THE LIGHT OF THE STARS 



BY EDWARD CHARLES PICKERING 



[Edward Charles Pickering, Director of Harvard College Observatory, b. July 

 19 1846. S.B. Lawrence Scientific School, 1865; LL.D. University of Califor- 

 nia, 1886; ibid. Michigan, 1887; ibid. Chicago, 1901; ibid. Heidelberg, 1903, 

 ibid. Harvard, 1903; D.Sc. Victoria University, Manchester, England. Thayer 

 Professor of Physics, Massachusetts Institute of Technology, 1867-77. Member 

 of National Academy ; Royal Astronomical Society; Societies of Lund, Mexico, 

 Palermo, Cherbourg, Lincei, Konig. Preuss.; Astronomical Society of France. 

 Author of Elements of Physical Manipulation; also of about fifty volumes of 

 the Annals of Harvard College Observatory.] 



IF an intelligent observer should see the stars for the first time, 

 two of their properties would impress him as subjects for careful 

 study, first, the irrelative positions, arid secondly, the irrelative 

 brightness. From the first of these has arisen the astronomy of 

 position, or astrometry. This is sometimes called the Old Astronomy, 

 since until within the last twenty years the astronomers of the world, 

 with few exceptions, devoted their attention almost entirely to it. 

 To the measure of the light should be added the study of the color 

 of the stars (still in its infancy), and the study of their composition, 

 by means of the spectroscope. In this way a young giant has been 

 reared, which has almost dwarfed its older brothers. The science of 

 astrophysics, or the New Astronomy, has thus been developed, 

 which during the last few years has rejuvenated the science and given 

 to it, by its brilliant discoveries, a public interest which could not 

 otherwise have been awakened. The application to stellar astronomy 

 of the daguerreotype in 1850, of the photograph in 1857, and of the 

 dry plate in 1882, has opened new fields in almost every department 

 of this science. In some, as in stellar spectroscopy, it has almost com- 

 pletely replaced visual observations. 



One department of the New Astronomy, the relative brightness of 

 the stars, is as old as, or older than, the Old Astronomy. An astro- 

 nomer even now might do useful work in this department without any 

 instruments whatever. Hipparchus is known to have made a catalogue 

 of the stars about 150 B. c. Ptolemy, in 138 A. D., issued that great 

 work, the Almagest, which for fourteen hundred years constituted 

 the principal and almost the sole authority in astronomy. It con- 

 tained a catalogue of 1028 stars, perhaps based on that of Hipparchus. 

 Ptolemy used a scale of stellar magnitudes which has continued in 

 use to the present day. He called the brightest stars in the sky the 

 first magnitude, the faintest visible to the naked eye, the sixth. More 

 strictly, he used the first six letters of the Greek alphabet for this 

 purpose. But he went a step further, and subdivided these classes. 

 If a star seemed bright for its class, he added the letter fj. (mu), stand- 



