September 2017 - Double Star of the Month
Four degrees south-west of ζ Cep and also 4 degrees SSW of the Garnet star μ Cephei is the fine pair STF 2840 (21 52 01.02 +55 47 48.3). First noted by William Herschel in 1782 this wide and easy pair of white stars can be seen in small telescopes.
The stars, of V magnitudes 5.6 and 6.4, are currently 18 arcseconds apart in position angle 192°, and they have closed up up by just 3 arcseconds since being measured by Struve at Dorpat in 1830.
The stars have been observed by the Gaia satellite which gives the distances for A and B as 5.18 mas and 6.06 mas but the error bars (± 0.57 mas) just overlap which does not rule out the possibility that they are physically connected, although the declination proper motions of the two stars are significantly different.
There is a mag 13 star 55 arcseconds away in position angle 346°.
29 Aqr (22 02 26.49 -16 57 57.8) is 4 degrees east and 1 degree south of Deneb Algedi (δ Cap). A discovery of Sir James South it boasts an interesting colour combination.
The bright components both of magnitude 7.2 have a rare coupling of spectral types, A2V and KOIII. The Dutch astronomer Willem van den Bos noted that they were orange and white in 1963 and Ernst Hartung, also observing from the southern hemisphere makes them white and yellowish.
If binary, then this is a long period system, the most recent relative position in the WDS from 2011 is 247° and 3".8, having closed up from 4".5 at the time of South. I measured the pair last year and got a distance of 4".4 over 3 nights but with a large scatter.
The star is very low from the UK so the magnitude 13 star at 291° and 143 arcseconds may well be a challenge too far.
Bob Argyle - Double Star Section Director