Double Star of the Month - September 2013

In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

As telescope makers of renown the Clarks (Alvan and his son Alvin George) were able to point large new telescopes at bright stars in order to test them and, if lucky, they discovered new companions. Such was the case of Sirius but here there was already significant evidence that the star was double based on Bessel's discovery of variable proper motion.

In the case of tau Cygni (21 24 47.35 +38 02 39.6), however, the discovery of duplicity was entirely serendipitous. Discovered by A. G. Clark using a 26-inch refractor, it has turned out to be a system of considerable interest. The period is 49.8 years and the separation varies from about 0".5 to 1".1 but the difficulty for the observer is the significant difference in brightness between the two stars. The WDS gives V mags of 3.8 and 6.6. The writer has seen the comes with the 8-inch OG at Cambridge and the system is widening again at present - in the autumn of 2013 the position will be 213° and 0".9 and so should be seen in 20-cm on a night of good seeing. The pair is easy to find as the southernmost of a trio of brightish stars 10 degrees following gamma Cygni and it is 1.5 degrees south following 61 Cygni. Recent investigations by the astrometric-based Palomar High-precision Astrometric Search for Exoplanet Systems (PHASES) have pointed out the possibility of a sub-stellar companion to one of the stars. The period may be 826 days and the mass may be 12.3 Jupiters but this is very much preliminary work.

About 10 degrees preceding the 3rd magnitude star beta Aquarii is 3 Aqr. Just south following is a pair of stars the preceding of which is 4 Aqr and the following 5 Aqr. William Herschel noted that 4 Aqr (20 51 25.69 -05 37 35.9) was double on Sept 3 1782 and listed it as number 44 is his Class I stars. Wilhelm Struve measured it at Dorpat in 1825 and at present the stars are in almost exactly in the same place having undergone a whole orbital revolution since then. This 187-year-period binary is not particularly easy from the UK due to its low declination and it is now closing again. For 2014.0 it will be at 30° and 0".7 making it a target for a superior night. The stars are mags 6.4 and 7.4.There are two distant and unconnected comites - C is 13.3 at 74" from AB and D is 9.7 at 136". In each case the separation is increasing.

Bob Argyle - Double Star Section Director