Double Star of the Month - August 2009

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.

100 Her (18 07 49.6 +26 06 04) is a bright and wide pair of white stars in eastern Hercules, about 15 degrees south preceding Vega. It forms a fine sight for the small telescope. The original separation derived by Herschel in 1777 showed the stars to be 17".0 apart whilst a more recent measure by the author in 1995 indicates that the stars have closed to 14".2 with very little change in angle.

These stars caused the Hipparcos satellite some difficulty as the errors in parallax, even in the revised version, are some ten times worse than might be expected but they still show that the parallaxes of the two stars are the same within the (large) errors and thus indicate that the two stars are probably physically related. In 1985 the CHARA team from Georgia State

University discovered that component A was a close binary. Subsequently, the period turned out to be less than 16 years. Perhaps the same situation might apply to star B, as an explanation for the large error in parallax. Both stars are noted as white by several observers.

HJ 5014 (18 06 49.9 -43 25 30) was another product of John Herschel's fecund search for new double stars at the Cape of Good Hope. Unfortunately there were few observations in the following 50 years when the pair moved through almost 180 degrees of position angle, widening considerably as it did so. Wierzbinski produced some orbital elements in 1958 with a period of 191.2 years. It was clear around the beginning of this century that the real period was much longer and Andreas Alzner produced an orbit in 2002 increasing the period to 450 years.

This is a beautiful pair of white stars (both A5 dwarfs) each of visual magnitude 5.7 which can be well seen in 10-cm aperture. The position for 2010.0 is 2.4 degrees and 1".72, and it will continue to widen until 2170 or so.

Bob Argyle - Double Star Section Director