June 2026 - Double Star of the Month
In mid-evening in June, Hercules is riding high in the sky and the more southerly constellation of Ophiuchus is approaching its best position for observing.
Hercules contains a good collection of visual double stars which we have sampled in this column in recent years. A pair which has so far eluded my attention but which I shall be attempting is 52 Her (16 49 14.21 +45 58 59.9).
This star first revealed its duplicity to S. W. Burnham in 1878 when he was using the Dearborn 18.5-inch refractor. The separation was then 1".8 and is now 2".0, but the position angle (PA) of the companion has also changed by 100 degrees to its current value of 45 degrees.
The main problem with resolution is the brightness of the companion. Burnham thought it was 10.5 and the Washington Double Star Catalog (WDS) gives 8.5 but that source also notes that The magnitude of B and C can be quite uncertain
. Considering the primary's V magnitude of 4.8 this pair becomes quite a challenge.
Gaia DR3 includes it and gives G = 8.96 but there is no astrometric information in that catalogue. There are three faint field stars of magnitudes 12.5, 12.7 and 14.0.
Tau Ophiuchi (18 03 04.91 -08 10 48.9) is a beautiful binary right on the border between Serpens and Ophiuchus and about 10 degrees south of that well-known pair 70 Oph.
Tau has a period of 279.5 years and is currently 1".4 apart in PA 292 degrees. The magnitudes are 5.3 and 5.9 and the stars are slowly closing whilst the PA is slowly increasing. The WDS notes a magnitude 11.2 field star at 125 degrees which is separating quickly from the main pair and is currently over 100" away. This pair has been measured on seven occasions since 1990 with the Cambridge 8-inch refractor and the PA has increased by just 13 degrees over a period of 29 years.
Within about 1 degree of tau are two globular clusters. To the NE is NGC 6539 and to the SSW is NGC 6517.
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.
