THE ancient philosophers taught that the celestial bodies were 'incorruptible and eternal,' not subject to change, as are all terrestrial 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 stars. 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. Nevertheless, 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 probable, 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 phenomena involved, and the search for the causes. The present genera-