July 2015 - Double Star of the Month

lambda Ophiuchi - STF2055 (16 30 54.84 +01 59 02.8) lies 20 degrees due south of the south-easternmost star in the Keystone of Hercules, beta Herculis. During the 19th century, measurements showed the two stars slowly separating and reaching a maximum distance of about 1".9 in the early 1880's. In 1906, Thomas Lewis, in his book on the Struve stars, considered that the period was about 130 years which as it turns out was a good attempt. The current period from observations of almost 2 revolutions gives a period of 129 years and a predicted position for mid-2015 of 42° and 1".43. The pair is not particularly easy to observe, partly because of its altitude in the sky - it is barely above the celestial equator - and partly because of the magnitude difference between the two components (4.2 and 5.2). Both stars are A0 dwarfs, according to the WDS, and lie at a distance of 173 light years. There are two much fainter stars at 120" and 308" which appear unconnected to the system.

DUN 219 (17 58 55.69 -36 51 30.2) lies not far from the spectacular open cluster Messier 7 and follows G Sco by about 2 degrees. It has not been included in either Hartung or the book by Sissy Haas but is nevertheless is a splendid sight. The stars are magnitudes 5.8 and 7.8 and the separation derived by Dunlop in 1836 of 47".1 has now increased to 53".5, (with the position angle changing from 265° to 252° over the same interval of time), making it a striking object object in small telescopes. The USNO include the pair in their linear elements catalogue, confirming that the two stars are entirely unrelated. A third star of mag 11.3 can be found at 40" from A. Just 16 seconds of time following and 8 arc minutes south is HJ 5000 AB, stars of magnitudes 7.1 and 8.9 separated by 7".3 but apparently in orbital motion. The writer measured this pair in 2010 and noted pale yellow and pale blue - pretty.

Bob Argyle - Double Star Section Director