July 2018 - Double Star of the Month
On a straight line between alpha Oph and 93 Her, but two-thirds of the way towards 93, and therefore just in Hercules, is STT 338 (17 51 58.46 +15 19 34.9) a neat close pair which was discovered by Otto Struve at Pulkova.
At the time of discovery the stars of magnitudes 7.2 and 7.4 were separated by only 0".6 in position angle (PA) 223 degrees. Since then orbital motion has taken them almost 60 degrees retrograde in angle and the separation has increased to 0".8.
Sissy Haas, recalling T. W. Webb, describes them as them gold and green white, but in fact the latter term was abbreviated in Webb to mean Greenwich. A recent orbit by Dr. Jean-Louis Prieur and colleagues assigns a period of 1276 years.
In a small rectangular area of about 20 x 9 degrees, just below the Teapot of Sagittarius, is the constellation of Corona Australis. It has a number of attractive double stars two of which HJ 5014 (August 2009) and gamma CrA (August 2010) have already been described in this column.
Kappa CrA (18 33 23.13 -38 43 33.6) is a fine pair which was noted by James Dunlop and is number 222 in his catalogue. The stars are magnitudes 5.9 and 6.2 and the current PA and separation are 358 degrees and 21".5.
Dunlop's 1826 separation of 30" must be an error, as the stars appear to have common proper motion and Gaia DR2 also indicates that they are both around 695 light-years away. Not connected however, are two fainter and more distant stars, a 13.1 magnitude at 202 degrees and 33", and an 11.6 magnitude at 247 degrees separated by 96".
About 2 degrees following is lambda CrA (COO 227), a pair of stars of magnitudes 5.1 and 10.0 at PA 213 degrees and separated by 30". It is, nevertheless, a physical pair and DR2 gives distances of 205 and 200 light-years respectively, with similar proper motions. A third star (mag. 9.9) is at 51 degrees and 43".
Bob Argyle - Double Star Section Director