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 2214:1188 (mag 11.0) and GSC 2214:622 (mag 11.5), respectively.
White Dwarf Star: WD 2148+286 GSC Catalog: N/A Constellation: Pegasus RA / Dec: 21h 51m 11s / +28° 51' 50" Magnitude: 10.5 (v) Distance: N/A Exoplanet: N/A Period: N/A Transit Duration: N/A Transit Depth: N/A Minimum Mass: N/A Radius: N/A Pred Transit Details:
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Date: Sep 03-04, 2011 21:25:00 - 04:43:20 UT+3 Location: Athens, Greece Equipment: AP 305/f3.8 Riccardi-Honders AP 1200GTO GEM SBIG ST-2000XM SBIG CFW-8A SBIG LRGB filters Integrations:
Temperatures:
Software: CCDSoft V5.00.201 AIP4Win V2.2 Processing: Reduction Differential Photometry |