Double Star of the Month - February 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.
With Gemini high in the sky in the early Spring sky, a number of binaries are on display for the small telescope user. One of the more difficult is STF1037 (07 12 49.08 +27 13 30.2), a pair of yellow stars which are locked in a highly eccentric orbit. They present a good test object because at present the separation is 1".00 according to the 116 year orbit which is given in the USNO 6th orbital catalogue. The magnitudes are listed in the WDS as 7.2, 7.3 and with the 8-inch refractor at Cambridge this pair has always been more difficult to see and measure clearly than the parameters would suggest. It is a good time to observe STF1037 - the pair is closing up again and will reach well below 0".1 in about 30 years time, and it not be this wide again until around 2063. In the 19th century, the German observer Madler was convinced that B was double again and the volume by Lewis on the Struve stars does show a loop in the apparent motion of B but no convincing evidence for a third component has come to light. Madler, and Dembowski failed to see the faint star C (V~13) found by Otto Struve. It is located at 78°, 14" but may be variable.
Puppis is a glorious constellation for the double star aficionado and one of the best objects is k Puppis (07 38 49.88 -26 48 14.0), a third magnitude star some 8° east of delta CMa. Discovered by William Herschel (H III 27) the stars are both hot blue dwarfs of spectral type B6 and might be expected to appear white in the eyepiece. Malin and Frew, in their revision of Hartung's book thought so but noted that Hartung himself had them as pale yellow. From his observatory in Victoria, Australia this object would have passed almost overhead. Haas also calls them white but gives the star name as kappa. There has been some angular motion since 1800 - star B has moved about 8° retrograde and can now be found at 318° and 9".9. Burnham added a faint, distant star, mag 13.7 at 7" from A, but as this has not been measured since 1927, it would appear to be a difficult object.
Bob Argyle - Double Star Section Director