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 570 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 PAWM (Pro-Am White Dwarf Monitor) project's purpose is to monitor a number of white dwarfs for possible exoplanet transits. Although no such example has been found to-date, white dwarfs hosting exoplanets are not only quite possible but offer dramatic transit opportunities where a transiting exoplanet would cause a significant drop in the magnitude of the host white dwarf and, quite possibly, eclipse it totally due to the small size of white dwarfs and the proximity of any orbiting exoplanet and, particularly, a super-earth in the habitable zone. Such transits would be expected to last one to two minutes and could conceivably involve a change of one full magnitude (or more) of the host star. Such a sizeable delta mag (and transit depth) also allows for the study of very dim white dwarf candidates (mag 16+), thus significantly expanding the available sample of candidate white dwarfs since such stars are very dim instrinsically.

Such a monitoring project is beyond the reach of professional observatories owing to the cost of telescope time but is ideally suited for advanced amateurs where both time and suitable large-aperture equipment are readily available. For further details in relation to the PAWM project, click here and/or here. For an excellent conceptual article on terrestrial earth-sized planets orbiting white dwarfs, click here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 4615:2261 (mag 12.9) and GSC 4651:2577 (mag 12.9), respectively.