August 2014 - Double Star of the Month
STF2375 (18 45 28.36 +05 30 00.4) is a pair of 6th magnitude stars found about 2.5 degrees north preceding the beautiful pair theta Serpentis. At discovery in 1825 F. G. W. Struve found 108° and 2".2. Orbital motion, for it appears to be a binary, has been rather slow. By 2010 the position angle had advanced to 120° and the separation to 2".6. Interest in the system was renewed in 1952 when Dr. William Finsen was observing the pair with his newly constructed eyepiece interferometer on the 26.5-inch refractor in Johannesburg. When an apparently single star is examined with the interferometer if it is in fact a close double there will be formed a set of fringes which disappear when the instrument is rotated so that the slits are parallel to the line joining the stars, in other words the position angle. Finsen was somewhat surprised when he found that there were fringes on both stars and they disappeared at exactly the same angle of the interferometer. It transpires that both stars were equally close pairs with identical position angles. It led Finsen to call them Tweedledum and Tweedledee. Since then orbital motion has destroyed the symmetry of the pairs and Aa-Ab is currently at 0".13, whilst Ba,Bb is now only separated by 0".08. Aa,Ab has a period of 27 years whilst that of Ba,Bb is 38 years.
DUN 224 (18 54 01.4 -47 16 27.4) is a beautiful triple star in the north of Telescopium, about 2 degrees south of the border with Corona Australis. Dunlop found the wide pair in 1826. The stars are mags 7.1 and 7.3 and currently separated by 87" so its likely that they could be seen in binoculars of sufficient aperture. R. T. A. Innes found the primary to have a closer, unequal companion - mag 9.1 at 1".8. There has been little change in separation since then but the PA has moved on to 192°. The two bright stars are unassociated. Both have parallaxes determined from Hipparcos; star A is 225 light years distant whilst B is 626 light years away. Ross Gould notes colours of yellow and white.
Bob Argyle - Double Star Section Director