Spectral Criteria in the Classification of Variable Stars

In a general study of variable stars the first problem is to formulate some method by which the stars may be divided into classes according to behavior or physical characteristics. The usual criteria are the form of the light-curve and the time elapsing between recurrences of similar phases. Variable-star observers have become exceedingly skillful in estimating the apparent brightness of variables relative to that of comparison stars of a graduated sequence. Both visual estimates and direct photography have been successfully used. For bright stars the photoelectric cell is practicable and gives the highest accuracy thus far attained in photometric observation. A plot of magnitude against time determines the light-curve and period, provided the observations are sufficient in number and well distributed.

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