September 2020 - Double Star of the Month

STF2872 (22 08 36.04 +59 17 22.2) is located in Cepheus about 1¼ degrees north and slightly west of zeta Cep (a distinctly reddish star of magnitude 3.4). To the small telescope it appears as a wide pair. The catalogue magnitudes are 7.1 and 8 and the current separation almost 22 arc seconds.

Image of a finder chart for the double star STF 2872 in Cepheus
A finder chart for the double star STF 2872 in Cepheus created with Cartes du Ciel.

When F. G. W. Struve examined the system during his great survey at Dorpat he noted that the companion was a close pair with a separation of around 0".5. Since then the position angle has decreased about 40 degrees and the separation has slowly increased giving a position angle of 296 degrees and separation 0".80 in autumn 2020. I was able to measure this pair in 2016 with the Cambridge 20-cm refractor and it may be divisible in 15-cm. The 840 year orbit shows the stars slowly closing, reaching 0".2 in 100 years time, but for the foreseeable future a moderate aperture will suffice. Gaia DR2 does not show BC as two stars, rather surprising as many pairs wider than 0".5 appear as two entries in that catalogue.

Image of a finder chart for the double star 57 Aql in Aquila
A finder chart for the double star 57 Aql in Aquila created with Cartes du Ciel.

Surprisingly missing from Hartung's Astronomical Objects for Southern Telescopes, 57 Aql (19 54 37.65 -08 13 38.3) is one of the finest pairs in Aquila. The stars are of magnitude 5.7 and 6.4 and both are late B dwarf stars.

It might be expected that both stars appear white but there have been wide variations in reported colours. Webb in 1851 thought they were distinctly contrasted pale yellow and pale blue whilst he notes in 1855 cols. entirely diff. Smyth though both were pale blue whilst Struve reported both stars as white. I have often wondered if the colour of a star can appear to change if it is a spectroscopic binary, especially if the components have different spectral types and the orbits are almost edge-on to the plane of sight. The WDS notes tell us that both components are, in fact, SBs, whilst the paper by Chini et al. in 2012 notes both stars have constant radial velocities.

There has been no change in relative position in the last 200 years with the two stars fixed at 171 degrees and 35".5. William Herschel noted 29".5 in 1781 which may suggest a misreading of his micrometer screw. Gaia DR2 places them 440 light years away; within the search radius of 150 arc-seconds, I also noted a magnitude 20 star with nominal parallax slightly greater than 57 Aql A and B but with an error of 30%.

Bob Argyle - Double Star Section Director