STUDY OF THE VARIABLE STARS 175 



THE STUDY OF THE VARIABLE STARS 



By Professor SOLON I. BAILEY, 



HARVARD COLLEGE OBSERVATORY 



fT^HE ancient philosophers taught that the celestial bodies were ' in- 

 -*- corruptible and eternal,' not subject to change, as are all ter- 

 restrial objects. In more recent times the stars "were regarded merely 

 as convenient points of reference for the determination of the motions 

 of the planets. In this way they became known as the fixed stais. 

 Relatively, they are indeed fixed; absolutely, all are in motion. Their 

 light remains constant, also, for the most part, so that, if Hipparchus 

 or Ptolemy should come back to earth after 2,000 years, he would 

 probably notice few changes in the positions or brightness of the stars. 



Any one who observes the sky carefully, through a period of years, 

 is sure to be deeply impressed with the absence of change. Neverthe- 

 less, there are many stars which undergo more or less regular changes 

 in brightness, and such objects are known as variable stars. In some 

 cases the whole cycle of change takes place within a few hours, while in 

 other cases it consumes months, or even years. The amount of the 

 variation, also, varies enormously, ranging all the way from zero to 

 many magnitudes, how many is not known. It is possible, even prob- 

 able, that at minimum the light of some variable may, for us at least, 

 be entirely extinguished. Mr. J. A. Parkhurst found that the variable 

 V Delphini was invisible at its minimum of 1,900 in the forty-inch 

 refractor of the Yerkes Observatory. This, it is estimated, would 

 make it fainter than the seventeenth magnitude. Since its light at 

 maximum is of about the seventh magnitude, this implies a range of at 

 least ten magnitudes. Other stars vary as much or more. A change 

 of ten magnitudes means that at maximum its light is 10,000 times 

 as great as that at minimum. To illustrate this we may imagine a 

 room illuminated by 1,000 ten-candle power electric lamps, and that 

 these are replaced by the light of a single candle. To reduce the light 

 of our sun by ten magnitudes would be equivalent to increasing its 

 distance 100 times, or to more than 9,000,000,000 miles. At such a 

 distance its apparent size would be less than the present mean size of 

 Jupiter or Venus. Fortunately our sun, if a variable star as seems 

 probable, has a small range of variation. 



The general problem of variable stars may be divided into three 

 parts — the discovery of the variables, the observation of all the phe- 

 nomena involved, and the search for the causes. The present genera- 



