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No 47 - December 2004

Eclipsing Binary Star - RZ Cassiopeiae

RZ Cas (see note by Tony Markham on the BAA website at is an Algol type eclipsing binary. It has an official period of 1.195274 days or 28.685928 hours. Each eclipse lasts about 4 hours. The star varies from 6.4 to 7.8 in magnitude. Because of the length of the eclipse opportunities to observe a full eclipse are fairly rare.

One such opportunity arose on the 16th/17th April 2004. An evening eclipse coincided with clear skies and good conditions. Observations were made with 10x50 binoculars from a deck chair in a back garden of suburban Edinburgh. Warm clothing is essential if it is intended to follow the whole four hour sequence of events. Observations are much easier if your eyes are continually dark adapted. The star is interesting to observe. It changes in magnitude quite fast as it approaches and leaves minimum.

light curve

The observations were recorded as shown on the diagram. The vertical axis is magnitude and the horizontal axis is hours. You will note that observations are made about every ten minutes in the two hours around mid eclipse. A good light curve can be constructed. Mid eclipse was around 1.03am.

There is a 'bump' at minimum (on the left). This may be real as it is thought that one of the stars in this system is a variable. Other observers, at other times, have seen 'bumps' in the light curve which have been attributed to 'starspots' or 'hotspot' where the gas stream in an Algol type binary hits the primary star. See the image of the system. I have looked at several eclipses since 2002 and not one is the same as another.

Algol binaries

The observations in April were of some value. They were sent to Tony Markham who is the Eclipsing Binary Secretary of the BAA Variable Star section. He replied:

A quick look at a light curve drawn up from your observations suggests that mid eclipse is occurring near phase 0.035 (based on the GCVS elements). This shows that the discrepancy that has been seen for some time between the observed eclipse times and the GCVS predicted eclipse times is continuing to increase and is now in the region of an hour.

If you wish to observe this star you should note that a finder chart and comparisons can be downloaded from the BAA Variable Star section website. I can also supply you with a copy. Contact me at < desloughney @ > or 0131 477 0718. The times of mid eclipse are printed in magazines such as Astronomy Now for each month.

Des Loughney
2nd December 2004


Cover page

From the President

Observing Is Not Believing

Eclipsing Binary Star - RZ Cassiopeiae

The BAA Honours our former Vice-President

The Astronomer's Drinking Song

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