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 light curve for exoplanet TrES-4b in Hercules depicted below is one of the latest transitting exoplanets, having being discovered in mid 2007, and which represents the fourth discovery of the Trans-Atlantic Exoplanet Survey team. It is characterized with a mass and radius of 84% and 167.4% that of Jupiter, respectively, thus making it one of the least dense exoplanets discovered so far and whose density is similar to cork! TrES-4b has an orbital period of only 85 hours and requires 216.0 minutes to transit its parent star at a depth of 10.5 millimag (or 1.05%). The parent star, GSC 2620:0648, is estimated to have a mass of 1.22 solar masses, a radius equivalent to 1.738 solar radii and a temperature of 6,100° K. Although the light curve is generally "V-shaped", indicative of a grazing transit, it is believed that TrES4-b does not graze its parent star. Further details regarding TrES-4 and TrES-4b are available in the paper published by the discovery team led by Georgi Mandushev et al here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 2620:0800 (mag 10.51) and GSC 2620:0483 (mag 11.07) respectively.