August 2025 - Double Star of the Month
About 3 degrees north of zeta Aql (V = 3.0) star atlases show a close pair of stars of around 6th magnitude which are aligned almost E-W. This is STTA 177 (19 12 34,45 +16 50 47.2). The stars were 121" apart in 1874 but are closing and are now near 97", still making them a good target for binoculars.
The A star is of interest as it is both a close visual binary but it is also an eclipsing binary of the Algol type with a period of 3.5 days. When S. W. Burnham was prowling the skies looking for new pairs with his 6-inch Clark refractor one of the stars he alighted on was this one. He found a pair of stars with magnitudes 7.1 and 8.0 separated by only 0".5. There has been little motion since then. I managed to measure them once in 2012 with the Cambridge 8-inch when the measure gave 0".7 but more recent measures put them at 0".6. There are two fainter and unrelated stars in the field.
Corona Australis sits below Sagittarius and on the sky it occupies a rectangle with sides of about 15 degrees in RA and 7 degrees in Dec. About 4 degrees east of the centre of this box and 1 degree south-east of mu CrA (V = 5.2) lies HJ 5066 (18 50 58.65 -41 03 45.7). It is marked as double in the second edition of The Cambridge Double Star Atlas but it is not labelled. The primary is a magnitude 6.5 B giant and its companion of magnitude 9.2 lies 10" away in PA 85 degrees.
Move another degree SSE and you will arrive at HDO 291, 6.1, 11.4, 341 degrees, 37" which is separating fairly rapidly due to the difference in proper motion between the two stars, the primary of which is a K giant. In one of his last discoveries, Robert Innes in 1926, using the then new Johanneburg 26.5-inch refractor, found B to a close equal pair (10.2, 12.2, 292 degrees, 0".6). A recent version of the Washington Double Star catalog (June 2025) records this discovery observation as the only observation.
Bob Argyle - Double Star Section Director
