File:Astronomy in a nutshell, the chief facts and principles explained in popular language for the general reader and for schools (1912) (14595879027).jpg

Identifier: astronomyinnutsh00serv (find matches)
Title: Astronomy in a nutshell, the chief facts and principles explained in popular language for the general reader and for schools
Year: 1912 (1910s)
Authors: Serviss, Garrett Putman, 1851-1929 Rouben Mamoulian Collection (Library of Congress) DLC
Subjects: Astronomy
Publisher: New York, London, G.P. Putnam's sons
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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h thenaked eye no stars fainter than the sixthmagnitude are visible, but very powerfultelescopes may show them down to theeighteenth magnitude. Each magnitude isabout two and a half times brighter than thenext magnitude below in the scale. A first-magnitude star is about one hundred timesbrighter than one of the sixth magnitude.But, in reality, the variation of brightnessis gradual, and for very accurate estimatesfractions of a magnitude have to be employed.There are about twenty first-magnitude stars,but they are not all of equal brightness.A more accurate photometry assumes azero magnitude, very nearly, representedby the star Arcturus, and makes the ratio2.512. Thus a star, nearly represented byAldebaran or Altair, which is 2.512 timesfainter than the zero magnitude, is of thefirst magnitude, and a star, nearly repre-sented by the North Star, which is 2.512times fainter than the first magnitude, is ofthe second magnitude. Counting in theother direction, a star, like Sirius, which is
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The Stars 221 brighter than the zero magnitude, is said tobe of a negative magnitude. The magnitudeof Sirius is — 1.6. There is only one otherstar of negative magnitude, Canopus, whosemagnitude is — 0.9. But for ordinary pur-poses one need not trouble himself with theserefinements. The stars are divided into five principaltypes, according to their spectra. Theseare: I. White stars, having a bluish tinge, inwhich the spectrum is characterised by broaddark bands, due apparently to an extensiveatmosphere of hydrogen, while there arebut few lines indicating the presence ofmetallic vapours. About half the starswhose spectra have been studied belong toType I. II. Yellowish-white stars, resembling thesun in having their spectra crossed with agreat number of lines produced by metallicvapours, while the hydrogen lines are lessconspicuous. These are often called solarstars, and they, too, are very numerous. III. Orange and slightly reddish stars,whose spectra contain mostly broad bandsinste

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