Double Star of the Month - June 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.

The two pairs in this month's column are similar in that they contain stars of spectral type A but there the similarity ends. Zeta Boötis is a close, bright binary and SHJ 179 is much fainter and considerably wider.

zeta Boötis (14 41 08.92 +13 43 42.0) is a white mag. 3.7 star south following Arcturus by about 8 degrees. Its binary nature was discovered by William Herschel on 1796, Apr 5 when he said that the pair was `very nearly in contact; I can, however, see a small division'. He had previously seen a mag. 11 optical companion in 1782 which became H VI 104. The proper motion of AB is taking it away from C and the distance has increased from 99 to 103 arc seconds over the last century or so.

The main pair is a bright, equal binary of high inclination and extreme eccentricity - in fact it appears to be the current record holder, surpassing even the value for 41 Dra (see Astronomy Now for June 2009). Having spent most of the last half century near PA 310 degrees and separation 1", it is now noticeably closing and the writer found it separated by 0".6 last Spring. If the 122.98 year orbit by Andreas Alzner is correct it will pass 0".5 in 2011 and then dip below 0".01 in the summer of 2021 when the angular motion will be 10 degrees per DAY. From there it will rapidly return to the 4th quadrant again the following year. The stars are both A0 dwarfs and the revised Hipparcos parallax is 19.00 mas.

SHJ 179 (14 25 29.91 -19 58 11.8) First measured in 1798 and placed in the catalogue of stars found by James South and John Herschel, this attractive wide pair of magnitude 6.6 and 7.2 stars makes an excellent target for the small telescope. Sissy Haas marks them as reddish white whilst Hartung from Australia notes them as pale yellow. They are low in the sky for the northern observer so this may explain the reddish tinge, as the WDS gives the spectral types as A2V and A4V. The proper motion of A seems to be shared by B and the relative position has changed little in the last 200 years, the current values being 205 degrees and 34.7. These are distant stars, the parallax of A placing it about 457 light years away.

In 1867 Burnham using his 6-inch Clark refractor found that B was double again and in the intervening period, the companion has moved about 15 degrees in a retrograde direction, remaining close to the discovery separation of 1".2. This pair is BU 225 BC in the WDS but also bears the appellation HDO 138 indicating that it was found later from Peru by the Harvard observers at Arequipa but who were unaware of Burnham's observation. The difference in magnitude is about 1.7 so this pair requires at least 100-mm of aperture to see well.

Bob Argyle - Double Star Section Director