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 700 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 DY Herculis is a fast pulsating variable star with a period of 214.03 minutes and which involves a delta mag of 0.51 (v) magnitudes during this brief time interval. DY Her is a delta Scuti-type variable star discovered by A. V. Solov'ev in 1950 with consequent studies in the mid 1950's and thereafter involving both photometry and spectroscopy. Longterm studies involving DY Her have shown a continuous decline in its period (see Szeidl et al). Further details from the International Variable Star Index are available here whereas an AAVSO finder chart is available here.