September 2025 - Double Star of the Month
About 1.5 degrees NW of gamma Sagittae, the bright star at the tip of the arrow of Sagitta, is STF I 48, or more recently STFA 48 (19 53 22.65 +20 20 14.1). This is a wide and bright double star which is easily seen in small apertures.
It forms part of the additional list of wider pairs compiled by Wilhelm Struve during his Dorpat survey. It was, however, noted by William Herschel forty years earlier who dubbed it H V 106 (now H 5 106). I measured it in 2003 when the relative position was 147 degrees and 42".
There is a 10.5 magnitude star some 222" distant and more recently the Hipparcos satellite resolved the B star into a pair of stars with magnitudes 7.6 and 9.4 but with a current separation of 0".2 this probably requires 40-cm and a night of good seeing. It is a binary with a period of 85 years.
The double stars found by James Dunlop are bright, wide and sometimes colourful. DUN 236 (21 02 12,73 -43 00 07.7) lies in the constellation of Microscopium about 1.5 degrees SSW of the magnitude 5.5 star eta Microscopii which is an orange giant.
The stars (magnitudes 6.7 and 7.0) were 67" apart when found in 1826 and since then the separation has reduced by 10" although the proper motions are almost identical. The spectral types suggest colours of pale yellow and pale orange. The parallaxes are similar but that of the brighter component has a very large error which may mean that there is another component close in.
Move the telescope 4 degrees due east of DUN 236 and you will alight on MLO 6 (see this column for Sept 2012), a fine and unequally bright pair.
Bob Argyle - Double Star Section Director
