Guide to RZ Cassiopeiae

RZ Cas is the best eclipsing variable for beginners.

Tracie Heywood’s light curve showing a primary eclipse

The eclipses of RZ Cas take less than 5 hours – indeed most of the ‘action’ takes place during the middle three hours.

This means that it is possible to observe a whole eclipse without needing to stay out all night.

The shortness of the eclipse also reduces the risk that the sky will cloud over part way through the eclipse.

Another helpful feature is that the eclipses are quite frequent – taking place very 29 hours.

Not only that – RZ Cas is also easily circumpolar for observers in the UK.

And the comparison star sequence is rather good – with mag 7.4, mag 7.7 and mag 8.0 comparison stars being located close to the variable. This contrasts with the situation for brighter eclipsing variables, such as Algol, for which the comparison stars may be a considerable distance away on the sky).

RZ Cassiopeiae is a similar type of eclipsing variable to Algol (Beta Persei). The orbital plane of the two stars in the RZ Cas system is edge on as seen from the Earth. The deep primary eclipse occurs when the brighter star is eclipsed by the fainter star. The secondary eclipse (in which the fainter star is eclipsed) is too shallow to be spotted visually. The brightness of RZ Cas is constant between eclipses.

Extreme brightness range 6.4 – 7.8
More typical range always the same
Period of variation 1.195247 days (approx 29 hours)
Frequency of observation Estimate the brightness every 20-30 mins during primary eclipse
Observe using 50-80mm binoculars
Visibility Can be observed all year round, but is lowish in the evening sky from April to June

Here are finder charts that shows the location of RZ Cassiopeiae:

RZ Cas
Chart for RZ Cas

This second chart, which is approx 8 degrees by 6 degrees, shows the area around RZ Cas in more detail.

You can make brightness estimates by comparing the brightness of RZ Cas with that of the lettered comparison stars.

Outside eclipses, RZ Cas will appear brighter than comparison B (and slightly fainter than SU Cas). Around mid eclipse it will be similar in brightness to comparison E, possibly slightly fainter if you catch it at mid eclipse. SU Cas is a low amplitude Cepheid variable star whose brightness variations (range 5.9-6.3) are too small to easily follow visually.

RZ Cas
Chart for RZ Cas