Double Star of the Month - February 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.

It was near delta Gem (07 20 07.39 +21 58 56.4) that Pluto was first seen on a plate taken by Clyde Tombaugh at Lowell Observatory in February 1930. By a strange coincidence, Herschel found the mag 8.2 companion to delta on March 13 1781, the very same night that he discovered Uranus and later recorded the system as H 2 27. The primary star is a late A9 giant, of visual magnitude 3.55, and at a distance of 60 light years according to Hipparcos. To see the companion comfortably requires 15-cm and a night of reasonable seeing. Some disagreement attaches to the colour of the secondary. It is given as K3V in the WDS and E. Hartung noted it as reddish, as it did T W Webb but some years before he had recorded the hue of the star as purplish. The primary is also a single-lined spectroscopic binary and has also been seen double at lunar occultation’s but attempts to resolve it directly using speckle methods have so far failed.

BU 332 (07 27 51.66 -11 33 24.7) is a multiple star which appears coincident with NGC2396 on Map 8 in Norton, close to the point where Monoceros, Canis Major and Puppis meet. Just after completing the notes on this system I read the Sky and Telescope for February 2013 and found that it featured in Sue French's column (page 57) under the name STF 1097. AC has mags 6.2, 8.7 and the components seem relatively fixed at 313° and 20". In 1865 Baron Dembowski suspected that A was double and it was later confirmed by Burnham using his 6-inch Clark refractor. Star B is magnitude 7.35 and there has been very little motion - amounting to 7° retrograde in PA, and the stars are possibly closing up. Small telescope users should be able to see the more distant D (V = 9.7) at 157o, 23". A 12.7 mag star at 32" will need at least 10-cm. Hartung notes that it is a beautiful field and that the close pair is deep yellow and white.

Bob Argyle - Double Star Section Director