Astrophotography by Anthony Ayiomamitis

Differential Photometry - HAT-P-8 in Pegasus

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 370 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-8b in Pegasus depicted below is one of the latest transitting exoplanets, having being announced in Dec/2008, and represents the eighth discovery by the Hungarian-based HATNet Project team. HAT-P-8b is characterized with a mass 1.52 times that of Jupiter while its radius is equivalent to 1.50 Jupiter radii, thus making this exoplanet one of the largest and most inflated "hot Jupiter" finds to-date. HAT-P-8b requires 216 minutes to transit its parent star at a depth of 7 mmag or 0.7%. The parent star, GSC 2757:1152, is a class F dwarf estimated to have a mass of 1.28 solar masses, a radius equivalent to 1.58 solar radii, a temperature of 6,200 K and to lie at a distance of 750 light-years away with a visual magnitude of 10.17. Further details regarding HAT-P-8 and HAT-P-8b are available in the paper published by the discovery team led by David W. Latham et al (click here).

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 2757:416 (mag 10.23) and GSC 2757:2126 (mag 10.40) respectively.

Image Details
Light Curve for Exoplanet HAT-P-8b
Imaging Details
Parent Star:

GSC/SAO Catalog:
GSC 2757:1152


RA / Dec:
22h 52m 09.85s /
+35 26' 49.5"


750 light-years

3.07632 + 0.000004 d

Transit Duration:
216 mins

Transit Depth:
7 mmag

Minimum Mass:
1.52 MJup

1.50 RJup

Pred Transit Details:
Ingress :  19:44 UT
Mid-trans :  21:32 UT
Egress :  23:20 UT
Aug 27-28, 2009
22:00:00 - 03:33:20 UT+3

Athens, Greece

AP 160 f/7.5 Starfire EDF
Astrodon Tru-Bal CRGB

Clear :  272 x 60 sec
Dark :  015 x 60 sec
Flat :  ~17,400 ADU
Binning :  1x1

Ambient : + 23.6 C
CCD Chip : - 10.0 C

CCDSoft V5.00.188
AIP4Win V2.3

Differential Photometry