Astrophotography by Anthony Ayiomamitis

Differential Photometry - Cygnus

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 EM Cyg is a quick eclipsing binary variable star with a period of 0.29090913 days and which involves a delta mag of 0.25 (V) magnitudes during this brief time interval. The variable nature of EM Cyg was first identified by Hoffmeister in 1928 and who reported a variability between magnitude 12.5 and 14.5 (see Burbidge and Burbidge, 1953). Originally classified as an Algol-type eclipsing binary, EM Cyg is now considered a cataclysmic variable of the Z Cam-type and, hence, for its current classification, namely, "UGZ+E". Z Cam variable stars are characterized with cyclical outbursts which range from 10 and 40 days and where the brightness post-outburst may not reach the pre-outburst levels. On the basis of their spectroscopic analysis, Burbidge and Burbidge (1953) suggest that EM Cyg in all likelihood represents a former nova. Further details from the International Variable Star Index are available here whereas an AAVSO finder chart is available here. For the entry involving EM Cyg in the The Catalog and Atlas of Cataclysmic Variables (Ronald Downes et al, 2006), click here.

Image Details
Light Curve for EM Cyg
Imaging Details
Variable Star:
EM Cyg

Other Designation:
GSC 2655-3329

RA / Dec:
19h 38m 40s /
+30° 30' 28"


Magnitude:
11.9 - 14.4 (p)

Period:
0.29090913 days

Variability:
UGZ+E

Comparison Star:
GSC 2655:3255

Check Star:
GSC 2655:3471


Date:
May 26, 2013
01:00 - 04:45 UT+3


Location:
Athens, Greece

Equipment:
AP 305/f3.8 Riccardi-Honders
AP 1200GTO GEM
SBIG ST-10XME
SBIG CFW10
SBIG LRGB filters


Integrations:
Lum :  184 x 60 sec
Dark :  010 x 60 sec
Flat :  ~ 23,400 ADU
Binning :  1x1

Temperatures:
Ambient : + 18.0 ° C
CCD Chip : - 15.0 ° C

Software:
CCDSoft V5.00.201
AIP4Win V2.4.0


Processing:
Reduction
Differential Photometry