March 2025 - Double Star of the Month
The spectral type of the primary of STF 1689 (12 55 30.32 +11 29 46.4) is given as M4III in the Washington Double Star catalog (WDS). One might expect to see a deep-orange, or red star but observations of the pair in the literature seem to indicate that the primary colour reflects an earlier spectral type.
T. W. Webb notes yellowish, whilst Admiral Smyth recorded pale white. All three observers noted that the companion was blue or bluish. More recently, I recorded orange and blue using a 10-inch reflector at x96 in 1970. Sissy Haas gives peach-white, and a note on TheSkySearchers website records colours of orange and bluish.
Gaia has observed both components and the stars are unrelated. The primary is 960 light-years away whilst B is 474 light-years distant. The components are slowly separating due to the different proper motions and in 2019 the relative position was 222 degrees and 30".3. The pair can be found 2 degrees NNW of epsilon Virginis.
R 213 (13 07 24.30 -59 51 37.8) was discovered in 1874 by Henry Chamberlain Russell. Russell was the Government Astronomer at Sydney Observatory from 1870 until 1905. He was using a 11.5-inch refractor made by Schröder to carry out a survey of some of the John Herschel double stars which the latter discovered at the Cape. It was one of the most difficult of his discoveries being found at 207 degrees, 0."3. His description of the stars was a very beautiful double star - both alike and orange coloured
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When I observed the pair in 2016 using the 67-cm refractor at Johannesburg, the PA was very similar but the distance has increased to almost 0".8. With WDS magnitudes of 6.6 and 7.0, the pair should be divided in 15-cm. R 213 lies in Centaurus close tothe border with Crux and can be swept up by moving 2.5 degrees due east from beta Crucis (Mimosa), which is DUN 125 - magnitudes 1.3, 7.2, 23 degrees, 373" - a fine binocular double.
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.
