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 510 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 TrES-3b in Hercules depicted below is one of the latest
transitting exoplanets, having being discovered in early 2007 and which represents the third discovery of the Trans-Atlantic
Exoplanet Survey team. It is characterized with a mass nearly twice that of Jupiter and a very brief orbital period of
only 31 hours and which is believed to be decaying. TrES-3b requires 77.4 minutes to transit its parent star at a depth
of 2.98%. The parent star, GSC 3089:929, is estimated to have a mass of 0.90 solar masses, a radius equivalent to 0.80 solar
radii and a temperature of 5,720° K. Although the light curve is generally "V-shaped", indicative of a grazing transit, it
is believed that TrES3-b does not graze its parent star. Further details regarding TrES-3 and TrES-3b are available in the
paper published by the discovery team led by Francis Donovan et al (click
here).
Note: The C- and K-stars used for the purposes of the differential photometry measurements
depicted below were GSC 2620:526 (mag 12.0) and GSC 3089:885 (mag 11.6) respectively.
Parent Star: TrES-3 GSC/SAO Catalog: GSC 3089:929 Constellation: Hercules RA / Dec: 17h 52m 07s / +37° 32' 46" Magnitude: 12.402 Distance: 1300 light-years Exoplanet: TrES-3b Period: 1.30619 + 0.00005 d Transit Duration: 77.4 mins Transit Depth: 29.8 mmag Minimum Mass: 1.92 MJup Radius: 1.295 RJup Pred Transit Details:
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Date: May 20-21, 2011 22:45:00 - 02:05:41 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 |