Double Star of the Month - June 2011
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.
Beta Serpentis (15 46 11.21 +15 25 18.9) lies about 11 degrees south of the crown of Corona Borealis. The primary is a mag 3.7 star of spectral type A2IV and many observers find it to be yellowish. The faint (mag 10.0) companion was found by the elder Herschel on 1781 Aug 13, and he did not allocate it a colour, noting it only as `extremely faint'. Today 15-cm aperture will show it clearly enough - a slighter harder test is the 10.7 mag star some 200" away in PA 212°. A very recent study has been using Hipparcos data to physically link apparently unrelated pairs of stars and in Beta's case the study found that there is a very high probability that the faint wide pair ROE 75 (mags 8.2, 10.7, PA 327°, separation 6".2) which can be found some 20 arc mins south preceding beta Ser is physically connected to Beta by dint of having a very similar parallax and proper motion.
The small constellation of Ara is seen against the Milky Way near the tail of the Scorpion, and Hartung lists numerous objects of interest within its borders. The coarse cluster NGC 6193 is involved with the emission nebula NGC 6188 and in addition the brightest star MLO 8 (16 41 20.42 -48 45 46.7) is a multiple system of some interest. In 1878 observers at Melbourne divided A (mag 5.1) and found a mag 8.4 companion at 2".1 distance. The distance between the two has been closing since and the last recorded measure in 1938 placed the companion 1".6 from the primary, although Hartung reports seeing the star clearly resolved with 10.5-cm in 1963. It is not clear if B is a background star or an orbital companion - the next positive observation will help to decide this question. For the small telescope user there is star C (mag 6.8) some 10" away and medium apertures should easily find two fainter (mags 10.5 and 11.4) and more distant comites found by Herschel and another of mag 12.5 discovered by Innes.
Bob Argyle - Double Star Section Director