Double Star of the Month - November 2006
In this new 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.
Well placed in mid-November in the north is the constellation of Aries, its three brightest stars nestling midway betweeen the Square of Pegasus and the Pleiades.
The brightest is Alpha Ari, V=2.0 also known as Hamal. It is a K2 giant at a distance of 20.2 parcsecs.
Beta Ari (Sheratan, V=2.65) is a close pair - it was first found as a spectroscopic binary over a century ago by Vogel at Potsdam. More recently, it has been resolved by long base-line interferometers. The apparent orbit is very eccentric and the separation varies between 4 and 60 milli-arcseconds.
The primary is an A5 dwarf and the difference in magnitude is 2.6. The system is 18.9 parsecs distant.
But for the small telescope user, the best double star in Aries is gamma, first found by Robert Hooke in 1664 whilst following a comet. Also known as Mesarthim the proper motion of star A is about 0".1 per year but as the relative positions have changed by less than 2 arc seconds over the past 170 years, it can be assumed that this is a long period binary. Both appear to be pure white and can be separated in a 2-inch telescope or possibly in stabilized binoculars. The separation has slowly diminished since the first measures (8".6 in 1830) and is now 7".6 with the position angle almost exactly 0 degrees.
Fornax occupies the area of sky from about 2 to 4 hours RA and between declinations -23 and -39 approximately. It is more notable for extragalactic objects - hosting as it does the Fornax cluster of galaxies, the Fornax dwarf gakaxy and NGC 1365, a spectacular barred spiral galaxy. The brightest star alpha For, a yellow F6 subgiant at 03 12.1 -28 59 (2000) is of considerable interest to the double star observer. Found by John Herschel in 1835 (HJ 3555) at PA 310 and separation 5".30, the pair continued to close and was single to Robert Innes at Johannesburg using a 9-inch refractor in the first years of the last century. It was not until 1925 that the companion appeared on the other side of the primary at a distance of 0".8 when van den Bos found it to be an easy object, but the following year, as it widened, it seemed to have faded by as much as 1 magnitude. Hipparcos notes a range of 0.04 magnitude in the visual brightness of the system, which, if it occurs only in star B implies that B varies about 0.13 mag. This is small compared to observed variations which perhaps support a period of variation of decades. The Hipparcos mission lasted only 3 years. The apparent orbit is a very narrow elongated ellipse and the companion is currently now near maximum separation which will occur in 2042 so should be easily visible in a 20-cm telescope.
The period is about 290 years and the pair is 14 parsecs distant. Burnham identifies the star as 12 Eridani in his General Catalogue. The colours have been noted as yellow and greenish.
Bob Argyle - Double Star Section Director