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 CoRoT-1b in Monoceros depicted below is the first discovery of the
French-led CoRoT (Convection Rotation and planetary Transits) satellite mission which was launched in December 2006 for the specific purpose
of studying 12,000 stars per session over 30- and 150-day periods using an on-board 27-cm telescope. Its main advantage being in (polar) earth
orbit is the ability to bypass the adverse effects of earth's atmosphere and the limited windows of opportunity available otherwise (due to
darkness etc). CoRoT-1b, announced in the spring of 2008, is characterized with a mass and radius 1.03 and 1.49 times that of Jupiter,
respectively, thus making this find a low-density but very large hot Jupiter and which defies current irradiated planet theory and modelling
since its radius is much bigger for its mass. CoRoT-1b requires 139 minutes to transit its parent star at a depth of approximately 25 mmag
(2.5%). The parent star, GSC 4804:2268, is of G0V spectral type and is estimated to have a mass of 0.95 solar masses, a radius equivalent to
1.11 solar radii, a temperature of 5,950° K and to lie at a distance of 1560 light-years away with a visual magnitude of 13.6. Further details
regarding CoRoT-1 and CoRoT-1b are available in the paper published by the discovery team led by Barge et al (click
here).
Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were
GSC 4804:1988 (mag 13.7) and GSC 4804:1936 (mag 13.7) respectively. Due to the low magnitude of CoRoT-1 (mag 13.6), the photometry involving
the transit by CoRoT-1b represents a most-challenging exercise for amateur equipment.
Parent Star: CoRoT-1 GSC/SAO Catalog: GSC 4804:2268 Constellation: Monoceros RA / Dec: 06h 48m 19.17s / -03° 06' 08" Magnitude: 13.60 Distance: 1560 light-years Exoplanet: CoRoT-1b Period: 1.5089557 + 0.0000064 d Transit Duration: 139 mins Transit Depth: 25 mmag Minimum Mass: 1.03 MJup Radius: 1.49 RJup Pred Transit Details:
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Date: Feb 04, 2010 19:30:02 - 23:53:51 UT+2 Location: Athens, Greece Equipment: AP 160 f/7.5 Starfire EDF AP 1200GTO GEM SBIG ST-10XME SBIG CFW-10 Baader Lum filter Integrations:
Temperatures:
Software: CCDSoft V5.00.188 AIP4Win V2.2 Processing: Reduction Differential Photometry |