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 (511 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 WASP-48b in Cygnus depicted below is one of the latest
transiting exoplanets, having being announced in Apr/2011, along with WASP-35b and WASP-51b, and represents
the fourty-eighth discovery by the WASP (Wide Angle Search for Planets) team. WASP-48b is characterized with a mass 0.98
times that of Jupiter while its radius is equivalent to 1.67 Jupiter radii, thus making this exoplanet one of the larger
and least-dense discoveries to-date. WASP-48b requires 191.08 minutes to transit its parent star at a depth of 10.8 mmag
or 1.08%. The parent star, GSC 03925-00739, is an evolved star estimated to have a mass of 1.09 solar masses, a radius
equivalent to 1.09 solar radii, a temperature of 5,920° K and a visual
magnitude of 11.66. Further details regarding WASP-48 and WASP-48b are available in the paper published by the discovery
team led by Enoch et al here.
Note: The C- and K-stars used for the purposes of the differential photometry measurements
depicted below were GSC 3925:786 (mag 11.3) and GSC 3925:077 (mag 11.3) respectively.
Parent Star: WASP-48 GSC/SAO Catalog: GSC 03925-00739 Constellation: Cygnus RA / Dec: 14h 24m 38.97s / +55° 28' 23.8" Magnitude: 11.66 Distance: N/A Exoplanet: WASP-48b Period: 2.143634 + 0.000003 d Transit Duration: 191.08 mins Transit Depth: 10.8 mmag Minimum Mass: 0.98 MJup Radius: 1.67 RJup Pred Transit Details:
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Date: July 07-08, 2011 22:45:00 - 03:14:56 UT+3 Location: Athens, Greece Equipment: AP 305/f3.8 Riccardi-Honders AP 1200GTO GEM SBIG ST-10XME SBIG CFW-8A SBIG LRGB filters Integrations:
Temperatures:
Software: CCDSoft V5.00.201 AIP4Win V2.2 Processing: Reduction Differential Photometry |