July 2026 - Double Star of the Month
Ten degrees west of Altair is an equilateral triangle of bright stars, the brightest being zeta Aql (V = 3.0), followed by epsilon = 13 Aql (V = 4.0) and finally 10 and 11 Aql (respectively V = 5.9 and V = 5.3) at the south-west vertex.
11 Aql (18 59 05.23 +13 37 21.2) was found by William Herschel and entered his first catalogue of double stars as H 3 32. Herschel noted that the S(outhern) star hardly visible with 227, but pretty strong with 460'.
The stars have magnitudes of 5.3 and 9.3 but the companion is 20" away from A so it can easily be seen in 15-cm. Observations with a 21-cm reflector showed the colours to be light yellow and blue. The stars are unconnected as the fainter component is about five times more distant than the primary.
Incidentally, epsilon (ε) is listed in the Washington Double Star Catalog (WDS). The primary is a red giant and a spectroscopic binary but is of marginal interest to the visual double star observer. It has two faint companions each more than 100" distant.
54 Sgr (19 40 43.34 -16 17 35.3) was found to be a bright, wide pair by John Herschel from Slough in 1828 and is catalogued at HJ 599 AC. For the visual observer the Herschel pair is an attractive sight with stars of magnitudes 5.4 and 7.7 at 41 degrees and 45".
However, this is quite a complex system. The primary is an occultation double and stars B and D are 13th magnitude field stars whilst C is a close pair at 0".2 found by the VLT/SPHERE instrument, a high resolution camera and coronographic device which has been used for direct detection of exoplanets.
Gaia DR3 shows that A and C have very similar parallaxes and are 24 light-years away. Forty five arc-minutes SW is S 722 - 7.2, 7.5 at 236 degrees and 10".3. The meaures given for HJ 599 AC and S 722 were made by the writer in 2016.
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.
