Double Star of the Month Archive 2020

In this series of short articles, a double star in both the northern and southern hemispheres will be highlighted for observation with small telescopes, with new objects being selected for each month.

January 2020 - Double Star of the Month

118 Tau (= STF 716) is well-placed for observing in mid-evening (RA 05 29 16.49 +25 09 01.1) and can be located almost half-way between the stars forming the points of the 'horns' of Taurus, (zeta and beta Tauri).

It is a distinctly neat pair for the small aperture - 10 or 15 cm will show it very well. First noticed by Herschel (H 2 75) this pair of B and A type dwarfs appeared white and pale blue or white and bluish to the early observers.

With little change of separation over 200 years (it is currently 4".7) the position angle has increased by 27 degrees to its current value of 210 degrees and Gaia DR2 shows that the parallaxes of the components are similar but not identical, being 8.96 mas ± 0.10 mas for the magnitude 5.8 A, and 8.42 ± 0.12 mas for B (magnitude 6.7).

In late 2002 Roberts and colleagues found faint companions to both A (at a distance of 1".7) and B at a distance of 1" but no confirmatory measures have yet been made. There is a 11.9 magnitude field star 140" away in PA 99 degrees.

The constellation of Lepus is unfortunately low in the sky for UK observers but repays some attention as there are attractive double stars to be found, especially if the seeing is good.

38 Leporis (RA 05 20 26.91 -21 14 23.1) is 2 degrees WSW of beta Leporis and was discovered by John Herschel (HJ 3750) from the Cape in November 1835 with the comment A most beautiful double star.

The magnitudes are 4.7 and 8.5, whilst the stars appearing to be slowly widening. In 2015 they were at 279 degrees and 4".0. Observing from Victoria in Australia, Ernst Hartung found them pale yellow and white, as did Sissy Haas adding easily seen with 75-mm.

If the seeing is good then try beta Lep itself. This Burnham pair is magnitude 2.9 and 7.5 at 8 degrees, 2".7 substantially different from the discovery PA 268 degrees indicating significant orbital motion. Gaia DR2 appears to show a third component with G mag 8.68 (similar to V) at a distance of 1".3 which may be new.

Bob Argyle - Double Star Section Director