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-3b in Ursa Major depicted below is one of the latest (and smallest) transitting exoplanets, having being announced the first quarter of 2008, and represents the third discovery by the Hungarian-based HATNet Project team. HAT-P-3b is characterized with a mass and radius 59.9 and 89.0% that of Jupiter with approximately one-third of its mass being comprised of heavy core elements as suggested by its much greater mass in relation to its radius. HAT-P-3b requires 123.5 minutes to transit its parent star at a depth of 14 mmag (1.4%). The parent star, GSC 3466:819, is a K-class dwarf star (with a faint binary companion) estimated to have a mass of 0.936 solar masses, a radius equivalent to 0.824 solar radii, a temperature of 5,185° K and to lie at a distance of 457 light-years with a visual magnitude of 11.561. Further details regarding HAT-P-3 and HAT-P-3b are available in the paper published by the discovery team led by G. Torres et al (click here).

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 3466:1158 (mag 10.88) and GSC 3466:860 (mag 11.23) respectively.