Over 2000 stars are known to be variable, and of these about 1000 are definitely periodic. These periodic variables fall into the two main classes of Cepheid and long-period variables.

It is still uncertain whether Cepheid and long-period variables are essentially different objects or varieties of essentially similar objects. If the latter, the varieties are quite distinct. Long-period variables have periods ranging from about 60 to 500 days, whereas no Cepheid is known whose period exceeds 38.7 days (U Carinae), and most have periods substantially shorter than this. Apart from their different ranges of period, the two classes of variables have many features in common. The light curve of Cepheid variables does not shew a regular symmetrical rise and fall, but rather a fairly rapid rise to maximum brightness followed by a slow decline to minimum, and many long-period variables shew the same features, although generally to a less degree. The Cepheid variables shew a very marked correlation between period and spectral type, shorter periods accompanying the earlier spectral types. Adams and Joy* find a similar correlation in the long-period variables, and this proves to be a direct extension of that already established for Cepheids. In a diagram in which spectral type and period are taken as co-ordinates, they find that a single smooth curve runs through the positions occupied by the long-period variables, the normal Cepheid variables and the cluster variables which form a special short-period group of Cepheids.