54 POPULAR SCIENCE MONTHLY. 



intervals grow faint, owing to the interposition of a dark companion. 

 Twenty years ago, when photography was first applied to the discovery 

 of variable stars, only about two hundred and fifty of these objects were 

 known. Since then, three remarkable discoveries have been made, by 

 means of which their number has been greatly increased. The first 

 was by Mrs. Fleming, who, in studying the photographs of the Henry 

 Draper Memorial, found that the stars of the third type, in which the 

 hydrogen lines are bright, are variables of long period. From this 

 property she has discovered 128 new variables, and has also shown how 

 they may be classified from their spectra. The differences between the 

 first, second and third types of spectra are not so great as those be- 

 tween the spectra of different variables of long period. The second 

 discovery is that of Professor Bailey, who found that certain globular 

 clusters contain large numbers of variable stars of short period. He 

 has discovered 509 new variables, 396 of them in four clusters. The 

 third discovery, made by Professor Wolf, of Heidelberg, that variables 

 occur in large nebula?, has led to his discovery of 65 variables. By 

 similar work, Miss Leavitt has found 295 new variables. The total 

 number of variable stars discovered by photography during the last 

 fifteen years is probably five times the entire number found visually up 

 to the present time. Hundreds of thousands of photometric measures 

 will be required to determine the light curves, periods and laws regu- 

 lating the changes these objects undergo. 



A far more comprehensive problem, and perhaps the greatest in 

 astronomy, is that of the distribution of the stars, and the constitution 

 of the stellar universe. ISTo one can look at the heavens, and see such 

 clusters as the Pleiades, Hyades and Coma Berenices, without being 

 convinced that the distribution is not due to chance. This view is 

 strengthened by the clusters and doubles seen in even a small telescope. 

 We also see at once that the stars must be of different sizes, and that 

 the faint stars are not necessarily the most distant. If the number 

 of stars was infinite, and distributed according to the laws of chance 

 throughout infinite and empty space, the background of the sky would 

 be as bright as the surface of the sun. This is far from being the case. 

 While we can thus draw general conclusions, but little definite infor- 

 mation can be obtained, without accurate quantitative measures, and 

 this is one of the greatest objects of stellar photometry. If we con- 

 sider two spheres, with the sun as the common center, and one having 

 ten times the radius of the other, the volume of the first will be one 

 thousand times as great as that of the second. It will, therefore, con- 

 tain a thousand times as many stars. But the most distant stars in the 

 first sphere would be ten times as far off as those in the second sphere, 

 and accordingly if equally bright would appear to have only one hun- 

 dredth part of the apparent brightness. Expressed in stellar magni- 

 tudes, they would be five magnitudes fainter. In reality, the total 

 number of stars of the fifth magnitude and brighter is about 1,500; 



