Double Star of the Month - September 2010

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.

The two pairs this month are both long period binaries, whose physical association was in some doubt for many years.

zeta Aqr (22 28 49.8 -00 01 12) is not strictly a northern double star being some 72 arc seconds south of the equator but it is such a bright and easy pair for the small telescope that it is worth seeking out. It is one of the bunch of bright stars which are 5 to 8 degrees following alpha Aquarii.

The stars that form zeta are magnitudes 4.3 and 4.5 and the spectral type of early F suggests that the colour of the primary should be yellowish. Webb in 1851 finds pale yellow, Hartung also notes yellow and Sissy Haas has pale citrus-orange.

According to Thomas Lewis, Zeta was found to be double by Christian Mayer in 1777 but it does not appear in his pioneering catalogue of 80 double stars published in 1780 according to Jurg Schlimmer. William Herschel observed it soon afterwards and includes it as H II 7. At this time the companion was about 4 arc seconds due north and for most of the nineteenth century the motion appeared linear, but in the first quarter century of the twentieth century the pair closed up and the companion began to swing around A. At present the position angle and separation are 170° and 2".1 and the stars are now separating and will continue to form an easy pair for centuries to come.

53 Aqr (22 26 34.3 -16 44 31.9) can also be seen from northern latitudes but it requires a night of good seeing to separate the two stars cleanly. These are almost identical GO dwarfs of mags 6.3 and 6.4 and when the pair was first found by South/Herschel the separation was more than 10 arc seconds. Over the last 200 years the stars have slowly closed, mostly in an apparently linear fashion but about 50 years ago the companion began a slow majestic swing around the primary.

Hale in 1994 computed an orbit with a period of 3500 years so our knowledge of the binary motion is cursory at best but regular and accurate measures over the next 50 years or so will define the periastron part of the orbit. If Hale's work is right, the maximum distance between the stars will be almost 27 arc seconds in 1800 yea’s time, but as luck would have it minimum distance of 1".27 is reached in 2014, so this is a very good time to watch this pair.

Bob Argyle - Double Star Section Director