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 275 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 discovery of the exoplanet around HD189733 in the constellation of Vulpecula by the Geneva Extrasolar Planet Search Programme in 2005 is perhaps the most exciting discovery to-date, for the parent star is very bright and easily within reach of amateur telescopes (see here). Exoplanet HD189733b is characterized with a mass and radius slightly greater than that of Jupiter (15% and 26% respectively) and a brief orbital period of only 53 hours. HD189733b requires 109.6 minutes to transit its parent star at a depth of 2.80%. The parent star, HD189733, is estimated to have a mass of 0.82 solar masses, a radius equivalent to 0.76 solar radii and a temperature of 5,050° K. For complete details as reported by the discovery team led by Francois Bouchy (Marseille Astrophysics Laboratory, France), click here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were SAO 88067 (mag 8.09) and SAO 88041 (mag 8.94), respectively.