August 2022 - Double Star of the Month

In the column for August 2019, I described 16 Cygni, a bright and easy double foundabout 6 degrees north of delta Cygni. Starting with 16 Cygni, and moving west about 5 degrees a coarse triple of 6th magnitude stars will appear in the finder field. The easternmost of these is STF 2486 (19 12 05.03 +49 51 20.7) which consists of stars with V = 6.5 and 6.7 which are currently 7".1 apart in position angle 202 degrees.

Image of a finder chart for the double star STF 2486
A finder chart for the double star STF 2486 created with Cartes du Ciel.

This is a relatively nearby (82.5 light-years) binary pair whose period has been found to be 1459 years but clearly this is a preliminary value, since the position angle has decreased but 21 degrees since 1819. The proper motion of the stars is more than 0".6 per year, so they are rapidly leaving behind star D (V = 11.1) at 196", but rapidly approaching star C which is V = 13.2 at 27".

Sir William Herschel's third and last catalogue of double star discoveries includes many pairs which are low in the sky from the UK, and two of them were found close to omicron Sagittarii.

Image of a finder chart for the double stars H N 126 and H N 129
A finder chart for the double star double stars H N 126 and H N 129 created with Cartes du Ciel.

The first of these, H N 126 (19 04 20.28 -21 31 53.7), is the more difficult of the two. It is about 15 arc-minutes NW of omicron and consists of stars with magnitudes 7.9 and 8.1 which are currently 1".3 apart in PA 183 degrees. This binary has almost completed 270 degrees of its apparent orbit since discovery, has an orbit of about 502 years, and lies 168 light-years distant.

About 1.25 degrees due south of omicron is H N 129 (19 04 14.20 -22 53 47.5), significantly easier at 309 degrees and 8".3, but with two components which are considerably unequal (V = 6.9 and 9.2).

Bob Argyle - Double Star Section Director