Double Star of the Month - December 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.
The conventional long-focus refractor user is at a disadvantage when it comes to examining that part of the sky near the celestial poles. This is a pity since there are a number of systems north of +75 which are worth looking out - amongst the binaries are STF2 and pi Cep and the beautiful optical pairs 19 Cam, STF1694 and kappa Cephei, not to mention the Pole Star itself.
The writer measured STF460 in Cepheus (04 10 02.74 +80 41 55.2) on three nights in 1994 but has not examined this slow moving binary since. The period appears to be about 415 years so it has moved almost half an orbital revolution since discovery in 1828. The position angle is increasing and anyone observing it in late 2012 should see the companion at about 149° and 0".69. Having reached a maximum apparent separation in the 1920s the pair is now closing and should reach 0".65 in around 2030. The stars are visual mags 5.6 and 6.3, and Webb gives colours of yellowish and bluish whilst Sissy Haas notes only that the primary is straw yellow.
32 Eri (93 54 17.49 -02 57 13.0) is another of William Herschel's discoveries, but being close to the equator is comfortably within reach of many latitudes and the smallest telescopes. The stars are mags 4.8 and 5.9 and the colours ascribed to stars by Hartung deep yellow and white seem to chime perfectly with the given spectral types of G8III and A2V. Only the earliest measures of the elder Herschel seem to disagree with the general finding that the separation is around 6".5. Is this an error by WH or evidence of rather swifter orbital motion which has not manifested itself since? Between the measures of Struve in 1822 and today there seems to have been no significant movement in separation or position angle but as the stars have similar proper motions there seems no doubt that they form a binary pair. At 165" there is a 10.5 magnitude third star - it too shows no apparent motion over 150 years, so is it also a member of the system?
Bob Argyle - Double Star Section Director
