Double Star of the Month - August 2010
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.
mu Cyg (21 44 08.57 +28 44 33.4) is a fine binary, currently well-seen in 10-cm aperture. It is number 72 in Christian Mayer's 1780 catalogue of double stars and since that time has been well followed by double star observers with the WDS listing more than 700 measurements. The apparent orbit is quite eccentric (0.66) and the companion spends about one-third of the orbital period of 789 years near periastron and the remaining two-thirds near apastron. A close approach occurred in 1936 when the distance dropped to about 0".5 and so the pair having reached an elongation will now start to close slowly, reaching around 1".2 in 40 years time and then widening to 7".2 in the 24th century.
According to the WDS the stars are both F dwarfs with the companion somewhat earlier than the primary. This is an unequal pair with the stars being magnitudes 4.8 and 6.2. The primary is a double-line spectroscopic binary and both the distant companions are field stars.
One of the finest sights in a small telescope in the southern hemisphere is gamma CrA (19 06 25.14 -37 03 48.5) whose binary nature was discovered by John Herschel from South Africa in 1834. With the separation ranging between 1".3 and 2".5 and a period of 122 years this is a good system to follow over a period of a few years. At the moment the companion is at 9 + and 1".35 with the position angle reducing at the rate of about 4 degrees per year. Like mu Cyg (above) it is an unequal pair of magnitudes 4.5 and 6.4. Hartung does not give any colours but Sissy Haas mentions that Gould finds pale yellow.
gamma CrA is 56 light years distant according to Hipparcos and located near the border with Sagittarius. The bright globular NGC 6723 appears in the same wide-field finder view when acquiring the star.
Bob Argyle - Double Star Section Director
