June 2025 - Double Star of the Month
Two degrees of tau Her (V = 3.9) is STF 2063 (16 31 47.73 +45 35 53.8). This is a rather wide but also rather unequal pair with the two stars shining at magnitudes 5.7 and 8.7 respectively.
There has been very little motion since William Herschel noted them on 1782 Nov 8. and the pair entered his catalogue as H 4 62 and which he called very or extremely unequal noting colours of white and 'br' although it is unclear what this actually is. Bright red? Bluish-red? Brown?
I checked with Bruce MacEvoy's excellent complete catalogue of the Herschel double stars and he does not comment on the colours. I observed the pair with a 21-cm reflector at x96 and found colours of white and blue.
The primary has a spectral type of A1V, and the Gaia DR3 catalogue reveals the stars are at the same distance (230 light-years) and have very similar proper motions. The companion lie at PA 195 degrees and the stars are currently separated by 16".3.
A rather similar pair can be found in the southern hemisphere in the constellation of Ara. HJ 4978 (17 50 28.40 -53 36 44.7) is a little more unequal (5.7 and 9.2) and somewhat closer (12".3 and PA 268 degrees). The system, noted as nu1 Arae by John Herschel, lies 1.5 degrees due east of the globular cluster NGC 6397.
The SIMBAD catalogue notes that the primary star is an Algol variable called V539 Ara, it has a period of 3.17 days and the eclipse depth is 0.53 magnitude, but SIMBAD gives no reference to the star as a wide visual double.
Whilst in the area, check out the pair R 303 which is one degree SW of HJ 4978. The stars are magnitudes 7.8 and 9.0 with a separation of 3".6 and a PA of 108 degrees.
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.
