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 925 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-5b in Cygnus depicted below is one of the latest transiting exoplanets, having being announced in mid-2011, and which represents the fifth discovery of the Trans-Atlantic Exoplanet Survey team. TrES-5b is characterized with a mass and radius of 1.778 and 1.209 that of Jupiter, respectively, thus making TrES-5b one of the denser exoplanets discovered so far. TrES-5b has an orbital period of only 1.4822446 days (35.6 hours) and requires 111.3 minutes to transit its parent star at a depth of 21.5 millimag (or 2.15%) in a non-eccentric orbit. The parent star, GSC 3949:0967, is a cooler G-class dwarf estimated to have a mass of 0.893 solar masses, a radius equivalent to 0.866 solar radii, a temperature of 5,171° K and a visual magnitude of 13.718 while lying 1,174 light-years away. TrES-5 is one of the dimmest exoplanet host stars discovered so far with a transiting exoplanet. Further details regarding TrES-5 and TrES-5b 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 3949:0689 (mag 12.5) and GSC 3949:1337 (mag 13.0) respectively.