Double Star of the Month - June 2012

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.

The two systems featured this month are both unequal and difficult pairs which need at least 30-cm to see well. Both stars are also at the same distance from us - about 140 light years.

gamma CrB (15 42 44.57 +27 17 44.3) is number 1967 in F. G. W. Struve's catalogue of 1837. This has always been a hard object to see in the 8-inch OG at Cambridge but on nights of good seeing the companion can be measured, most recently in the summer of 2010 when the separation was found to be 0".7. The stars are in a very elongated and inclined 91 year orbit so that most of the motion is in distance. At the time of writing the pair is closing again and the separation has reached 0".56 at PA 111° but the stars are magnitude 4.0 and 5.6 which makes resolving the pair more difficult. Check the nearby eta CrB first before attempting this pair. It is slightly wider and more equally bright. If this does not jump out as a clear double then its unlikely that gamma will!

See 258 (16 03 32.22 -57 46 29.5) is also known as iota 1 Normae and it sits half way between zeta Arae and beta Circini. It has a short period for a visual binary - 26.9 years and at the middle of 2012 can be found at 213° and 0".33 but after this the stars close further. The two components are relatively bright (5.2 and 5.8), even so a night of good seeing will be necessary to see any sign that this is a double star. For the smaller telescope user there is a third star which forms HJ 4825 and which is currently at 241° and 11".2 having slowly decreased from 251° and 15" at discovery in South Africa in 1835. John Herschel measured this system again in 1837 and on the latter date the close pair would have been 0".42 apart but Herschel made no mention of it. Hartung notes that C is reddish compared with AB which are probably a pair of late A stars.

Bob Argyle - Double Star Section Director