July 2017 - Double Star of the Month
The northern star (STF 1575, 11 51 57.57 +08 49 48.0) is one of the few doubles to be found in Virgo north of the equator. It is a William Herschel discovery (H 4 49) and since 1782 there has been little change in separation which is currently 30".4.
Hipparcos demonstrates that both stars have similar parallaxes and proper motions and Shaya and Olling in their 2011 paper have indicated that there is a near 100% chance of the stars being physical.
The magnitudes are 7.4 and 7.9 and the pair can be found a little more than 5 degrees due south of Denebola (beta Leo) and it forms the vertex of an isoceles triangle of stars which include 4 and 6 Virginis.
Thomas Lewis (1906) incorrectly calls this pair H III 51 and notes both components are white, which are the colours derived by F. G. W. Struve. In fact both stars have spectral type K0. Richard Harshaw using 20-cm finds orange and white.
About 1.3 degrees below the bright star Kaus Australis (lambda Sgr) there is a triangle of 5th and 6th magnitude stars. Each of these is a visual pair which can be seen in 15-cm. At the peak of the triangle is WNO 6 (18 28 57.76 -26 34 55.0) with magnitudes of 6.7 and 8.0 and which appears to be unchanged in over 100 years. The writer found 182.0 degrees and 41".83 in September 2016.
About 15 arcminutes preceding are the other two members of this triangle.
The northernmost star is BU 133 found by the great Americam observer on 1873, July 6 using his famous 6-inch Clark refractor. He estimated that the stars were both 7.5 magnitude and indeed all the early estimates show no difference in brightness between the stars and yet the WDS gives 6.6 and 8.5. The writer recently measured it (September 2016) and found 233.2 degrees and 0".77 which would make the pair a stiff test in 15-cm.
The remaining star is magnitude 6.7 and has a magnitude 8.7 companion at 54" and PA 135 degrees. This component was mentioned by Burnham and subsequently ignored.
Bob Argyle - Double Star Section Director