Double Star of the Month - July 2008
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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 (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