About two hundred and fifty stars have been formally registered as variable, and many more are open to the like suspicion. Gore's ‘Revised Catalogue’ includes 243 entries, besides 39 provisional additions; Chandler's nearly contemporaneous list enumerates 225 objects. Of these 160 are reckoned as ‘periodical,’ the rest as ‘irregular’ or ‘temporary.’ Periodical stars are further divided into those with ‘long,’ and those with ‘short’ periods. Nor is the distinction by any means arbitrary. The stars seem to separate of themselves into two principal groups, undergoing fluctuations in cycles of respectively less than fifty, and between two and four hundred days. The paucity of stars with periods of intermediate lengths is shown graphically in fig. 11, where the height of the curve represents the numbers of stars subject to changes proportionate in duration to the horizontal distance from left to right.

Variations requiring several months for their completion differ both in degree and kind from those run through in a few days. They are of much greater amplitude, ranging over five to eight instead of, at the most, two magnitudes; they are accomplished with less punctuality; and they are frequently attended by symptoms of atmospheric ignition entirely foreign to quicker vicissitudes. Most important of all, they affect bodies of peculiar constitution. Nearly all long-period variables are red stars with banded spectra; those of short period are white or yellowish in colour, and display Sirian or solar spectra.