March 2014 - Double Star of the Month

24 CBe (12 35 07.76 +18 22 37.4) sits at the north edge of the great Coma cluster of galaxies about 3 degrees following M88, but it is in a poor naked-eye star field so that locating it is not straightforward. One way would be to find beta CVn (V = 4.3) and move 23 degrees south. The effort is worth it - this is a very pretty, bright and easy pair for the small telescope. The contrasting colours have been noted by many observers. Webb thought the stars yellow and very blue, Smyth found orange and emerald and Sissy Haas made them citrus orange and fainter royal blue. The pair also serves another purpose - as a scale and orientation calibrator. At present the position angle is 270 degrees and the separation 20" (actually 270.3 and 20.14 for 2015.0). Many observing guides list the pair as optical but the evidence is not very persuasive. The Hipparcos satellite gives the parallaxes as 7.24 +/- 2.74 milliarcseconds (mas) for A and 19.29 +/- 14.58 mas for B. The proper motions are small but very similar. A is a K2 giant and B is a metallic-lined A9 dwarf which is also a spectroscopic binary.

x Vel = DUN 95 (10 39 18.39 -55 36 11.8) is in a rich area of the southern Milky Way just 5 degrees north of the Eta Carina Nebula, NGC 3372. This pair was found by John Dunlop at Paramatta in 1826 and is a glorious sight in small telescope. The stars (V = 4.38 and 6.06), according to Ross Gould using a 35-cm reflector, are yellow and deep-yellow and the low power field contains two small asterisms. Andrew James, on the other hand, is an experienced Australian observer with a very extensive website devoted to double stars and especially those of Dunlop. He reports that Russell in 1873 made the colours straw-yellow and greenish blue and around 1980 members of the AS of New South Wales reported orange and pale blue. Given the spectral type of B is B8V, the reported deep yellow is rather unexpected. There has been little motion between the two components over the last two centuries. In 2000 the position angle was 105° and the separation 51".7. The primary is an early G-type supergiant which is also a semi-regular pulsating star. Hipparcos puts the primary at a distance of 840 light years. In 1834 John Herschel found a faint companion to B, V = 11.9, some 15" away in position angle 178°. The distance has widened to 20" today.

Bob Argyle - Double Star Section Director