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 345 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-12b in Canis Venatici depicted below is one of the latest transitting
exoplanets, having being announced in Apr/2009, and represents the twelveth discovery by the Hungarian-based HATNet Project team. HAT-P-12b is
characterized with a mass 0.211 times that of Jupiter while its radius is equivalent to 0.959 Jupiter radii, thus making this exoplanet
the least dense gas-dominated exoplanet found to-date and which is more characteristic of a "hot Saturn". HAT-P-12b requires 140 minutes to
transit its parent star at a depth of 25 mmag or 2.5%. The parent star, GSC 3033:706, is an K4 dwarf estimated to have a mass of 0.73 solar
masses, a radius equivalent to 0.70 solar radii, a temperature of 4,650° K and to lie at a distance of 465 light-years away with a visual
magnitude of 12.84. Further details regarding HAT-P-12 and HAT-P-12b are available in the paper published by the discovery team led by J.D.
Hartman et al (click here).
Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were
GSC 3033:765 (mag 12.6) and GSC 3033:875 (mag 12.7) respectively.
Parent Star: HAT-P-12 GSC/SAO Catalog: GSC 3033:706 Constellation: Canis Venatici RA / Dec: 13h 57m 33.47s / +43° 29' 36.8" Magnitude: 12.84 Distance: 465 light-years Exoplanet: HAT-P-12b Period: 3.2130598 + 0.0000021 d Transit Duration: 140 mins Transit Depth: 25 mmag Minimum Mass: 0.211 MJup Radius: 0.959 RJup Pred Transit Details:
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Date: May 13-14, 2009 23:05:02 - 03:10:03 UT+3 Location: Athens, Greece Equipment: AP 160 f/7.5 Starfire EDF AP 1200GTO GEM SBIG ST-10XME SBIG CFW-10 Astrodon Tru-Bal LRGB Integrations:
Temperatures:
Software: CCDSoft V5.00.188 AIP4Win V2.3 Processing: Reduction Differential Photometry |