopists have agreed in regarding the white stars, with spectra characterized by the predominance of the series of hydrogen lines (secchi's first type), as representing an early stage of development, corresponding to a condition of low density. Through the continued action of gravity, accompanied by loss of heat, the absorbing metallic vapors increase in density, producing a marked increase in the number and strength of the metallic lines, while the hydrogen lines become narrower and less conspicuous (secchi's second type). The reduction of light caused by the greater absorption is most marked at the violet end of the spectrum, causing the color of the star to change from white to yellow. After passing this, the solar, stage, further operation of the same causes results in the production of red stars, whose spectra might be expected to indicate comparatively low temperature and high density of the absorbing vapors.

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