Double Star of the Month - April 2009
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.
25 CVn = STF1768 (13 37 27.70 +36 17 41.4) is a beautiful, unequal pair situated about 11 degrees south of M51 in Ursa Major. It was missed by the elder Herschel and found by Struve at Dorpat in 1827. The eccentric nature of its apparent orbit(e = 0.80) was such that it disappeared from view to all observers between 1859 and 1876. The current orbit by Soderhjelm gives a period of 228 years and predicts a separation of 0.2 arc seconds for 1864. The star is currently just starting another run into periastron so it is well placed for observers with small telescopes. The magnitudes are 5.0 and 7.0 and the position for 2009 is 97° 1".7. The revised Hipparcos parallax is 16.45 mas putting it at a distance of 61 parsecs. The primary is an A7 subgiant and there are few colour estimates in the literature. Webb makes them white and blue, as does Chambers is his revision of Smyth. The original Bedford catalogue does not contain an entry for this star.
beta Hya = HJ 4478 (11 52 54.56 -33 54 29.3) was found by the younger Herschel during one of his sweeps at the Cape in 1834. The star is given as both beta Crateris and beta Hydrae in Herschel's 1847 volume and beta Crateris in Chambers version of Smyths Bedford catalogue from 1881. This pair is now a challenge for the 20-cm telescope and it likely that it will be many years before it opens up again so it is worth making an effort to see it if possible whilst it is well-placed in the sky for the southern observer. Hartung gives the colours of both stars as pale yellow but the WDS lists the spectrum of the primary as B9III. The separation has decreased from 1".7 at discovery to about 0".7 now and the position angle is increasing. magnitudes are 4.7 and 5.5 and the distance is 95 parsecs.
Bob Argyle - Double Star Section Director