Astrophotography by Anthony Ayiomamitis

Differential Photometry - TrES-3 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 (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 TrES-3b in Hercules depicted below is one of the latest transitting exoplanets, having being discovered in early 2007 and which represents the third discovery of the Trans-Atlantic Exoplanet Survey team. It is characterized with a mass nearly twice that of Jupiter and a very brief orbital period of only 31 hours and which is believed to be decaying. TrES-3b requires 77.4 minutes to transit its parent star at a depth of 2.98%. The parent star, GSC 3089:929, is estimated to have a mass of 0.90 solar masses, a radius equivalent to 0.80 solar radii and a temperature of 5,720 K. Although the light curve is generally "V-shaped", indicative of a grazing transit, it is believed that TrES3-b does not graze its parent star. Further details regarding TrES-3 and TrES-3b are available in the paper published by the discovery team led by Francis Donovan et al (click here).

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 3089:883 (mag 12.25) and GSC 3089:1137 (mag 12.10) respectively. Five measurements were discarded due to thin clouds which rendered analysis impossible.

Image Details
Light Curve for Exoplanet TrES-3b
Imaging Details
Parent Star:

GSC/SAO Catalog:
GSC 3089:929


RA / Dec:
17h 52m 07s /
+37 32' 46"


1300 light-years

1.30619 + 0.00005 d

Transit Duration:
77.4 mins

Transit Depth:
29.8 mmag

Minimum Mass:
1.92 MJup

1.295 RJup

Pred Transit Details:
Ingress :  23:35 UT
Mid-trans :  00:32 UT
Egress :  01:30 UT
May 03, 2008
01:45:00 - 04:46:55 UT+3

Athens, Greece

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

Lum :  225 x 45 sec
Dark :  015 x 45 sec
Flat :  ~18,500 ADU
Binning :  2x2

Ambient : + 14.0 C
CCD Chip : - 15.0 C

CCDSoft V5.00.188
AIP4Win V2.2

Differential Photometry