Department of Physics AstroLab

A Study of a Pulsating Variable

A small percentage of stars vary in brightness periodically. So far over 100,000 have been catalogued. An important class of variable stars are the pulsating variables. As these stars periodically expand and contract their luminosity changes. Different types of pulsating variables can be distinguished by their light curve. Well-known types include the Cepheids and RR Lyrae stars.

The aim of the project is to determine the light curve and hence the period of an RR Lyrae star. By choosing variable stars with adjacent non-varying stars the light curve can be easily determined from measurements of the relative brightness. How well can the light curve and period be determined? Using archival and published data for this system is there any evidence for a change of period?

An example of the type of observations required and the type of results achieved for this project is given below.

This is a 30-second V-band image of the RR Lyrae star SW And (lower right) and three adjacent stars of similar brightness. The brightness of SW And is measured relative to these three stars. By making a series of such observations over the term the brightness variations of the RR Lyrae star can be determined.

Below is the light curve for RR Gem as measured by Astrolab studentts (Jane and Paul). The curve is based on 434 60-second V-band exposures taken with an ST6 on one of our 10-inch Meade telescopes. A model for the light curve is shown as the solid line. Using this data they determined a period for RR Gem of 0.39727 +/- 0.00001 days.

By using literature measurements of maxima times the long term trends in the pulsation period can be mapped. The figure below displays the observed minus calculated (O - C ) maxima times for the RR Lyrae TZ Aur; Durham measurements are displayed as red points whereas the literature values for the maxima as listed in GEOS RR Lyrae database are showed as open squares. The pulsation period for TZ AUR has been constant over the last ~100 years, e.g. for over 93,000 cycles.


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