Astrophotography by Anthony Ayiomamitis

Differential Photometry - Ursa Major

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 U UMa is a quick eclipsing binary variable star with a period of 0.19667128 days and which involves a delta mag of 1.50 (V) magnitudes during this brief time interval. The variable nature of UX UMa was first reported by Kukarkin et al. in 1969 and various studies later have suggested that UX UMa is a cataclysmic variable (for example, see Williams, 1983). UX UMa is an Algol-type eclipsing system with spherical or slightly ellipsoidal components and, hence, for its classification, namely "EA". It is believed that a pulsating white dwarf is involved in this (nova-like) binary system (see Nather and Robinson, 1974). Further details from the International Variable Star Index are available here whereas an AAVSO finder chart is available here. For the entry involving UX UMa in the The Catalog and Atlas of Cataclysmic Variables (Ronald Downes et al, 2006), click here.

Image Details
Light Curve for UX UMa
Imaging Details
Variable Star:
UX UMa

Other Designation:
GSC 3469-1302

RA / Dec:
13h 36m 41s /
+51° 54' 50"


Magnitude:
12.57 - 14.15 (V)

Period:
0.19667128 days

Variability:
EA/WD+NL

Comparison Star:
GSC 3469:0651

Check Star:
GSC 3469:0395


Date:
May 25-26, 2013
21:50 - 00:50 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 :  145 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