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 425 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-4b in Bootes depicted below is one of the later
transitting exoplanets, having being discovered late 2007, and represents the fourth discovery by the Hungarian-based
HATNet Project team. HAT-P-4b is characterized with a mass 68% times that of Jupiter and in spite of a larger radius
(1.27 RJup), thus making this find a low-density hot Jupiter. HAT-P-4b requires 254 minutes to transit its
parent star at a depth of 8 mmag (0.80)%.
The parent star, BD+36°2593, is estimated to have a mass of 1.26 solar masses, a radius equivalent to 1.59 solar radii,
a temperature of 5,860° K and to lie at a distance of 1011 light-years away with a visual magnitude of 11.21. Further
details regarding HAT-P-4 and HAT-P-4b are available in the paper published by the discovery team led by G. Kovacs et
al here.
Note: The C- and K-stars used for the purposes of the differential photometry measurements
depicted below were GSC 2569:1310 (mag 11.3) and GSC 2569:1092 (mag 11.5) respectively.
Parent Star: HAT-P-4 GSC/SAO Catalog: BD+36°2593 Constellation: Bootes RA / Dec: 15h 19m 58s / +36° 14' 37" Magnitude: 11.21 Distance: 1011 light-years Exoplanet: HAT-P-4b Period: 3.056536 + 0.000057 d Transit Duration: 254 mins Transit Depth: 8 mmag Minimum Mass: 0.68 MJup Radius: 1.27 RJup Pred Transit Details:
|
|
Date: Apr 22-23, 2010 23:30:01 - 05:15:34 UT+3 Location: Athens, Greece Equipment: AP 160 f/7.5 Starfire EDF AP 1200GTO GEM SBIG ST-10XME SBIG CFW10 SBIG Lum filter Integrations:
Temperatures:
Software: CCDSoft V5.00.195 AIP4Win V2.2 Processing: Reduction Differential Photometry |