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 710 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-25b in Aries depicted below is one of the latest transiting exoplanets, having being announced in Aug/2010, and represents the twenty-fifth discovery by the Hungarian-based HATNet Project team. HAT-P-25b is characterized with a mass 0.567 times that of Jupiter while its radius is equivalent to 1.19 Jupiter radii, thus making this exoplanet one of the lesser density "hot Jupiter" finds to date. HAT-P-25b requires 169.1 minutes to transit its parent star at a depth of 20.4 millimag or 2.04%. It has an eccentric orbit which is statistically insignificant with a period of 3.652836 days and represents a growing sample of gas giants whose mass is between Saturn and Jupiter. The parent star, GSC 1788:1237, is a G5 dwarf estimated to have a mass of 1.01 solar masses, a radius equivalent to 0.96 solar radii, a temperature of 5,500° K and to lie at a distance of 969 light-years away with a visual magnitude of 13.19.

Further details regarding HAT-P-25 and HAT-P-25b are available in the paper published by the discovery team led by Quinn et al here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 1788:1192 (mag 12.6) and GSC 1788:971 (mag 13.0) respectively.