Astrophotography by Anthony Ayiomamitis

Differential Photometry - HAT-P-5 in Lyra

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-5b in Lyra depicted below is one of the latest transitting exoplanets, having being announced late 2007, and represents the fifth discovery by the Hungarian-based HATNet Project team. HAT-P-5b is characterized with a mass slightly greater that of Jupiter and in spite of a larger radius (1.257 RJup), thus making this find a slightly low-density hot Jupiter. HAT-P-5b requires 175 minutes to transit its parent star at a depth of 1.32% with ingress requiring approximately 21 minutes. The parent star, GSC 2634:1087, is estimated to have a mass of 1.16 solar masses, a radius equivalent to 1.167 solar radii, a temperature of 5,960 K and to lie at a distance of 1100 light-years with a visual magnitude of 11.95. Further details regarding HAT-P-5 and HAT-P-5b are available in the paper published by the discovery team led by G. Bakos et al (click here).

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 2634:955 (mag 11.7) and GSC 2634:687 (mag 11.7) respectively.

Image Details
Light Curve for Exoplanet HAT-P-5b
Imaging Details
Parent Star:

GSC/SAO Catalog:
GSC 2634:1087


RA / Dec:
18h 17m 37s /
+36 37' 17"


1100 light-years

2.788491 + 0.000025 d

Transit Duration:
175 mins

Transit Depth:
13.2 mmag

Minimum Mass:
1.06 MJup

1.257 RJup

Pred Transit Details:
Ingress :  20:52 UT
Mid-trans :  22:25 UT
Egress :  23:58 UT
May 28-29, 2008
23:22:01 - 04:02:15 UT+3

Athens, Greece

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

Lum :  382 x 40 sec
Dark :  015 x 40 sec
Flat :  ~18,500 ADU
Binning :  2x2

Ambient : + 23.0 C
CCD Chip : - 07.5 C

CCDSoft V5.00.188
AIP4Win V2.2

Differential Photometry