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 HAT-P-16b in Andromeda depicted below is one of the
latest transitting exoplanets, having being announced July/2010, and represents the sixteenth discovery by the
Hungarian-based HATNet Project team. HAT-P-16b is characterized with a whopping mass 4.193 times that of Jupiter and
in spite of a larger radius (1.289 RJup), thus making this find a high-density hot Jupiter. HAT-P-16b
requires 184 minutes to transit its parent star at a depth of 10.1 millimag (1.01%) while orbiting its host star
every 66.62 hours in an eccentric orbit.
The parent star, GSC 2792:1700, is an F8 dwarf estimated to have a mass of 1.22 solar masses, a radius equivalent to
1.24 solar radii, a temperature of 6,158° K and to lie at a distance of 766 light-years away with a visual magnitude
of 10.77. Further details regarding HAT-P-16 and HAT-P-16b are available in the paper published by the discovery team
led by Lars Buchhave et al here.
Note: The C- and K-stars used for the purposes of the differential photometry measurements
depicted below were GSC 2792:1737 (mag 10.5) and GSC 2792:1220 (mag 10.7) respectively.
Parent Star: HAT-P-16 GSC/SAO Catalog: GSC 2792:1700 Constellation: Andromeda RA / Dec: 00h 38m 18s / +42° 27' 48" Magnitude: 10.88 Distance: 766 light-years Exoplanet: HAT-P-16b Period: 2.775960 + 0.000003 d Transit Duration: 184 mins Transit Depth: 10.1 mmag Minimum Mass: 4.193 MJup Radius: 1.289 RJup Pred Transit Details:
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Date: Nov 07-08, 2013 19:35:00 - 00:40:24 UT+2 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 Processing: Reduction Differential Photometry |