July 2016 - Double Star of the Month

39 Dra (17 57 12.56 -30 22 24.80) can be found about 4 degrees following and slightly north of xi Dra in the 'head' of Draco. It is also known as b Dra.

The binocular user will see a pair of mag 5.1 and 8.0 stars separated by about 89". Any aperture larger than 6-cm will show the companion to A which William Herschel found in 1780 and allocated to his class 1 i.e. between 0 and 2". Since that time the pair has widened with increasing position angle and in 1993 was found at by the writer at 351° and 3".9. The AB stars are magnitudes 5.1 and 8.1 so it requires good seeing to see B clearly.

This is a quintuple system since all 3 bright stars are physically connected and both A and B are spectroscopic binaries. Surprisingly, although the motion in AB amounts to just 25 degrees in 230 years, a highly speculative orbit with a period of 3962 years appears in the USNO Sixth Orbit Catalogue.

In October 2011, John Nanson found the primary was a distinct yellow watered down by a weak touch of white, I could see a slight tinge of blue in "B" and "C" was just weakly white. There is much more information on this system on the Starsplitters website.

PZ 6 (17 59 05.28 -30 15 10.8) is a beautiful pair which lies about 1.5 degrees due west of gamma Sagittarii. In the 1st edition of the Cambridge Double Star Atlas it is given as PZ 4 but is corrected to PZ 6 in the second edition.

An observation of this pair on the evening of 5 September 2013 using the Johannesburg refractor showed the colours of the stars to be deep yellow and lilac.

The WDS gives spectral types M1Ib and G8II so this is a rare pairing of a supergiant and a giant. There is little motion between the two stars and the primary star sits a little over 1,000 light years away but with an uncertainty in the distance of 30%.

The stars are magnitudes 5.4 and 6.0, and the current separation of 5".7 appears to indicate that the stars have been slowly closing since the first measure in 1826. A 13.2 mag star at 25" was measured by the writer in September 2013.

By moving the telescope another 1.5 degrees further west, and crossing the border from Sagittarius into Scorpio, the observer will come across PZ 5 - another bright wide pair easily resolvable in a small aperture. The stars are mags 6.7 and 8.2 and both white - the spectral types are A3 and B9. Note, however, that PZ 5 is not labelled in CDSA 2 whilst it is in the first edition. The position in 2013 as determined by the writer was 104° and 5".7. On the same night a mag 11 companion was also noted at a distance of 49".

Bob Argyle - Double Star Section Director