Double Star of the Month - April 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.
The two stars in this month's column both have early A type primary stars and are each located around 125 light years from Earth but as objects for observation, one can be seen in stabilized binoculars, the other needs at least a 30-cm telescope.
alpha CVn (12 56 01.67 +38 19 06.2) is the leader of the northern constellation of the Hunting Dogs and sits in an desolate part of the sky to the naked eye, below Ursa Major and above the faint coarse grouping of stars which form Coma Berenices. It was found by William Herschel and the magnitudes are 2.8 and 5.5. Hipparcos, which has measured both components, has had trouble with the distance to star B but even though the error in the trig. parallax is some 30% the distance still agrees with that of star A within the mutual errors. The clincher here is the proper motion of each star - around 0".2 annually and in the same direction. The separation has reduced from 22" in 1777 to around 19".1 today - slow enough to be used by the writer as a standard for micrometer calibration. Both stars are brilliant white and A is the prototype of the alpha2 CVn variables - it possesses lines of rare earths in its spectrum and the amplitude of variability is some 0.14 mag in a period of 5.5 days. The WDS states that both stars are spectroscopic binaries but neither appears in the 9th catalogue of SB orbits.
gamma Cen (12 41 31.20 -48 57 35.6) can be found by extending the line between alpha and gamma Crucis by about the same distance again. In 1847 John Herschel recorded his observation of the star in sweep 553 - "A star 4m. which I am very much inclined to believe close double, but could not verify it owing to bad definition. Tried 320 but it will not bear that power". He noted that there was indisputable evidence of rapid orbital motion (5°.4 in just over one year to 1836.28). The stars then closed rapidly to about 0".13 over the next 11 years and reached their closest point again in 1933. With a period of 84.5 years these two almost identical A stars are closing rapidly again and at the time of writing can be found at 311°, 0".26 - wait another year and the distance will be 0".18.
Bob Argyle - Double Star Section Director