Astrophotography by Anthony Ayiomamitis

Differential Photometry - 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 750 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 eclipsing binary star BX Peg is a quick eclipsing binary variable star with a period of 0.2804208 days and which involves a delta mag of 0.69 (r) magnitudes during this brief time interval. The variable nature of BX Peg was discovered by Shapley and Hughes (1934) and has been the source of various studies since then including a photometric analysis by Samec (1990). BX Peg lies in the same field of view as KW Peg. BX Peg is a W Ursae Majoris-type eclipsing system where the binary components involve main sequence stars with ellipsoidal components virtually in contact with each other and, hence, for its classification, namely "EW/KW". Further details from the International Variable Star Index are available here whereas an AAVSO finder chart is available here.

Note: The minima observed below represent the secondary and primary minima, respectively.

Image Details
Light Curve for BX Peg
Imaging Details
Variable Star:
BX Peg

Other Designation:
GSC 2197-1458

RA / Dec:
21h 38m 49s /
+26 41' 34"

10.65 - 11.35 (v)

0.2804208 days


Comparison Star:
GSC 2197:1871

Check Star:
GSC 2197:1485

Sep 25-26, 2012
20:55 - 02:55 UT+3

Athens, Greece

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

Lum :  294 x 60 sec
Dark :  010 x 60 sec
Flat :  ~ 24,000 ADU
Binning :  1x1

Ambient : + 24.0 C
CCD Chip : - 10.0 C

CCDSoft V5.00.201
AIP4Win V2.4.0

Differential Photometry