May 2014 - Double Star of the Month
The northern target for this month's column is one of the most popular of all double stars - zeta UMa (13 23 55.42 +54 55 31.5). Known equally well as Mizar, this beautiful pair of early A stars has the longest history of all telescopic pairs. First seen by Castelli in 1617, the two stars, whose magnitudes are 2.2 and 3.4 and which are separated by 14".5 were also the first double star to be photographed by Bond in 1857, and Mizar A was the first star to be shown to be a spectroscopic binary in 1889. Mizar A was also high on the list of pairs observed by Michelson with the interferometer on the 100-inch telescope at Mount Wilson in the 1920s, and was duly resolved. Some 706" distant is Alcor (mag 4.0) which forms a naked-eye pair with the Mizar system. It seems likely that the three stars are moving through space together in common with other members of the UMa moving group. In addition to Mizar A, Mizar B is also a single-lined spectroscopic binary and Alcor was also suspected a number of times of being a close binary but no proof has yet been put forward. There is, however, a faint, low-mass star only 1".1 distant from Alcor, discovered in 2009 which does seem physically connected, making the whole group a sextuple system.
Another easy pair of B stars can be found in 3 Cen (13 51 49.58 -32 59 38.6). Discovered by William Herschel it was also picked up by James Dunlop in Australia and appears in his catalogue as Dun 148. With 32.5-cm Gould finds the colours are pale yellow and white. The stars have closed slightly since discovery and are now separated by 7".8 in 104° as measured by the author in 2013. The parallax of A, determined by Hipparcos, puts the star at a distance of 343 light years and the system is part of the Sco/Cen moving group, as is the neighbouring 4 Cen which is barely 1° north following. Another Herschel discovery, in 2013 these stars were 14".8 apart in PA 185°. Hartung notes the colours as pale yellow and ashy. Hartung notes both stars are spectroscopic binaries but the WDS only mentions the duplicity of the brighter component which has a period of 6.93 days.
Bob Argyle - Double Star Section Director