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

September 2015 - Double Star of the Month

Nestling in the region about 4 degrees north following the upper left-hand corner of the Square of Pegasus, AC 1 (HIP1669) (00 20 54.10 +32 58 40.9) seems to have been a little neglected by the double star community. It doesn't even have a note in the WDS catalogue but things may change now that Henry Zirm has published an orbit for it.

It was discovered by Alvan Clark using a 7.125-inch object glass of his own make in October 1856, and published in MNRAS a year later with additional notes by W. R. Dawes. At the time of discovery it was 0".4 apart and Dawes was of the opinion that it escaped the attention of F. G. W. Struve at Pulkovo because it was too close at that time.

This view has been borne out - the stars are now easily measurable with a 20-cm refractor. The magnitudes are 7.3 and 8.3. Hartung notes that the stars are 'deep-yellow' and also points out the presence of a orange-red star some 4'.5 SW (HR 59 - spectral type K5III). Zirm's orbit with a projected period of 525 years is clearly provisional as the stars are close to maximum separation and only 12 degrees has been described in the apparent orbit. The author found 289°s; and 1".9 in autumn 2014.

Psi 1, 2, and 3 Aquarii are three bright stars of visual magnitude 4.2, 4.4 and 5.0 respectively. They can be found about 3 degrees east of the centre of the line joining alpha Peg to Fomalhaut, in a region rather low down from the UK but filled with interesting visual doubles.

Some 4 degrees directly below psi 3 is 94 Aquarii (23 19 06.51 -13 27 30.4) a fine, wide pair which is worth seeking out. Its mag 5.3 and 7.0 stars appeared yellow and orange to Hartung who was able to observe them close to the zenith whilst the Reverend Webb noted a reddish glare in A whilst B appeared greenish.

The proper motion of the pair amounts to more than 0".3 per year so the fact that the change recorded in separation over 200 years amounts to only 2 arc seconds tells us that this is a physical pair, and it is located 69 light years away. In 1976 McAlister and colleagues discovered that B was a close pair (MCA 74) and it has subsequently turned out to be a binary of short period. It rotates every 6.3 years and the separation is never more than 0".2.

Bob Argyle - Double Star Section Director