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 925 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-5b in Cygnus depicted below is one of the latest
transiting exoplanets, having being announced in mid-2011, and which represents the fifth discovery of the Trans-Atlantic
Exoplanet Survey team. TrES-5b is characterized with a mass and radius of 1.778 and 1.209 that of Jupiter, respectively,
thus making TrES-5b one of the denser exoplanets discovered so far. TrES-5b has an orbital period of only 1.4822446 days
(35.6 hours) and requires 111.3 minutes to transit its parent star at a depth of 21.5 millimag (or 2.15%) in a non-eccentric
orbit.
The parent star, GSC 3949:0967, is a cooler G-class dwarf estimated to have a mass of 0.893 solar masses, a radius
equivalent to 0.866 solar radii, a temperature of 5,171° K and a visual magnitude of 13.718 while lying 1,174 light-years
away. TrES-5 is one of the dimmest exoplanet host stars discovered so far with a transiting exoplanet.
Further details regarding TrES-5 and TrES-5b are available in the paper published by the discovery team led by Georgi Mandushev et al
here.
Note: The C- and K-stars used for the purposes of the differential photometry measurements
depicted below were GSC 3949:0689 (mag 12.5) and GSC 3949:1337 (mag 13.0) respectively.
Parent Star: TrES-5 GSC/SAO Catalog: GSC 3949:0967 Constellation: Cygnus RA / Dec: 20h 20m 53s / +59° 26' 55" Magnitude: 13.718 Distance: 1,174 light-years Exoplanet: TrES-5b Period: 1.4822446 + 0.0000007 d Transit Duration: 111.312 mins Transit Depth: 21.5 mmag Minimum Mass: 1.778 MJup Radius: 1.209 RJup Pred Transit Details:
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Date: Aug 06, 2013 00:15:00 - 04:10:26 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 V2.4.0 Processing: Reduction Differential Photometry |