Astrophotography by Anthony Ayiomamitis

Differential Photometry - Corona Borealis

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 345 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 XO-1b in Corona Borealis depicted below is one of the latest transitting exoplanets, having being discovered in 2006, and is particularly unique since it represents a clear collaboration between professional and amateur astronomers where the research team led by Peter McCullough (Space Telescope Science Institute, USA) identify a number of potential candidates using widefield imaging and amateurs were asked to pursue the photometry thereafter of promising candidates. XO-1b is characterized with a mass and radius 0.90 and 1.184 that of Jupiter, respectively, and an orbital period of only 95 hours. XO-1b requires 179.5 minutes to transit its parent star at a depth of 17 mmag (1.70%). The parent star, GSC 2041:1657, is of spectral type G1V and is estimated to have a mass of 1.00 solar masses, a radius equivalent to 0.928 solar radii and a temperature of 5,750 K. Further details regarding XO-1 and XO-1b are available in the paper published by the discovery team led by Peter McCullough et al (click here).

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 2041:1282 (11.2) and GSC 2041:186 (mag 11.4), respectively.

Image Details
Light Curve for Exoplanet XO-1b
Imaging Details
Parent Star:

GSC Catalog:
GSC 2041:1657

Corona Borealis

RA / Dec:
16h 02m 11.8s /
+28 10' 10"


652 light-years

3.94153 + 0.00003 d

Transit Duration:
179.5 minutes

Transit Depth:
17 mmag

Minimum Mass:
0.902 MJup

1.05 RJup

Pred Transit Details:
Ingress :  20:56 UT
Mid-trans :  22:26 UT
Egress :  23:55 UT

12-days old
Jun 04-05, 2009
22:55:00 - 04:00:28 UT+3

Athens, Greece

AP 160 f/7.5 Starfire EDF
Baader IR Pass Filter

InfraRed :  224 x 75 sec
Dark :  015 x 75 sec
Flat :  ~16,000 ADU
Binning :  2x2

Ambient : + 25.4 C
CCD Chip : - 15.0 C

CCDSoft V5.00.188
AIP4Win V2.3

Differential Photometry