A variable star, as its name suggests, is a star whose magnitude varies intrinsically, in contrast to eclipsing binaries whose magnitude varies as a result of one star in the binary system eclipsing the other. True variables are one of five types, namely Mira stars, semiregular stars, cepheids, eruptive variables and, finally, cataclysmic variables. Minimum to maximum magnitude can range from days to many months with some variables displaying irregular periods.

A popular method for the study of variable stars, particularly short-term variables, is by the use of the technique known as "differential photometry". Rather than measure the (variable) magnitude of a variable star on an absolute scale, measurements are made over time relative to one or more non-variable star(s) and these differences are then plotted so as to study and illustrate the relative or differential change in magnitude. Due to the very large number of variables stars, the field of differential photometry represents one of the key fields in astronomy whereby the amateur astronomer can make a meaningful and long-lasting contribution to both science and astronomy.

More recently, the search for extrasolar planets (over 750 discovered so far) has identified yet another interesting application for the practice of differential photometry whereby the minute drops in magnitude of a star hosting an exoplanet are studied. Further details for the interested party are available here.

Note: The large-amplitude short-period pulsating star GSC 03850-00137 is a fast pulsating variable star with a period of 91.16 minutes and which involves a delta mag of 0.66 (CR) magnitudes during this brief time interval. GSC 03850-00137 is a HADS variable discovered by Damerdji et al in 2007 while mining data from the TAROT (Telescope a Action Rapide pour les Objets Transitoires) remote observatory and whose primary purpose is to monitor for gamma-ray bursts. Their initial candidate list of 1175 potential variable stars was reduced to the most promising candidates and published as TSVSC1 (TAROT Suspected Variable Star Catalog, ver. 1). Further details from the International Variable Star Index are available here.