Double Star of the Month - September 2007
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.
61 Cygni (21 06 50.84 +38 44 29.4) Although observed by Flamsteed in 1690, the duplicity of this star seems to have been first noted by Bradley in 1753. A recent orbit by Kiseleva indicates that the separation of the two stars would have been around 10 arc seconds for 1690 - certainly within Flamsteed's resolving power. He did, for instance, see the third star in the zeta Cancri system in1680 when it was 6 arc seconds from AB.
Piazzi discovered the large common proper motion in 1806, and Bessel then made his pioneering measurement of the parallax of 61 Cyg A indicating that it was nearby. Even so, as late as 1891, Burnham was not convinced that the pair was a binary one, being discouraged by the large separation which amounted to 21 arc secs at that time. At present this has widened to almost 31 seconds and, according to Kiseleva, reaches 34.3 secs in 2106 before closing to a minimum of 9 arc seconds around 2350. Long series of astrometric plates taken during the last century led to speculation that star A was accompanied by a planetary mass companion in a short period but this has not yet been confirmed.
The pair is a very attractive one for the small telescope user. The stars are both late K dwarf stars and hence orange in hue, although Hartung gives colours of orange and red. Hipparcos finds parallaxes 287.1 and 285.4 milliarcsecond for A and B with errors of 1.5 and 0.7 mas respectively.
theta Indi (HJ 5258) (21 19 51.9 -53 26 57.4) John Herschel remarked `Beautiful' as he recorded this pair found with the 20-foot reflector at Feldhausen on sweep 468 dated 1834 Jul 8. He gives the magnitudes as 6 and 10 and notes that it is number 7003 in the Brisbane catalogue. He later made four measures of position angle and two of separation to give a weighted mean measure for 1834.51 of 307°.0, 3".67. Modern catalogues such as the WDS list the magnitude as 4.5 and 6.9, a significant difference in the brightness of star B, but the distance has now almost doubled and it may be this is why the companion looks brighter in recent years.
There is no doubt that this is a physical system as the proper motion, common to both stars is 0.13 arc seconds per year and the parallax is 33.58+/- 0.76 mas equivalent to a distance of 97 light years. Hartung found the stars were pale yellow and distinctly reddish.
Bob Argyle - Double Star Section Director