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 (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 HAT-P-7b in Cygnus depicted below is one of the latest
transitting exoplanets, having being announced in the spring of 2008, and represents the seventh discovery by the
Hungarian-based HATNet Project team. HAT-P-7b is characterized with a mass and radius 1.78 and 1.36 times that of Jupiter,
respectively, thus making this find one of the largest and hottest Jupiters discovered so far owing to its high mass and
proximity to its hot parent star. HAT-P-7b requires 243 minutes to transit its parent star at a depth of only 0.62% with
ingress and egress requiring approximately 21-22 minutes. The parent star, GSC 3547:1402, is estimated to have a mass of
1.47 solar masses, a radius equivalent to 1.84 solar radii, a temperature of 6,350° K and to lie at a distance of 1043
light-years away with a visual magnitude of 10.46. Since exoplanet HAT-P-7b lies in the Kepler zone, a region of the sky
which will be the focus of the upcoming NASA Kepler mission, it is
anticipated that many primary (and secondary) transits of HAT-P-7b will be studied! Further details regarding HAT-P-7 and
HAT-P-7b are available in the paper published by the discovery team led by Andras Pal et al (click
here).
Note: The C- and K-stars used for the purposes of the differential photometry measurements
depicted below were GSC 3547:1807 (mag 10.55) and GSC 3547:1007 (mag 10.52) respectively.
Parent Star: HAT-P-7 GSC/SAO Catalog: GSC 3547:1402 Constellation: Cygnus RA / Dec: 19h 28m 59s / +47° 58' 10" Magnitude: 10.46 Distance: 1043 light-years Exoplanet: HAT-P-7b Period: 2.2047299 + 0.000004 d Transit Duration: 242.6 mins Transit Depth: 6.2 mmag Minimum Mass: 1.776 MJup Radius: 1.363 RJup Pred Transit Details:
|
|
Date: June 03-04, 2008 23:15:00 - 04:15:55 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 |