Double Star of the Month - November 2011
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 constellation of Andromeda now rides high in the evening sky stretching across some 50 degrees of right ascension. One of its most famous double stars is gamma (see DSOTM for Dec 2006) but some 6 degrees north of gamma and slightly following is the binary system STF 228 (021402.43 +472903.3). The spectral types of the stars are given as F2V and F7V in the WDS with visual magnitudes of 6.56 and 7.21. The period is 144 years and at present the separation is beginning to decrease with the relative position for 2012.0 given as 295°, 0".8. By 2032 the distance will have reached 0".32. Hipparcos gives the parallax of 25.23 milli-seconds of arc (mas) with an error of 0.66 mas which places the system at a distance of 129 light years.
Whilst Andromeda is one of the largest constellations, Reticulum (The Net) is one of the smallest. Extend a line from alpha through delta Reticuli a little more than the separation of those two stars and you come to Dunlop 14 (033810.24 -594635.0). Visible in binoculars it repays examination through the telescope. The two stars, magnitudes 7.00 and 8.34, are both main sequence F stars and yellow in colour. Their separation was 45" at discovery in 1826 and by 2008 this has increased to 57".4. However, looking at the Hipparcos Catalogue reveals the fact that both the proper motions and parallaxes of the two stars are identical to within the given errors which makes this a bound system and it is located about 247 light years away. In Sissy Haas's book Double Stars for Small Telescopes, Ross Gould observing from Canberra with a 7-inch refractor, noted a faint third star of magnitude 12.8 close to B and which is not yet in the WDS.
Bob Argyle - Double Star Section Director