August 2018 - Double Star of the Month
Embedded in the Milky Way in Cygnus, about 2 degrees East and slightly south of 22 Cyg, is HJ 1470 (20 03 39.5 +38 19 38.3) a deep-red star which lies at a distance of 1630 light years (with an error of about 30 light years) according to the latest results from the Gaia mission (DR2). Simbad gives the spectral type as M0III and the star is about 225 times as bright as the Sun.
John Herschel noted a distant companion of magnitude 9.3. The Cloudy Nights website contains drawings of HJ 1470 and three nearby pairs which together form an arc of stars about 22' across and known as Chaple's Arc or the Fairy Ring. The other pairs are considerably less impressive.
An observer in the US using an 8-inch at x53 noted that the primary was strong yellow-orange/reddish and greyish-blue. At the beginning of 2005, I measured the pair with the Cambridge 8-inch. The result was 340 degrees and 28".6.
Browsing though Sissy Haas' excellent descriptive guide to visual double stars, I came across the pairs S 715 (19 17 39.96 -15 58 01.7) and S 716 (19 18 05.55 -15 57 13.4) which can be found in Sagittarius.
The brighter pair is S 715 where the two components have magnitudes of 7.1 and 7.9 and they are currently at 17 degrees and 8".4. Just 6 arc minutes preceding and 1 arc minute north is S 716 with magnitudes 8.4 and 8.6 at 194 degrees and 5".0.
Gaia has observed both pairs; each appears physically connected but the components of S715 are 480 light-years away whilst the stars in S 716 are both 1030 light-years distant.
S 716 is also known as Stone 46. Ormond Stone (1847-1933) was Director of Cincinnati Observatory where he found a number of pairs using the 11-inch refractor, in this case about 40 years after South first noted it.
I measured S 715 in 2016 with the Johannesburg telescope, but S 716 was not noticed, although it should have been clear in the 6-inch finder.
Bob Argyle - Double Star Section Director