Double Star of the Month - December 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.
gamma And (02 03 53.9 +42 19 48) is one of the most impressive double stars in the northern hemisphere. It first appears in the list of pairs found by Christian Mayer and published in the Astronomische Jahrbuch in 1784. William Herschel first measured it in the late 1770's when he found 70.4 degs and 9".25 `a mean of two years observation'. The colours are striking - yellow and blue green, the latter a contrast effect - reflecting the spectral types of K3 and B8. The relative positions of the two bright stars has changed little since then - the modern values being 63 and 9.5 In 1842, Otto Struve, using the new 15-inch refractor at Pulkova found the companion to be a close pair. Subsequent observations show that BC is a 61 year binary with a highly inclined and eccentric orbit, closing from 0".4 at discovery to become single in all telescopes by 1892. It is currently closing again (the latest orbit showing 0".33 for 2007.0); between 2014 and 2016 it will swing through 280 degrees of position angle and then start to widen again, having reached a minimum distance of just over 0.01 arc seconds in late 2015. This may be the last opportunity for some time for owners of 25-cm telescopes to see the pair elongated.
theta Eri (Acamar - 2 58 15.7 -40 18 17) is a brilliant white A star some 20 degrees north following Achernar (alpha Eridani). The parallax as determined by Hipparcos is 20.72 mas yielding a distance of 157 light years. The small proper motion of about 0.06 arc seconds per year of star A would have changed the separation of the pair by some 10 arc seconds over the last 200 years. No such change appears in the relative measures so the pair is clearly a long period binary. The WDS shows that the PA has changed by 8 degrees between 1835 and now whilst the separation has edged closer from 8".7 to 8".4. The notes to that catalogue also indicate that the primary star is a spectroscopic binary but it does not appear in the 9th Spectroscopic Binary Catalogue. The WDS has it under the catalogue name PZ 2 and in Piazzi's Praecipuarum Stellarum Inerrantium Positiones Mediae of 1814 the stars are given magnitudes 4.5 and 5.6. The modern magnitudes are 2.9 and 4.0. The fainter star of this beautiful pair is also an A star so both components appear white. It is visible in small telescopes and possibly larger stabilized binoculars. It reminds the writer of the pair theta Aql.
Bob Argyle - Double Star Section Director