May 2026 - Double Star of the Month
Draw a line between iota and alpha Draconis. Take the midpoint and then move south-west by 1 degree. This will bring you to a field containing two F. G. W. Struve doubles, some 20 arc-minutes apart.
The brighter of the two and also the most western is STF 1882 (14 44 03.55 +61 05 53.7) which consists of stars with visual magnitudes 6.9 and 9.2 currently separated by 11".3 in position angle (PA) 0 degrees. Gaia DR3 indicates that the stars are at similar distances but the the errors on the parallaxes are several times larger than is normal for stars of this brightness which might indicate that there is further structure in the system. The Washington Double Star Catalog also lists a magnitude 10.5 at 37 degrees and 7".2.
The other pair is STF 1878 (14 42 03.25 +61 35 42.9) which is very similar to STF 1882 in terms of the component brightnesses. Here we have magnitudes 6.3 and 9.2 at 313 degrees and 4".0 with the stars closing with decreasing PA since discovery. In this case the components have almost identical parallaxes and small errors indicating that they form a physical system. A is DL Dra, a delta Scuti variable.
Zeta (ζ) Lupi (15 12 17.0 -52 05 57.7), also DUN 176, is a spectacular pair. Zeta is the most southerly of the bright naked-eye stars in Lupus lying about three degrees north of the border with Circinus.
It is a K giant of visual magnitude 3.5 and is accompanied by a F8 dwarf of magnitude 6.7 which is 72" away in PA 249 degrees. The stars lies 114 light-years away using the Gaia DR3 parallax for the fainter component. Although the primary has a similar parallax it is not the same as B within the observed errors which may be due to the brightness of the star or it may have an as-yet undisclosed duplicity. The almost identical proper motions would seem to mark this out as a physical pair. The primary is a KO giant and its companion an F8V which led Ross Gould using 175-mm at x100 to report colours of deep yellow and dull brown-yellow.
Closely north and east is the pair HLD 121 which is now closing from the 3" separation which was measured at discovery in 1882. In 2016 the stars were 2".2 apart in PA 214 degrees.
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.
