Double Star of the Month - September 2012
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.
beta Cep (21 28 39.58 +70 33 38.5) was observed as a double star by both William Herschel and Piazzi. The mag 3.2 primary is accompanied by a companion of magnitude 8.6 some 14" distant in position angle 251 and there has been little change in this relative position in more than 200 years. Beta Cep is a distant star - Hipparcos puts it at almost 700 light years. It is clearly also a very luminous star, the WDS catalogue marks it out as a giant of spectral class B3. Interest in beta was reawakened in the 1970s when Antoine Labeyrie, the father of speckle interferometry, found the brighter star to be a close but unequal binary. Paul Couteau the famed visual observer tried to resolve it with the telescope at Nice but failed and subsequent observations have shown the magnitude difference in the visual to be more than 3. The orbit of Aa is very highly inclined and motion is almost all in distance - ranging from 0".33 at greatest separation (to be reached in 2035) to less than 0".01 near periastron. AB is noted as a showcase pair by Sissy Haas and she gives the colours as brilliant white and green. Smyth notes white and blue and also that there is a coarse but very minute double star preceding. Beta Cephei is probably more well-known as the prototype of a class of pulsating variable stars similar to Cepheids but with lower amplitude and range of magnitude. In this case the period is about 4 hours and the star varies by about 0.03 in V.
MLO 6 (21 27 01.62 -42 32 52.5) is located in Microscopium - close with the border with Grus and sadly too far south to be seen from the UK. Hartung gives colours of deep yellow and white which is surprising given that the primary is an A star but with metallic lines in its spectrum. The distance is 184 light years and the small change in relative position (146°, 4".2 in 1879, to 150°, 2".9 in 2008) suggest that this is a long period physical system. The magnitudes of 5.63 and 8.15 means that a good view of both stars would be gained with apertures of 15-cm or more.
Bob Argyle - Double Star Section Director