August 2019 - Double Star of the Month

16 Cyg (19 41 49.1 +50 31 32) is a beautiful, wide double star easily found by heading due north from delta Cygni, itself a bright although unequal and much closer pair (see the column for Aug 2011).

16 Cyg is a much measured pair and the Washington Double Star (WDS) observations catalogue has almost 600 entries.

It is clear from the astrometry of both stars given by the Gaia DR2 mission that they at the same distance from Earth (A is 68.8 light years away whilst B is 69.2). It seems certain that they form a binary system of long period where here long is taken to mean anywhere from 18 centuries to 485 centuries. This is the range of possible solutions from three different research groups.

In 1998 a star of visual magnitude 13, thought to be a M dwarf was found just 3 arc seconds from A and it is clear that it shares the large space motion of the bright stars and is therefore physically associated with A. In 1996 a planet associated with A was independently found at Lick and McDonald observatories which has a period of 2.19 years.

16 Cygni is a binocular pair but best seen in telescopic apertures. The components are spectral types G1 and G3, so slightly larger and more luminous than the Sun, and they shine with magnitudes 6.0 and 6.2. Smyth calls them pale fawn colour, whilst Webb just notes that they are yellow.

AC11 (18 24 57.2 -01 34 46) is in Serpens, about 2 degrees north-west of the 3.3 mag eta Serpentis. It was found by Alvan Clark on 30th July 1854, and reported by W. R. Dawes to the Monthly Notices of the RAS. "A very difficult object", he reported, "though decidedly elongated with a 7.5-inch aperture".

The stars are of magnitudes 6.7 and 7.2 and at present this 248 year binary is just closing from maximum separation. The orbital position in late 2019 is 354 degrees and 0".8 and it is well seen in the Cambridge 20-cm refractor. It remains above 0".6 for another 20 years or so and then dives down to about 0".03 towards the end of this century.

Continue another 2 degrees NW to find 59 Ser, a pretty pair separated by just under 4".

Bob Argyle - Double Star Section Director