April 2014 - Double Star of the Month
STT 235 (11 32 20.76 +61 04 57.9) is in UMa close to the bowl of the Big Dipper and about 5 degrees slightly south preceding alpha UMa. The pair has a period of 72.7 years and is presently opening, reaching maximum separation of 1" in 2027. At the time of writing the stars are separated by 0".88 so this is a good opportunity to resolve this pair. The components have visual magnitudes of 5.7 and 7.6 so pick a night when the seeing is good and use at least 20-cm, although 15-cm, if the optics are particularly fine, would probably show the object as double. The star appears in the Hipparcos catalogue as HIC 56290 and it has an annual proper motion in declination of about 0".1 towards the south. The mag. 11.3 star some 195" away would seem to be travelling through space with a similar motion, and was noticed by Helmut Abt. STT 235 has a parallax of 35.73 mas putting it at a distance of 91 light years.
Far down in the southern sky, epsilon Cha (11 59 37.58 -78 13 18.5) is the brightest member at the centre of a small cluster of stars some 111 pc distant. Its nearby co-moving companion, HD 104237 (mag 6.6) is also called DX Cha and is the nearest Herbig Ae star. This is a stellar quintet with most of the companions being very young stars. In 1836, John Herschel divided eps Cha itself into two components 1".6 apart, and the pair is known as HJ 4486. The WDS gives magnitudes of 5.3 and 6.0 but orbital motion has taken the fainter star to within about 0".4 of A.
The author made a measure of this pair from Johannesburg in 2008 and obtained 210° and 0".37 very similar to the last measure in the WDS dated 1997. A substantial aperture will be required to see this pair and it would be interesting to have a confirmatory sighting.
Unlike many of the stars in the cluster and a wider association which are spectral class M, eps Cha is a late B star.
Bob Argyle - Double Star Section Director