July 2014 - Double Star of the Month
zeta Her (16 41 17.46 +31 36 07.0) is part of the Keystone of Hercules, the south preceding component of the four stars in the pattern. Only 35 light years from us, it has long been known to be a close and difficult pair, but at the time of writing it is opening up and will soon be as easy to resolve as it gets. The ephemeris for the 34.5 year period orbit shows that in summer 2014 the stars are at 140° and 1".20. The difficulty comes with the large difference in magnitude - in the visual the components are magnitudes 2.95 and 5.40. The writer has followed this pair since 1990 and has been able to measure it each year since apart from 2001 - 2004 inclusive when it was too difficult for the 20-cm Cooke refractor at Cambridge. Over many years there have been suggestions of a sub-period due to one of the stars being a close, unresolved pair, and in 1983 a third component was detected in the infra-red but since then no confirming observations have been made and at present it is assumed that zeta is a simple binary star. The primary star is of spectral type early G and sometimes appears orange to observers with the companion appearing green by contrast.
eta Oph (17 10 22.66 -15 43 30.5) was one of S W Burnham's later discoveries and is also known as BU 1118. This bright, twin pair of white early A stars of visual magnitudes 3.05 and 3.27 was separated by 0".4 at discovery in 1889. Like zeta Her above this pair is now close to its maximum separation and is actually starting to close. In summer 2014 it will be found at 232° and 0".57. This needs 30-cm on a night of very good seeing as it is low from the UK. The orbital motion accelerates rapidly as periastron approaches in 2024 at which time the stars are 0".006 apart and moving at 15 degrees per DAY. The period of this highly inclined and very eccentric (e = 0.95) orbit is 88 years. There are two faint comites of magnitude 11.2 and 12.4 both about 100" distant. Eta (combined magnitude 2.4) can be found about 15 degrees north following Antares.
Bob Argyle - Double Star Section Director