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 860 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-22b in Ursa Major depicted below is one of the latest transiting exoplanets, having being announced in Aug/2010 along with three other exoplanet finds, and represents the twenty-second discovery by the Hungarian-based HATNet Project team. HAT-P-22b is characterized with a sizeable mass 2.147 times that of Jupiter while its radius is equivalent to 1.080 Jupiter radii, thus making this exoplanet a somewhat massive "hot Jupiter". HAT-P-22b requires 172.22 minutes to transit its parent star at a depth of 9.7 millimag or 0.97%. It has a circular orbit with a period of 3.212220 days. The parent star, GSC 03441-00925, a G5 dwarf with an M-dwarf companion, is estimated to have a mass of 0.92 solar masses, a radius equivalent to 1.04 solar radii, a temperature of 5,302° K and to lie at a distance of 267 light-years away with a visual magnitude of 9.732. Further details regarding HAT-P-22 and HAT-P-22b are available in the paper published by the discovery team led by Gaspar Bakos et al here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 3441:0569 (mag 11.30) and GSC 3441:1113 (mag 11.30) respectively.