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.
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 |
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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:
Temperatures:
Software: CCDSoft V5.00.201 AIP4Win V2.4.0 Processing: Reduction Differential Photometry |