310 POPULAR SCIENCE MONTHLY. 



and no permanent changes have yet been noted. Variable stars, on 

 the contrary, are changing before our eyes; and they repeat their 

 fluctuations continually. They present opportunities for discoveries 

 of the greatest interest in themselves, and of remarkable utility in the 

 study of the problem of stellar evolution. 



It is a conservative statement that in nineteen variable stars out 

 of twenty we have little idea as to the causes of variability. The 

 causes of the variations have been determined in the case of Algol 

 and a few others of that class: large dark companions revolve around 

 these stars, and once in every revolution the companions pass between 

 us and the principal stars, thus preventing a portion of their light from 

 reaching us. In Zeta Geminorum and three or four others of its class 

 the spectroscope has shown that massive dark companions are close to, 

 and rapidly revolving around, the principal stars. These invisible 

 companions produce disturbances in the extensive atmospheres of the 

 stars, and cause the observed variations in brightness; but the nature 

 of the disturbances is still a matter of conjecture. Omicron Ceti 

 and other stars of its class have given no evidence of companions. 

 Brightness variations in them seem to be due to internal causes. Per- 

 haps they have reached the age when solid crusts attempt to form 

 on their surfaces, just as one day a crust struggled to form on the 

 liquid earth. A crust formed one month may be melted or sink to a 

 lower level a few months later. Perhaps there are ' sun-spots ' on these 

 stars, in scale vastly more extensive and in period shorter than those 

 on our sun ; but these suggested explanations may be far from the truth. 



For more than half a century a great many astronomers have de- 

 voted themselves assiduously to making photometric observations of 

 variable stars. There are a dozen observatories, both large and small, 

 which are systematically devoting some of their resources to this 

 work. By common consent of the profession, or by appointment 

 from learned societies, there have for some fifty years been individual 

 astronomers, or committees of astronomers, who systematize results, 

 call attention to the need for observations of certain neglected objects, 

 and in many other ways encourage the photometric study of variable 

 stars. Photometers are inexpensive, the methods are simple, and 

 results have rapidly accumulated. 



Observations of variable stars with slit-spectrographs, on the con- 

 trary, are surprisingly meager and fragmentary. Not a single insti- 

 tution, not a single telescope, not a single observer, is working con- 

 tinuously or even extensively on the subject. Yet the method is a very 

 powerful one: the few isolated studies made on variable stars have 

 led to results of remarkable richness. The subject is one of great diffi- 

 culty. Photographic spectra require much time for accurate meas- 

 urement and reduction. And, finally, powerful and expensive instru- 

 ments are demanded. 



