Astronomical Society of Edinburgh
Journal 44

Two Variables in Lyra

The very good book "Observing Variable Stars - A Guide for Beginners" by David Levy (which is available in the Society's library) suggests that a good star for beginners is W Lyrae. Another variable, which is highlighted in the handbook "Stars and Planets" by Ian Ridpath and Wil Tirion, is RR Lyrae. The third edition of the latter book has just been published and is well worth buying as a general reference at the price of £14.99.

RR Lyrae

Finder chart for RR Lyrae

This star is the prototype of an important class of variables used as 'standard candles' for indicating distances in space. Related to Cepheid variables they are giant stars that pulsate in size varying by about one magnitude in less than a day. RR Lyrae varies from 7.1 magnitude to 8.1 magnitude in 13.6 hours. Thus it is possible to see significant changes in an evening's observing session as the rise from minimum to maximum takes less than three hours.

The changes can be followed from suburban Edinburgh using 10 by 50 binoculars on a clear night that is free from haze. It is fairy straightforward to find the star. You first spot the naked eye star delta Cygni (2.9 magnitude). RR Lyrae is in the same binocular field of view of this star. If delta Cygni is at the top left RR Lyrae is at the bottom right. A finder chart indicating the neighbouring patterns of stars and the non variables, including their magnitudes, that are used for comparison is shown. The pattern of the three stars on the right is particularly distinctive.

Light curve of RR Lyrae

RR Lyrae (RA 19.25, Dec 42.8) has been observed since the middle of August and a rough light curve determined. This is illustrated in the further diagram below. However, the star does not seem to be totally predictable. The observed times of maxima are not those indicated by data obtained from the American Association of Variable Star Observers (www.aavso.org). The maxima seem to be a few hours later than predicted. In addition the times between maxima and minima seem to vary by an our within a constant period of 13.6 hours.

At the time of writing I suspect that this apparent variation may be due to observing errors so it is planned to continue observations throughout the autumn.

W Lyrae

This star is a long period variable of 196.4 days. It varies from 7.7 magnitude to 12.3. Observations therefore require a telescope and I use a 200mm reflector. It is difficult to observe from a suburban location at minimum as only on exceptional nights have I seen stars of 12 magnitude or dimmer. On the best night this year I could just see, near the zenith, stars of 12.5 magnitude. Nevertheless this is not too great a problem with this star as it is only at 12 or dimmer for about twenty days. Because of lack of contrast with a light polluted sky I have never seen it at less than a magnification of 50. The best view is at time a hundred. Observing it at a higher magnification is not much use. You cannot see the comparator stars as the field of view is too small. It is only worth observing this star about once a week as you will not notice any change over a lesser interval.

Finder charts can be downloaded from the website of the American Association of Variable Star Observers. If obtaining the charts is a problem I would be happy to make copies for you (ring 0131 477 0817). I usually start by finding kappa Lyrae (magnitude 4.2). Using a 40mm eyepiece W Lyrae is in the same field of view about midway between kappa Lyrae and a pair of stars in Hercules of magnitudes 5.7 and 5.9. Kappa Lyrae is at a declination of 36 degrees and W Lyrae at 36.40 (RA 18.15). The big challenge is finding it for the first time. Once discovered the location is not forgotten as the local assemblage of stars is quite distinctive.

Below are the observations of this star this year from June onwards. It reached minimum during the first week of July and is due to reach maximum in the middle of October.

Light curve for W Lyrae

An advantage of observing these stars in Lyrae is that they are circumpolar. They are low down for three months in the evening but for the rest of the year they are well placed for study. I have created specific record sheets for both these stars. I would be happy to supply copies of these to anyone interested.

Although these stars are relatively faint they do represent celestial spectacles in their own right. Imagine living in planet revolving around RR Lyrae and watching your sun pulsate on a 13 hour cycle varying in brightness every day by 2.5 times? Imagine being anywhere near a star such as W Lyrae which varies by nearly a hundredfold over 96 days?

Note

I could not find 'official' finder charts for RR Lyrae from either the AAVSO or the British Astronomical Association (Variable Star Section). My finder chart was constructed from data obtained from the planetarium computer software product "Starry Night Pro". The data on the magnitude of the comparison stars is not 100% reliable. This is, apparently, because the data was obtained from the analysis of photographic plates and not by direct photometric analysis of each individual star.

Des Loughney
9th October 2001