Astrophotography by Anthony Ayiomamitis

Differential Photometry - Hydra

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 500 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 large-amplitude short-period pulsating star KZ Hya is a fast pulsating variable star with a period of 85.70 minutes and which involves a delta mag of 0.80 (V) magnitudes during this brief time interval. KZ Hydrae has been studied fairly intensely during the past few decades and ever since its discovery in 1975 by Przybylski and Bessell. It has been noted that its period is continuously increasing with the passage of time, thus suggesting that a degenerate companion which is not visible must be present. Further details from the General Variable Star Search Gateway are available here whereas an AAVSO finder chart is available here.

Image Details
Light Curve for KZ Hya
Imaging Details
Variable Star:
KZ Hya

Other Designation:
HD 94033

RA / Dec:
10h 50m 54s /
-25° 21' 15"


Magnitude:
9.46 - 10.26

Period:
0.0595104212 days

Variability:
SXPHE

Comparison Star:
GSC 6638:764

Check Star:
GSC 6638:483


Date:
Feb 08-09, 2011
23:15 - 02:54 UT+2


Location:
Athens, Greece

Equipment:
AP 160 f/7.5 Starfire EDF
AP 1200GTO GEM
SBIG ST-10XME
SBIG CFW10
SBIG LRGB filters


Integrations:
Lum :  200 x 60 sec
Dark :  010 x 60 sec
Flat :  ~ 24,200 ADU
Binning :  2x2

Temperatures:
Ambient : + 11.0 ° C
CCD Chip : - 25.0 ° C

Software:
CCDSoft V5.00.201
AIP4Win V2.4.0


Processing:
Reduction
Differential Photometry