182 ANNUAL REPORT SMITHSONIAN" INSTITUTION, 1928 



binary stars are so close together that they appear single as seen from 

 the earth, even with the most powerful telescopes, and their duplicity 

 is known only by the fact that their orbital motions in the line of 

 sight are shown by the spectroscope. Many of these spectroscopic 

 binaries have periods less than a day, and the two stars of the pair 

 are so close together that they almost touch each other. Our Milky 

 Way sj^stem, comprising billions of stars, is made up of such smaller 

 groups, while it, in turn, seems to be a larger unit, a cluster or perhaps 

 a spiral nebula, with a definite motion as a whole. 



Anotlier activity of the stars wliich has had an increasing import- 

 ance in astronomy since it was first discovered is the variation of 

 total luminosity as seen from the earth. This enticing field of obser- 

 vation has been so attractive to all interested in the heavens that 

 hundreds of amateurs all over the world are devoting their time and 

 talents to the observation of stellar light changes. The thrill of see- 

 ing one of the lucid stars of the sk}^ periodically dimming or en- 

 hancing its brightness is an experience never to be forgotten. The 

 study of stellar variation has been not only a hobby or useful diver- 

 sion for the amateur, but has commanded the efforts of nearly all the 

 greatest astronomers of the last hundred years. The brighter stars 

 have been observed with the naked eye ; the fainter ones have required 

 the most powerful instruments that could be had. Much time is being 

 given to these objects at the largest observatories. About one-third of 

 the time of the 100-inch telescope is now being devoted to variable 

 stars of different kinds. 



The first scientific record of variation among the stars seems to he. 

 that concerning the appearance of Tycho Brahe"s new star, or nova, 

 in 1572. This was a very bright nova, appearing in Cassiopeia at a 

 point where no star had been known before. It was as bright as 

 Venus at its brightest and could be seen in tlie daytime. After a 

 few months it disappeared from view, as there were no telescopes at 

 that time to folloAv it fur flier. On account of the lack of accurate 

 observations of its position, it can not certainly be located at the pres- 

 ent time, but it is probably identical with a faint star of the foar- 

 teentli magnitude. It has fherefore decreased in brightness at least 

 17 magnitudes, or six million times, since its historic and record- 

 breaking outburst. 



Some 50 novte or temporary stars liave appeared since that time. 

 The number has inci'cased with the use of better instruments and 

 with the aid of photograpliy. Many recall Nova Aquihe wliich was 

 discovered on June 8, 1918, as a first-magnitude star. Photographs 

 show that for 30 years previous it was an unktiown and inconspic- 

 uous eleventh-magnitude star in the Milky Way. Some time about 

 June 6 the crisis of its history occurred. The next day it had 

 increased in brightness a hundredfold and was doubling its bright- 



