August 2026 - Double Star of the Month
The constellation of Equuleus sits between Delphinus and Pegasus. Its brightest star, alpha, is only visual magnitude 3.9. Move one degree south and four degrees west and you will alight on epsilon, or 1 Equulei (20 59 04.54 +04 17 37.8). This is a fine pair for the small aperture with star B appearing 10.6 arc-seconds away in PA 68. The magnitudes are 5.4 and 7.0.
However, when I started measuring this system in 1990, it appeared triple in the 8-inch refractor at Cambridge with a magnitude 6.3 star visible about 1" away from A. Over the next two decades the distance decreased until in 2011, I found that the stars were 0".4 apart and were subsequently seen as one image.
The period of AB is 98 years and the close pair is still closing and approaching a separation of 0".1. Ground-based interferometers in California and Chile have resolved the A component into two with a period of 2.3 days. The separation of these two components reaches a maximum of about 0.1 milli-arcsecond.
Just 1 degree west and slightly north is STF 2735—a beautiful pair (see the column for Sept 2018).
BU 245 (18 10 05.81 -30 43 43.3) can be found in the spout of the 'Kettle' that forms the outline of the brightest stars in Sagittarius, just 1 degree ESE of gamma Sgr (Alnasi).
An observation with the 26.5-inch refractor at Johannesburg in 2016 revealed colours of yellow and blue (the primary is a K1 giant) and measures on 2 nights gave a mean of 345 degrees and 4".1 showing a small increase in separation since discovery in 1874 by S. W. Burnham using his 6-inch Clark refractor. The position angle has also decreased by 10 degrees over the same interval. The two stars lie at a distance of 410 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.
