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 exoplanet HD 209458b in Pegasus depicted below is one of the earlier transitting
exoplanets, having being discovered in the late 1990's by the radial velocity method and later via transit (in fact, it is the first exoplanet
transit discovery), and whose atmosphere contains water vapour. It is characterized with a density slightly less than that of Jupiter owing
to a mass equivalent to 0.66 Jovian masses and a radius equivalent to 1.32 Jovian radii. HD 209458b has an orbital period of just under 85
hours and requires 184 minutes to transit its parent star at a depth of 1.65%. The parent star, HD 209458, is estimated to have a mass of 1.1
solar masses, a radius equivalent to 1.2 solar radii and a temperature of 6,000° K. Further details regarding HD 209458 and HD 209458b are
available in the paper published by the transit discovery team led by David Charbonneau et al (click
here).
Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were
GSC 1688:1982 (mag 8.32) and GSC 1688:1820 (mag 9.82) respectively. Due to extremely poor seeing, this particular
exoplanet will be revisited at the first available opportunity for better results.
Parent Star: HD 209458 GSC/SAO Catalog: SAO 107623 Constellation: Pegasus RA / Dec: 22h 03m 10.8s / +18° 53' 04" Magnitude: 7.65 Distance: 154 light-years Exoplanet: HD 209459b Period: 3.52474541 + 0.00000025 d Transit Duration: 184.2 mins Transit Depth: 16.5 mmag Minimum Mass: 0.657 MJup Radius: 1.320 RJup Pred Transit Details:
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Date: Sept 02, 2008 21:30 - 02:00 UT+3 Location: Athens, Greece Equipment: AP 160 f/7.5 Starfire EDF AP 1200GTO GEM SBIG ST-2000XM SBIG CFW-10 SBIG LRGB + IR-block Integrations:
Temperatures:
Software: CCDSoft V5.00.188 AIP4Win V2.2 Processing: Reduction Differential Photometry |