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Double Star of the Month - July 2011
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.
Alpha Herculis or Rasalgethi (17 14 38.86 +14 23 24.9) is a red supergiant star (M5Ib/II) and the leader of Hercules. It has long been known as a visual binary and is particular attractive to small telescope users because of the contrast offered between the primary and its G5 giant companion some 4".6 distant. The primary is usually seen as orange/red but the companion has variously been reported as white (Hartung), bluish-turquoise (Haas) and sage green (Perez). The primary is a semi-regular pulsating variable whose brightness varies from about magnitude 3.1 to 3.9, and the main period of oscillation appears to be 1343 days. The companion is about 5.4. The motion is orbital but extremely slow and the period is thought to be about 3600 years, but this is complicated by the fact that each component is double again. The primary has been resolved occasionally by speckle interferometry at the 0".02 level whilst the secondary has been known as a spectroscopic binary for many years; the period is 51.6 days. There is also another companion to A with a period of 10 years which does not appear to be the speckle companion so alpha is at least quintuple. For the larger telescope user two faint companions mag. 11.1 at 79", and mag. 15.5 at 19" appear to be merely optical.
Located in Ara, just on the border with Scorpius, HJ 4949 AB (17 26 51.98 -45 50 34.7) is the primary, of the wide pair DUN 216 (5.6, 7.1) which was found by Dunlop in 1826 at 30O and 33".1. By 1999 the pair had widened to 102" thanks to the proper motion of C. AB is a beautiful pair barely 2" apart which has moved about 20 degrees retrograde since being found by John Herschel in 1834. The stars, of magnitudes 5.6 and 6.3, are both late B in spectral type and hence appear white and afford "a beautiful contrast with the surrounding starry field" according to Hartung. Hipparcos places them at distance of 613 light years.
Bob Argyle - Double Star Section Director
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Deep-Sky Objects and Observations
