Astrophotography by Anthony Ayiomamitis

Differential Photometry - WASP-12 in Auriga

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-12b in Auriga depicted below is one of the latest (and largest) transitting exoplanets, having being announced in Dec/2008, and represents the twelventh discovery by the WASP (Wide Angle Search for Planets) team. WASP-12b is characterized with a mass 1.41 times that of Jupiter while its radius is equivalent to 1.79 Jupiter radii, thus making this exoplanet the largest discovery at the time. What really sets WASP-12b apart is the fact that models suggest a surface temperature of 2516 Kelvin and which makes it the hottest exoplanet discovery so far owing to a very brief orbital period of 1.09 days (the shortest orbital period yet detected). WASP-12b requires 175.7 minutes to transit its parent star at a depth of 15.0 mmag or 1.50%. The parent star, 2MASS J063032.79+294020.4, is an F9V star estimated to have a mass of 1.35 solar masses, a radius equivalent to 1.57 solar radii, a temperature of 6,300 K and to lie at a distance of 871 light-years away with a visual magnitude of 11.69. Further details regarding WASP-12 and WASP-12b are available in the paper published by the discovery team led by Hebb et al here.

Note: Due to the proximity of WASP-12b to its host star, namely 2% that of earth's distance from the Sun, the life expectancy of WASP-12b is approximately 10 million years owing to the fact WASP-12 continually strips away WASP-12b's atmosphere and the host star will eventually devour the exoplanet.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 1891:324 (mag 11.3) and GSC 1891:876 (mag 11.7) respectively.

Image Details
Light Curve for Exoplanet WASP-12b
Imaging Details
Parent Star:
2MASS J063032.79+294020.4

GSC/SAO Catalog:


RA / Dec:
06h 30m 32.79s /
+29 40' 20.4"


871 light-years

1.091423 + 0.000003 d

Transit Duration:
175.7 mins

Transit Depth:
15.0 mmag

Minimum Mass:
1.41 MJup

1.79 RJup

Pred Transit Details:
Ingress :  19:50 UT
Mid-trans :  21:18 UT
Egress :  22:46 UT
Jan 14-15, 2011
21:00:00 - 01:46:50 UT+2

Athens, Greece

AP 160 f/7.5 Starfire EDF
SBIG Lum filter

Lum :  180 x 90 sec
Dark :  015 x 90 sec
Flat :  ~24,800 ADU
Binning :  2x2

Ambient : + 15.0 C
CCD Chip : - 25.0 C

CCDSoft V5.00.201
AIP4Win V2.2

Differential Photometry