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 370 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-13b in Ursa Major depicted below is one of the latest transitting exoplanets, having being announced in Oct/2009, and represents the thirteenth discovery by the Hungarian-based HATNet Project team. HAT-P-13b is characterized with a mass 0.853 times that of Jupiter while its radius is equivalent to 1.281 Jupiter radii, thus making this exoplanet one of the largest and most inflated "hot Jupiter" finds to-date. HAT-P-13b requires 194 minutes to transit its parent star at a depth of 6.2 mmag or 0.62%. The parent star, GSC 3416:543, is a class G4 dwarf estimated to have a mass of 1.22 solar masses, a radius equivalent to 1.56 solar radii, a temperature of 5,653° K and to lie at a distance of 698 light-years away with a visual magnitude of 10.622.

HAT-P-13 is a very interesting find, for it has a second (high-mass) exoplanet (HAT-P-13c) orbitting the parent star. This double-planet system is the object of intense scrutiny during April-June, 2010, for attempts will be made to possibly confirm whether the outer planet transits its parent star as well as to constrain orbit parameters related to its inclination relative to its star as well as the inner companion (HAT-P-13b) for its highly eccentric orbit. Further details regarding HAT-P-13, HAT-P-13b and HAT-P-13c 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 3416:1608 (mag 10.63) and GSC 3416:171 (mag 11.1) respectively.