Astrophotography by Anthony Ayiomamitis

Differential Photometry - HAT-P-14 in Hercules

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 560 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-14b in Hercules depicted below is one of the latest transiting exoplanet, having being announced in Apr/2010, and represents the fourteenth discovery by the Hungarian-based HATNet Project team. HAT-P-14b is characterized with a mass 2.232 times that of Jupiter while its radius is equivalent to 1.150 Jupiter radii, thus making this exoplanet one of the largest and most dense "hot Jupiter" finds to-date. HAT-P-14b requires 131.4 minutes to transit its parent star at a depth of 5.4 millimag or 0.54%. The parent star, GSC 3086:152, is a class F dwarf estimated to have a mass of 1.386 solar masses, a radius equivalent to 1.468 solar radii, a temperature of 6,600 K and to lie at a distance of 670 light-years away with a visual magnitude of 9.98.

HAT-P-14 is a very interesting discovery, for its transit is characterized as near-grazing and which permits for the study of transit duration anomalies suggesting the presence of exomoons and/or the identification of orbit element changes indicative of an additional exoplanet around HAT-P-14. Further details regarding HAT-P-14 and HAT-P-14b are available in the paper published by the discovery team led by Torres et al here.

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 3086:1752 (mag 10.44) and GSC 3086:78 (mag 10.53) respectively. Due to the extremely shallow transit depth for HAT-P-14b, namely 5.4 millimag, the successful capture of the transit light curve is quite a formidable exercise.

Image Details
Light Curve for Exoplanet HAT-P-14b
Imaging Details
Parent Star:

GSC/SAO Catalog:
GSC 3086:152


RA / Dec:
17h 20m 27.88s /
+38 14' 32.4"


670 light-years

4.627669 + 0.000005 d

Transit Duration:
131.4 mins

Transit Depth:
5.4 mmag

Minimum Mass:
2.232 MJup

1.150 RJup

Pred Transit Details:
Ingress :  20:42 UT
Mid-trans :  21:47 UT
Egress :  22:53 UT

17-days (phase 89.4%)

June 18-19, 2011
22:45:00 - 03:00:09 UT+3

Athens, Greece

AP 305/f3.8 Riccardi-Honders
SBIG LRGB filters

Lum :  209 x 60 sec
Dark :  015 x 60 sec
Flat :  ~22,000 ADU
Binning :  1x1

Ambient : + 23.0 C
CCD Chip : - 15.0 C

CCDSoft V5.00.201
AIP4Win V2.4.0

Differential Photometry