April 2020 - Double Star of the Month

Six degrees NE of delta Leonis is 54 Leo (10 55 36.80 +24 44 59.0). It is an attractive and brightbut unequal pair (V magnitudes 4.5 and 6.3) which is well seen in small apertures.

It is H 3 30 in William Herschel's catalogue and the great observer noted the colours as white and ash-colour or greyish-white. Struve catalogues it as STF 1487 and Admiral Smyth found them white and grey whilst Webb noted greenish-white and blue and in 1972, I recorded white and blue using a 25-cm reflector.

Gaia DR2 finds that the parallaxes for A and B are respectively 9.83 and 10.17 milliarcseconds corresponding to distances of 332 and 321 light-years, although the error on the A component parallax is 12 light-years, indicating the possibility of another star in the system. In 2018, a measure made with the Cambridge 8-inch refractor put the stars at 114.3 degrees and 6".5.

The most southerly double star to appear in this series so far is iota Octantis (12 54 58.80 -85 07 24.1).

It came to light as a double star in April 1935 when Robert Rossiter was surveying the sky with the 27.5-inch refractor at the Lamont-Hussey Observatory in Bloemfontein, South Africa. He found a pair of stars with magnitudes 5.5 and 6.3 at 0".67 and numbered the pair RST 2819.

Unless the stars have widened significantly, it is difficult to understand why they were not found before by southern observers. Perhaps the closeness of the stars to the southern celestial pole where traditional refractors are hard to maneouvre played a part.

Since discovery the stars have moved just 13 degrees and shown little motion in separation. The last observation in 2006 put them at 240 degrees and 0".7, and they were elongated by Ross Gould with 17.5-cm; he noted the primary was orange. In 1941 Willem van den Bos added a third star of mag. 10.9, currently at 53 degrees and 62".

Bob Argyle - Double Star Section Director