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 510 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 HAT-P-18b in Hercules depicted below is one of the latest transiting exoplanet discoveries, having being announced late July/2010, and represents the eighteenth discovery by the Hungarian-based HATNet Project team. HAT-P-18b is characterized with a low mass of only 0.197 times that of Jupiter and in spite of a larger radius (0.995 R_Jup), thus making this find a low-density Saturn-mass exoplanet with a negligible core mass. HAT-P-18b requires 163 minutes to transit its parent star at a depth of 18.3 mmag (1.83%). The parent star, GSC 2594:646, is a K2 dwarf estimated to have a mass of 0.77 solar masses, a radius equivalent to 0.75 solar radii, a temperature of 4,803° K and to lie at a distance of 541 light-years away with a visual magnitude of 12.759. Further details regarding HAT-P-18 and HAT-P-18b are available in the paper published by the discovery team led by Hartman et al here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 2595:392 (mag 11.7) and GSC 2594:102 (mag 12.5) respectively.