S Persei is one of the supergiant semiregular pulsating variables of type SRc. These luminous stars, supergiants of the upper HR diagram, undergo a variety of different behaviors all caused by pulsation of one kind or another. Some, like Betelgeuse, vary inconspicuously by no more than a magnitude away from their means; others, like S Per, can sometimes undergo dramatic changes in brightness as shown here, over hundreds or thousands of days. However, all seem to have periods between several hundred and a few thousand days, many have multiple periods present at the same time, and none of them vary regularly over long periods of time. They're not at all regular (as a Mira might be) but neither are they irregulars as the L-types are. This particular span of S Persei's light curve also seems to show a drop in mean magnitude as well.
Prepared By: Matthew Templeton