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 400 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-24b in Virgo depicted below is the latest exoplanet
discovered, having being announced on April 14/2010 along with another four new discoveries, and represents the
twenty-fourth discovery by the WASP (Wide Angle Search for Planets) team. WASP-24b is characterized with a mass
1.032 times that of Jupiter while its radius is equivalent to 1.104 Jupiter radii, thus making this exoplanet a very
typical "hot Jupiter". WASP-24b requires 155.5 minutes to transit its parent star at a depth of 12 mmag (1.2%).
The parent star, GSC 339:329-1, is an F8-9 star estimated to have a mass of 1.129 solar masses, a radius equivalent
to 1.147 solar radii, a temperature of 6,075° K and to lie at a distance of 1080 light-years away with a visual magnitude
of 11.3. Further details regarding WASP-24 and WASP-24b are available in the paper published by the discovery team
led by Rachel Street et al here.
Note: The C- and K-stars used for the purposes of the differential photometry measurements
depicted below were GSC 339:432 (mag 10.47) and GSC 339:520 (mag 10.65) respectively. The light curve below is the
first amateur result of this newly discovered exoplanet in the constellation of Virgo.
Parent Star: USNO-B1.0 0923-0348089 GSC/SAO Catalog: GSC 339:329-1 Constellation: Virgo RA / Dec: 15h 08m 51.72s / +02° 20' 36.1" Magnitude: 11.3 Distance: 1080 light-years Exoplanet: WASP-24b Period: 2.3412083 + 0.0000049 d Transit Duration: 155.5 mins Transit Depth: 12 mmag Minimum Mass: 1.032 MJup Radius: 1.104 RJup Pred Transit Details:
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Date: Apr 22, 2010 00:00:01 - 04:30:58 UT+3 Location: Athens, Greece Equipment: AP 160 f/7.5 Starfire EDF AP 1200GTO GEM SBIG ST-10XME SBIG CFW-10 SBIG Lum filter Integrations:
Temperatures:
Software: CCDSoft V5.00.195 AIP4Win V2.2 Processing: Reduction Differential Photometry |