Double Star of the Month - January 2007

In this new 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.

With Orion as well-placed as it gets in northern latitudes, it is worth taking a look at lambda Ori (05 35 08.9 +09 56 03) in the head of the Hunter. In binoculars, lambda forms a coarse triangle with phi 1 and phi 2 Ori. In larger telescopes and those operating in the infra-red, there appears to be a cluster of about a dozen B stars and a number of low-mass stars. Lambda Ori makes its own H II region by ionising the surrounding cool gas which appears to be in the form of an expanding ring. It may be that this is the site of an ancient supernova, some 300,000 years ago because the small proper motion of lambda itself does not project to the centre of the ring and that it may have been given a kick by a putative binary partner after that star went supernova. The young neutron star Geminga is also though to have been in that area at the time of the explosion. For the small telescope user, lambda is a fine pair with the 3.5 and 5.5 mag stars both brilliant white, reflecting their spectral types of 09.5II and B0.5V. Never less several observers have seen colour including Webb who thought they were yellowish and purple, whilst Olcott considered them yellow and red. The separation of 4".4 is virtually unchanged since records began.

In the sprawling southern constellation of Puppis, there are many fine pairs for the small and medium telescope. A particularly noteworthy quadruple system can be found in Dunlop (Delta) 30 (06 29 49.1 -50 14 20). First listed by Dunlop in 1826 this unequally bright pair of stars is given by Hartung as yellow and reddish. The WDS lists the magnitudes as 5.97 and 7.98 and the separation is currently 12 arc seconds, making it an easy object. In later surveys, first Russell, and then the Harvard College expeditions to Arequipa in Peru found that both components were close visual doubles. The brighter component is known as R65 and has a period of 52.9 years. It is currently 0".7 apart and closing. The fainter component of the wide pair is known as HDO 195 and has a period of 101 years - it is also closing and in 2007 its separation is just below 0".4. Both systems should be resolvable in a 30-cm telescope.

Bob Argyle - Double Star Section Director