Double Star of the Month - December 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.

Epsilon Per (03 57 51.2 +40 00 36) With the newly invigorated Comet Holmes causing a stir in observational circles, this month's northern double star is in Perseus where the comet has spent much of the last few months. Epsilon is located in the east of the constellation about 15 degrees following Algol. Coincidentally, its primary star - a hot B dwarf is also an eclipsing system in the Algol class. Almost due north, about 9 arc seconds distant is the companion, an A2 dwarf and so it might be expected that both stars would appear white in a small telescope. Hartung, however, notes star B to be slate gray whilst Smyth in 'Celestial Cycle' records that it is lilac. The latter also informs the reader that John Herschel suggested putting a piece of paper at the centre of the objective, (Smyth used a 2-inch stop on his 5.9-inch refractor), in order better to see the faint companion which is 6 magnitudes fainter than A. This is a relatively distant system - the revised Hipparcos catalogue, produced recently by Dr. Floor van Leeuwen gives a parallax of 5.12 +/- 0.22 mas corresponding to 630 light years.

Jc 8 (03 12 25.7 -44 25 11) In 1835 John Herschel found a close double star at this position and gave it his catalogue number 3556. He had trouble in making out that it was wedge-shaped and estimated the position as 230 degrees+/- and 1.5 arc seconds. In March 1856 Captain W. S. Jacob in Madras, whilst making measures of southern pairs with his 6.3-inch refractor made by Lerebours and Secretan, suspected the A component of being double in the S direction. This has turned out to be a rapid visual binary with a period of 45.2 years with the separation varying from 0.04 to 0.75 arc seconds. Currently the separation of AB is 0".70 and the pair is beginning to close again, whilst that of AB-C has widened to 3".7 making the whole group an attractive sight in a 20-cm telescope or bigger.

Bob Argyle - Double Star Section Director