September 2026 - Double Star of the Month
The second edition of the Cambridge Double Star Atlas shows that southern Cepheus is quite rich in visual double stars. Starting from the brightest star (alpha, V = 2.5) move 2.5 degrees south and slightly west and you will alight on an unequal, wide pair of stars. The brighter of the two is STF 2780 (21 11 48.24 +59 59 11.8) which can be divided at present with 15-cm.
The stars have magnitudes 6.1 and 6.8 and are undergoing majestically slow orbital motion, the position angle has reduced from 225 degrees in 1828 to 211 degrees today. The separation has remained unchanged at 1".0. The same 15-cm will catch the 8.9 magnitude field star at 211 degrees and 119" and the 10.3 magnitude field star at 46 degrees and 62" but not the 14.2 star at 260 degrees and 96".
Rather disappointingly Gaia DR3 has no parallax data for this system but Hipparcos placed the stars at a distance of 4,300 light-years but with an uncertainty of about 2,400 light-years.
53 Aquarii (22 26 34.30 -16 44 31.9) has been described before in this column in September 2010.
It displays a remarkably large apparent orbit since the stars were more than 10" apart when found by James South and John Herschel. Since then the separation had reduced to around 1".3 and at present the stars are near their closest and consequently moving most quickly. In 2010 the PA was 45 degrees and the separation was 1".4 but a measurement made last year put them at 117 degrees and 1".3 with the two components now just beginning to widen. The stars are almost equally bright (6.3 and 6.4), even so they are often difficult to measure from the UK as the low altitude usually results in poor seeing.
Again, rather surprisingly, given the separation there is no parallax in Gaia DR3 but Hipparcos tells us that this is relatively nearby system with a measured distance of 63 light-years.
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.
