Double Star of the Month - March 2008
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.
The two stars this month are both relatively difficult pairs to see. They have periods of over 100 years but the apparent separations vary quite widely throughout the complete orbit.
omega Leo (09 28 27.4 +09 03 24) William Herschel found this pair in 1782 and catalogued it as number 26 in his first class of double stars. He gave it a position angle of 110.9 degrees and estimated the separation at 1". By the time that F. G. W. Struve observed the star in 1825 the star had advanced 43 degrees in angle with unchanged distance. By 1838 Struve could only elongate it and the modern orbit of 118.227 years by van Dessel predicts a separation of 0".71. In 2008 the star is almost back to its discovery position so here is a chance to see it as Herschel would have done. The magnitudes are 5.69 and 7.28 which add considerably to the difficulty of measuring it, and the revised Hipparcos parallax is 30.12 mas with an uncertainly of 0.71 mas.
delta Vel (08 44 44.2 -54 42 31) When Robert Innes lived in Sydney at the end of the 19th century, he used a small refractor in a search for new double stars. One of the first was delta Velorum, a mag 1.9 star which turned out to have a 5th magnitude companion at a distance of about 2" and a PA at about 170 degrees. In fact, delta Argus (as it was then) was first found by Solon Bailey in Arequipa, Peru in 1894 using the 13-inch Harvard refractor but Innes was first into print and thus gained priority. The star closed slowly until the early 1950's after which there were no observations until Hipparcos in 1991 with exception of one observation in 1978 which it is now believed is of the Innes companion but at first was thought to be a 3rd component. The Hipparcos observations showed the pair at 0".7 and widening, having been close in the 1980's. An orbit by Andreas Alzner with the benefit of a speckle measure made in 1999 showed the companion swinging around the end of its long apparent ellipse and heading back for its discovery position. The period is 142 years and in 2008 it will be at 319 degrees and 0".66, a difficult object for a 30-cm telescope. It is also now known to be the brightest eclipsing binary in the sky, a 45.16 day period with a primary dip of about 0.4 mags having been found by Sebastian Otero in 1997. A faint John Herschel pair at a distance of 69 arc seconds shares the proper motion of delta so this is a quintuple system some 25 parsecs from us.
Bob Argyle - Double Star Section Director