Astrophotography by Anthony Ayiomamitis

Differential Photometry - HD 209458 in 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 275 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 light curve for exoplanet HD 209458b in Pegasus depicted below is one of the earlier transitting exoplanets, having being discovered in the late 1990's by the radial velocity method and later via transit (in fact, it is the first exoplanet transit discovery), and whose atmosphere contains water vapour. It is characterized with a density slightly less than that of Jupiter owing to a mass equivalent to 0.66 Jovian masses and a radius equivalent to 1.32 Jovian radii. HD 209458b has an orbital period of just under 85 hours and requires 184 minutes to transit its parent star at a depth of 1.65%. The parent star, HD 209458, is estimated to have a mass of 1.1 solar masses, a radius equivalent to 1.2 solar radii and a temperature of 6,000° K. Further details regarding HD 209458 and HD 209458b are available in the paper published by the transit discovery team led by David Charbonneau et al (click here).

Note: The C- and K-stars used for the purposes of the differential photometry measurements depicted below were GSC 1688:1982 (mag 8.32) and GSC 1688:1820 (mag 9.82) respectively. Due to extremely poor seeing, this particular exoplanet will be revisited at the first available opportunity for better results.

Image Details
Light Curve for Exoplanet HD 209458b
Imaging Details
Parent Star:
HD 209458

GSC/SAO Catalog:
SAO 107623

Constellation:
Pegasus

RA / Dec:
22h 03m 10.8s /
+18° 53' 04"


Magnitude:
7.65

Distance:
154 light-years
Exoplanet:
HD 209459b

Period:
3.52474541 + 0.00000025 d

Transit Duration:
184.2 mins

Transit Depth:
16.5 mmag

Minimum Mass:
0.657 MJup

Radius:
1.320 RJup

Pred Transit Details:
Ingress :  19:19 UT
Mid-trans :  20:51 UT
Egress :  22:23 UT
Date:
Sept 02, 2008
21:30 - 02:00 UT+3


Location:
Athens, Greece

Equipment:
AP 160 f/7.5 Starfire EDF
AP 1200GTO GEM
SBIG ST-2000XM
SBIG CFW-10
SBIG LRGB + IR-block


Integrations:
Blue :  776 x 08 sec
Dark :  015 x 08 sec
Flat :  ~17,000 ADU
Binning :  1x1

Temperatures:
Ambient : + 24.0 ° C
CCD Chip : - 07.5 ° C

Software:
CCDSoft V5.00.188
AIP4Win V2.2


Processing:
Reduction
Differential Photometry