Double Star of the Month - April 2012
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.
2 CVn = STF1622 (12 16 07.55 +40 39 36.6) is a beautiful pair with components of contrasting hues which sits near the western border of Canes Venatici with Ursa Major and about 5 degrees following beta CVn. The primary star, an early M giant is accompanied by a late F dwarf and many writers have commented upon the colours to be seen here: - Webb called them very gold and blue, Dembowski thought them yellow and azure, Franks made them orange and blue and more recently Sissy Haas recalled brick red and silvery sapphire. The stars are magnitudes 5.9 and 8.7 and are currently separated by about 11".4 which makes them an easy target even for the small telescope.
mu Crucis (12 54 35.66 -57 10 40.4) is simply one of the most beautiful doubles in the sky. A pair whose components of visual magnitude 3.9 and 5.0 share common proper motion and distance, this system belongs with the Scorpio OB2 association of young hot stars, and Hipparcos places both stars about 412 light years away. Shatsky and Tokovinin used the ADONIS near infrared adaptive optics system on the ESO 3.6-metre reflector to search for faint, close companions and they found two objects within 5" of component B. No magnitudes or proper motions are available so it is too early to say if these are physically connected but the two bright stars certainly form a very long period binary. Mu Crucis was found by Dunlop in 1826. The spectral types are both B and Hartung record them as both white whereas Richard Jaworski sees a tinge of yellow in the fainter star. The current separation of 35" and the brightness of the stars almost makes this a southern equivalent of Albireo but without the colours.
Bob Argyle - Double Star Section Director