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 cataclysmic variable star AH Her has a period of 0.258116 days and which involves a delta mag of about 0.25 (V) during this brief time interval on the basis of the light curve below but can vary between magnitude 10.8 at maximum to about magnitude 14.7 at minimum. AH Her is characterized as a Z Cam-type variable due to its cyclical outbursts which cycle every 19.8 days. AH Her is the focus of a recent observation campaign by AAVSO in anticipation of a forthcoming HST study due to recent anomalous behaviour. Perhaps the first reference in the academic literature related to AH Her is the study by Elvey and Babcock in 1943 where two spectra were obtained (see Robinson, 1973). Further details from the International Variable Star Index are available here whereas an AAVSO finder chart is available here. For the entry involving AH Her in the The Catalog and Atlas of Cataclysmic Variables (Ronald Downes et al, 2006), click here.