June 2018 - Double Star of the Month
Easily found four degrees due west of alpha CrB, STF 1932 (15 18 20.19 +26 50 24.7) is a visual binary whose period is 203 years, so the current aspect of the stars closely resembles that at the time of discovery.
The separation around the cycle ranges from 0".6 to 1".6 and at the moment the stars are as widely separated as they get (2018.5, 266 degrees, 1".62). The magnitudes are nearly equal (7.3 and 7.4) and both stars are F-class giving a yellowish aspect to the observed colours.
Whilst in the area look at eta CrB (STF 1937), four degrees NNE which is now a test for 25-cm. The position angle is currently increasing 20 degrees per year and by the middle of 2018 will be at 246 degrees and 0".42.
Beta Scorpii (16 05 26.23 -19 48 19.4) skirts the southern horizon during the short northern summer nights. It was first seen as double by Benedetto Castelli in 1627 and was later catalogued by William Herschel as H 3 7. With the two bright components of magnitude 2.6 and 4.5 separated by 13".7 and 20 degrees the pair is not difficult even low down.
A century after Herschel, S. W. Burnham noticed a close and very faint companion to A about an arc second away and of magnitude 10. More modern measures show that this system has closed in considerably and that the estimated period is 610 years.
Slipher found that A was a spectroscopic binary and a lunar occultation observation of A in 1976 indicated another component at a distance of 0".1, but no further observations of this pair have been forthcoming. McAlister found that C was also a close binary with a period of 39 years and a separation of about 0".1. Take into account the distant magnitude 7.5 at 519" and 30 degrees from A and this is a physical sextuple star.
Bob Argyle - Double Star Section Director