Double Star of the Month - October 2013

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.

Delta Cephei (22 29 10.25 +58 24 54.7) is the prototype pulsating variable the period of which holds the key to its distance. Thanks to Hipparcos this distance has also been determined geometrically and it comes out at 865 light years with a formal error of 37 light years. Regrettably few Cepheids were in range of the satellite during its operation between 1989 and 1993 but its successor, GAIA, will sweep up many more when it launches later this year. Delta is also a most attractive double star and its mag 6.1 companion is probably visible in regular binoculars. It can be found 41" distant from delta in PA 191°. This is formally star C in the system as S. W. Burnham found a much fainter (mag 13) and somewhat closer star (B) using the 18.5-inch refractor at Dearborn. Matt Heijen using a 30-cm Orion records the colours on his website and notes 'yellow, almost orange and bluish white'. There are three more distant comites between mag 13.5 and 14. C was noted as a spectroscopic binary by Belopolsky more than 100 years ago but little seems to be known about it now and it does not appear in the Ninth Catalogue of Spectroscopic Binary Orbits. Recent radio observations of delta reveal an extended nebula of ionized hydrogen surrounding the star which infers that mass loss is taking place at a level of around 10-6 solar masses per year. This may go some way to explaining the discrepancy between the mass derived from stellar evolutionary models and that obtained from stellar pulsation and dynamical techniques.

STF2944 in Aquarius (22 47 50.19 -04 13 44.8) is a nice triple star which contains a visual binary system. It is 2.5 degrees following the pretty, coarse pair kappa Aqr which has an orange primary star. The closer AB pair consists of stars with magnitudes 7.3 and 7.7 which have closed up considerably since discovery in 1782. In the same time the position angle has increased by almost 60°. For the end of 2013 the stars can be found at 303° and 1".86. C which is mag 8.6 is 60" away in position angle 86°. This is not physically connected to AB and is being left behind by the rather considerable proper motion of the close pair, some 0".4 per year. It is large enough to have attracted the attention of Willem Luyten and both components are in the NLTT (Not Less than Two-Tenths) Catalogue. The colours in these stars are not too obvious given that they are rather faint, but at low power Sissy Haas gets beige white and arctic blue for AB combined and C. Hipparcos confirms the proximity of the system to the Sun - the parallax yields a distance of 105 light years.

Bob Argyle - Double Star Section Director