Double Star of the Month - July 2012
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.
With the summer constellations now becoming dominant, the two pairs selected this month are to be found in the constellations of Hercules and Ophiuchus.
STF2194 (17 41 05.50 +24 30 43.2) sits in the preceding edge of a glorious 1 degree field which includes the bright stars 83 and 84 Herculis. The pretty pair itself is an optical system and since 1783 it has widened gradually from 14".3 to 16".3. The main stars are magnitudes 6.5 and 9.3 and the current position angle is near 7°. Webb noted them as orange and blue in 1850, using his 3.7-inch refractor whilst Franks early in the last century found them to be yellow and lilac. The primary star is a K0 giant which is also a spectroscopic binary. About 169" away in PA 163° is a third star, nominally the same brightness in the visual as B.
STF2173 (17 30 23.78 -01 03 46.2) is a visual binary with a period of 46.4 years. Its highly inclined apparent orbit means there are two opportunities for small apertures to see the two stars before orbital motion takes them beyond the range of most amateur telescopes. The writer measured this pair in 1992 and 1997 with the Cambridge 8-inch Cooke telescope when the separation was respectively 1".1 and 0".7. The two stars are now separating reaching a separation of 0".8 in the south-eastern quadrant in 2012, they then close rapidly and open out again, reaching 1".1 in the north-west in 2037. Both stars are yellow and close to V = 6.0 so this pair forms a beautiful test object at present for 15-cm aperture. Whilst in this area, check out another target some 4 degrees slightly preceding this pair - 41 Oph, a close unequal binary which is one of R. G. Aitken's last discoveries. It needs at least 15-cm on a good night.
Bob Argyle - Double Star Section Director