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 370 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-8b in Pegasus depicted below is one of the latest transitting
exoplanets, having being announced in Dec/2008, and represents the eighth discovery by the Hungarian-based HATNet Project team. HAT-P-8b is
characterized with a mass 1.52 times that of Jupiter while its radius is equivalent to 1.50 Jupiter radii, thus making this exoplanet
one of the largest and most inflated "hot Jupiter" finds to-date. HAT-P-8b requires 216 minutes to transit its parent star at a depth of 7 mmag
or 0.7%. The parent star, GSC 2757:1152, is a class F dwarf estimated to have a mass of 1.28 solar masses, a radius equivalent to 1.58 solar
radii, a temperature of 6,200° K and to lie at a distance of 750 light-years away with a visual magnitude of 10.17. Further details regarding
HAT-P-8 and HAT-P-8b are available in the paper published by the discovery team led by David W. Latham et al (click
here).
Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were
GSC 2757:416 (mag 10.23) and GSC 2757:2126 (mag 10.40) respectively.
Parent Star: HAT-P-8 GSC/SAO Catalog: GSC 2757:1152 Constellation: Pegasus RA / Dec: 22h 52m 09.85s / +35° 26' 49.5" Magnitude: 10.17 Distance: 750 light-years Exoplanet: HAT-P-8b Period: 3.07632 + 0.000004 d Transit Duration: 216 mins Transit Depth: 7 mmag Minimum Mass: 1.52 MJup Radius: 1.50 RJup Pred Transit Details:
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Date: Aug 27-28, 2009 22:00:00 - 03:33:20 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 CRGB Integrations:
Temperatures:
Software: CCDSoft V5.00.188 AIP4Win V2.3 Processing: Reduction Differential Photometry |