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 700 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 Qatar-1b in Draco depicted below is one of the latest transiting exoplanets,
having being announced in Dec/2010, and represents the first discovery by the Alsubai Project exoplanet transit survey. Qatar-1b is
characterized with a mass 1.09 times that of Jupiter while its radius is equivalent to 1.164 Jupiter radii, thus making this exoplanet a very
typical hot Jupiter. Qatar-1b requires 96.7 minutes (1.61 hrs) to transit its parent star at a depth of 20.4 mmag (2.04%). The parent star,
GSC 4240:470 (3UC311-087990), is a K dwarf-star estimated to have a mass of 0.85 solar masses, a radius equivalent to 0.823 solar radii, a
temperature of 4,861° K and with a visual magnitude of 12.843. Further details regarding
Qatar-1 and Qatar-1b are available in the paper published by the discovery team led by K. A. Alsubai et al (click
here).
Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were
GSC 4240:236 (mag 10.95) and GSC 4240:905 (mag 11.5) respectively.
Parent Star: Qatar-1 GSC/SAO Catalog: GSC 4240:470 Constellation: Draco RA / Dec: 20h 13m 31.61s / +65° 09' 43.4" Magnitude: 12.843 Distance: N/A Exoplanet: Qatar-1b Period: 1.420033 + 0.000016 d Transit Duration: 96.7 mins Transit Depth: 20.4 mmag Minimum Mass: 1.09 MJup Radius: 1.164 RJup Pred Transit Details:
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Date: Jul 04-05, 2012 22:51:00 - 02:30:46 UT+3 Location: Athens, Greece Equipment: AP 305/f3.8 Riccardi-Honders AP 1200GTO GEM SBIG ST-10XME SBIG CFW-10 SBIG LRGB filters Integrations:
Temperatures:
Software: CCDSoft V5.00.201 AIP4Win V2.2 Processing: Reduction Differential Photometry |