Astrophotography by Anthony Ayiomamitis

Differential Photometry - HAT-P-7 in Cygnus

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 (275 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-7b in Cygnus depicted below is one of the latest transitting exoplanets, having being announced in the spring of 2008, and represents the seventh discovery by the Hungarian-based HATNet Project team. HAT-P-7b is characterized with a mass and radius 1.78 and 1.36 times that of Jupiter, respectively, thus making this find one of the largest and hottest Jupiters discovered so far owing to its high mass and proximity to its hot parent star. HAT-P-7b requires 243 minutes to transit its parent star at a depth of only 0.62% with ingress and egress requiring approximately 21-22 minutes. The parent star, GSC 3547:1402, is estimated to have a mass of 1.47 solar masses, a radius equivalent to 1.84 solar radii, a temperature of 6,350 K and to lie at a distance of 1043 light-years away with a visual magnitude of 10.46. Since exoplanet HAT-P-7b lies in the Kepler zone, a region of the sky which will be the focus of the upcoming NASA Kepler mission, it is anticipated that many primary (and secondary) transits of HAT-P-7b will be studied! Further details regarding HAT-P-7 and HAT-P-7b are available in the paper published by the discovery team led by Andras Pal et al (click here).

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 3547:1807 (mag 10.55) and GSC 3547:1007 (mag 10.52) respectively.

Image Details
Light Curve for Exoplanet HAT-P-7b
Imaging Details
Parent Star:

GSC/SAO Catalog:
GSC 3547:1402


RA / Dec:
19h 28m 59s /
+47 58' 10"


1043 light-years

2.2047299 + 0.000004 d

Transit Duration:
242.6 mins

Transit Depth:
6.2 mmag

Minimum Mass:
1.776 MJup

1.363 RJup

Pred Transit Details:
Ingress :  20:32 UT
Mid-trans :  22:37 UT
Egress :  00:42 UT
June 03-04, 2008
23:15:00 - 04:15:55 UT+3

Athens, Greece

AP 160 f/7.5 Starfire EDF
SBIG LRGB + IR-block

Green :  413 x 35 sec
Dark :  020 x 35 sec
Flat :  ~17,500 ADU
Binning :  1x1

Ambient : + 18.0 C
CCD Chip : - 12.5 C

CCDSoft V5.00.188
AIP4Win V2.2

Differential Photometry