Double Star of the Month - July 2008

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.

At this time of year the constellations of Hercules and Ophiuchus straddle the northern hemisphere meridian in the evening. Ophiuchus contains one of the finest binaries in the sky in 70 Oph (18 05 27.21 +02 30 08.8) - a pair with a period of 88 years and a long history of observation. Its proximity to the Sun means that the apparent orbit is large and the two components can be seen in small telescopes at any point in the orbital cycle. The last periastron occurred in 1991 and the telescopic distance between the stars has more than

tripled since then. In mid-2008 the apprent position of B is 132.9 and 5".50. The writer has been following this pair every year since 1990 since when the position angle has decreased from 217 to 134 degrees. Early attempts to compute the orbit led to suggestions that there was a third body in the system. Later and more accurate measurements, along with substantial radial velocity investigations, have not shown any evidence for this idea.

Herschel found the pair in 1779 and called it H II 4. It is number 2272 in Struve's Dorpat catalogue. Hipparcos gives the distance as 5.1 pc and the magnitudes are 4.22 and 6.17. The colours are particularly splendid, the spectral types being K0V and K4V.

Triangulum Australe crosses the southern meridian about an hour after alpha Centauri on July nights. The brightest stars are magnitudes 1.9, 2.8 and 2.9. About 3 degrees north of beta is Dunlop 194 (15 54 52.64 -60 44 37.1), two stars of magnitude 6.4 and 10.0 separated by about 44 arc seconds and having changed little since John Herschel's measures from Feldhausen. R. P. Sellors using an 11-inch refractor at Sydney found that A was a closer double, the 8.1 mag companion being located about 0.6 arc seconds E of the primary. In this system too there has been little change since discovery. A is a luminous B star which Hipparcos places 500 parsecs away with an uncertainty of perhaps half this distance.

Bob Argyle - Double Star Section Director