Double Star of the Month - July 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.
S W Burnham discovered 26 Dra (17 34 59.58 +61 52 28.4) using the 18.5-inch refractor at Dearborn Observatory in 1879. It was soon apparent that this was a binary system because the large proper motion of the primary star (almost 0".6 per year) was clearly shared by the faint and close companion. Along with many of the other similar pairs he discovered, Burnham substantially underestimated the brightness of the companion, and gave the magnitude of the stars as 5.5 and 10.1 is his General Catalogue of 1906. The WDS gives 5.28 and 8.54 and I saw the comes perfectly well with the 8-inch Cooke refractor at Cambridge in summer 1999. At that time the separation was 1".6 but the pair is now closing quickly and it will take a larger aperture to see them in 2013 when the separation is 0".65. The large proper motion is a consequence of the proximity of this star system. The Hipparcos catalogue gives a revised parallax of 70.47 mas which corresponds to 46.3 light years with a quoted error of 0.24 light years. To find it, draw a line between beta and nu Draconis in the head of the Dragon and extend the line twice as far again.
See 342 (17 53 23.47 -34 53 42.5) is also a close pair requiring a reasonable aperture to resolve, but it does have the additional attraction of being embedded in the open galactic cluster M7, close to the tail of Scorpius. Since discovery in 1897, the pair has moved in retrograde fashion by 80 degrees whilst the separation has remained close to 0".4. A provisional orbit by Andreas Alzner puts the period at 700 years and its parallax places it at the distance of M7. A good chart for identifying See 342 is given in Burnham's Celestial Objects Volume 3, page 1712. It is #1 and is located at the SW corner of the cluster. Robert Burnham quotes magnitudes of 6.5 for each star whilst the WDS lists 5.85 and 7.89 which also seems too unequal. Recent measures by Tokovinin give a delta m of 0.3 to 0.4. The primary star is a K giant and a spectroscopic binary.
Bob Argyle - Double Star Section Director