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 light curve for XO-2b in Lynx depicted below is one of the latest transitting exoplanets, having being announced mid-2007, and is particularly unique since it represents a clear collaboration between professional and amateur astronomers where the research team led by Peter McCullough (Space Telescope Science Institute, USA) identifies a number of potential candidates using widefield imaging and amateurs are asked to pursue the photometry thereafter of promising candidates. XO-2b is characterized with a mass and radius 0.57 and 0.973 times that of Jupiter, respectively, and an orbital period of only 62.8 hours. XO-2b requires 162 minutes to transit its parent star at a depth of 12.0 mmag (1.20%). The parent star, GSC 3413:0005, is an K0V dwarf star estimated to have a mass of 0.98 solar masses, a radius equivalent to 0.97 solar radii and a temperature of 5,340° K. As is the case with HAT-P-1, XO-2 has a close binary twin (GSC 3413:210) with a similar spectrum and magnitude which lies only 31" away. Further details regarding XO-2 and XO-2b are available in the paper published by the discovery team led by Christopher Burke et al (click here).

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 3413:104 (11.2) and GSC 3413:11 (mag 11.2), respectively. Due to extremely volatile seeing during the session, approximately sixteeen integrations involved a single saturated pixel for XO-2 near the predicted mid-transit point and which adversely affected the result below.