Double Star of the Month - June 2007

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.

sigma CrB (16 14 41.04 +33 51 31.8) was discovered by William Herschel on 1780 August 7 and appears as the third entry in his class I pairs (separation 0 - 4"). The components passed within about 1".2 seconds of each other in 1830 and have been widening ever since. The maximum distance according to the 889 year orbit of Scardia occurs in 2245 at 9". Victorian observers suspected changes of colour and magnitude. Webb, in `The Intellectual Observer' for 1863 (page 134) offers the following summary: `Creamy-white and smalt-blue. There is much discrepancy about the smaller star. Struve calls it ``certainly not blue'', and differing very little from the other, 1825; Struve, white 1836.69; Dembowski, yellow, ashy, and doubtful blue, 1854 to 1857; Secchi, sometimes blue, sometimes yellow, 1855 to 1857. I fancied it, with a 3.7-inch object-glass, at one time ruddy, at another bluish, from 1850 to 1855, apparently changing even while being looked at; a versatility of hue which I have remarked on other stars similarly circumstanced, and which may possibly depend upon equal sensitiveness to colour in different conditions of the retina; during a short glimpse with 5.5-inches, 1862.57, the companion seemed bluish; at the same time I thought, as I had done in former years, that there was more than 0.5 mag of difference. Struve gave more than 1 mag. Secchi's discordances are considerable, ranging between 0.5 and 2 mags, from 1855 to 1857; but the honesty of that excellent observer, in recording every temporary impression, must be allowed for'. Small telescopes will show a fainter third star which does not belong to the system. In 1829 it was 44" distant from the main pair and last year measures put the distance at 90", due to the motion of nearby sigma (the Hipparcos parallax yields a distance of 21.68 parcsecs).

mu Lupi (15 18 32.05 -47 52 30.7) This beautiful triple system can be well-seen in apertures of 10-cm and above. Strangely John Herschel does not mention the third component is any of his observations made with the 7-foot and 20-foot telescopes from Feldhausen, yet star C is mag 7.1 compared with the 5.1 and 5.2 of A and B. The close pair is clearly binary but the period is several hundred years since the motion in the last 180 years is 40 degrees retrograde whilst separation has halved to its current value of about 1".0. Hartung thought the distant star reddish. This star is also a physical member of the system since Hipparcos assigns it the same parallax as AB - 11.22 mas - corresponding to a distance of 90 parcsecs.

Bob Argyle - Double Star Section Director