August 2024 - Double Star of the Month
33 Cygni lies in northern Cygnus, near the border with both Cepheus and Draco. It forms one apex of a triangle whose sides are 1.5 degrees long with two other stars, both of which are visual doubles. The star which is directly east of 33 is STF 2687 (20 26 23.47 +56 38 19.3). This is an easy pair for the small aperture with the stars of magnitudes 6.3 and 8.1, separated by 25".
The other member of the triangle is STF 2671 (H I 95). This is a brighter but closer pair than STF2687. The main components have magnitudes 6.1 and 7.5 and the current position of the fainter component is 337 degrees and 3".8, coordinates which have changed very little over 200 years. There is a 12.4 magnitude star at 53 degrees and 90" and two further 13th magnitude field stars.
The second edition of The Cambridge Double Star Atlas shows the pair HJ 2975 in Capricornus about 5 degrees south of, and slightly following, the bright triangle of visual double stars formed by omicron (ο) Cap, rho (ρ) Cap, and pi (π) Cap (see this column for August 2015, 2016 and 2017 respectively). It is a fairly difficult pair for small telescopes as the Washington Double Star catalog (WDS) gives magnitudes of 7.5 and 11.6 and a separation of 11".
The Atlas also shows that the star immediately north preceding by about 30 minutes is also double. This is a brighter and easier object (magnitudes 7.8 and 8.1) which does not warrant a label because the stars are 39" apart, but it is HJ 2973 (20 32 13.27 -22 09 17.6), a pair of yellowish F stars, one of which was found to be a close double at a lunar occultation in 1983.
These Herschel pairs were discovered using the 20-foot reflector from Slough in 1830/1, and John Herschel allocated magnitudes of 8 and 14 to HJ 2975 and 8.9, 8.9 to HJ 2973.
Bob Argyle - Double Star Section Director
If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Double Stars of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.
