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 Qatar-2b in Virgo depicted below is one of the latest transiting exoplanets, having being announced in Oct/2011, and represents the second discovery by the Alsubai Project exoplanet transit survey. Qatar-2b is characterized with a mass 2.49 times that of Jupiter while its radius is equivalent to 1.14 Jupiter radii, thus making this exoplanet a dense hot Jupiter. Qatar-2b requires 108.6 minutes (1.81 hrs) to transit its parent star at a depth of 37.4 mmag (3.74%). The parent star, GSC 4974:0112 (3UC167-129863), is a K dwarf-star estimated to have a mass of 0.713 solar masses, a radius equivalent to 0.740 solar radii, a temperature of 4,645° K and a visual magnitude of 13.3. A second orbiting planet (Qatar-2c) is suspected to exist and with a much greater period. Further details regarding Qatar-2 and Qatar-2b are available in the paper published by the discovery team led by Marta Bryan et al here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 4974:0498 (mag 10.99) and GSC 4973:0005 (mag 12.5) respectively.