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 710 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 WASP-43b in Sextans depicted below is one of the latest transiting exoplanets, having being announced in Apr/2011 and represents the fourty-third discovery by the WASP (Wide Angle Search for Planets) team. WASP-43b is characterized with a mass 1.78 times that of Jupiter while its radius is equivalent to 0.93 Jupiter radii, thus making this exoplanet one of the heavier hot-Jupiter discoveries to-date. WASP-43b requires 69.55 minutes to transit its parent star at a depth of 28.9 mmag or 2.89%. WASP-43b requires a mere 19.44 hours to orbit its host star owing to the smallest orbit distance (0.014 AU) of any hot-Jupiter discovery to-date and will be devoured by the host star in a few million years owing to tidal interaction effects. The parent star, GSC 05490-00141, is an K7V star estimated to have a mass of 0.58 solar masses, a radius equivalent to 0.598 solar radii, a temperature of about 4,400° K and a visual magnitude of 12.40. WASP-43 is the smallest host star to a hot-Jupiter to-date. Further details regarding WASP-43 and WASP-43b are available in the paper published by the discovery team led by Coel Hellier et al here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 5490:153 (mag 10.7) and GSC 5490:134 (mag 12.4) respectively.