Double Star of the Month - May 2012

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.

Nine degrees south of Arcturus is 15 Boo - a naked eye star which is a difficult double star for the small aperture but which can be well seen in a 20-cm OG on a good night. Track another 3 degrees south-east and the eye alights on a crooked line of 3 stars with the brightest of them being the most northerly. This star is STF1835 (14 23 22.74 +08 26 47.8) and because it is almost half a magnitude brighter than 15 Boo it is unusual in that it has no Bayer number or Flamsteed letter. For the small telescope the pair offers a very pleasing sight with an AO primary being accompanied by an F2 secondary some 6" away to the south. Hartung makes the colours white and deep yellow, Sissy Haas has goldish-white and powder blue whilst W. S. Franks in 1916 made them white and lilac. Angular change has amounted to 7 degrees in the last 230 years but it seems certain that the two stars are physically connected. In 1889 Burnham found that B was a close double (BU 1111) and indeed it turns out to have a binary period of 40 years. The separation ranges from 0".15 to 0".3 so could be seen in 30-cm or more when at its widest in 2022. For 2012.0 the distance is 0".2.

About 3 degrees following beta Crucis, although actually in Centaurus, is a group of bright double stars - R 213, I 424 and CorO 152 - the latter two of which are in the same field. I 424 (13 12 187.63 -59 55 13.9) is a very unequal and rather close pair which needs 20-cm on a good night to see well, but which Hartung could just catch with 10.5-cm. The magnitudes are 4.8 and 8.4 and a recent measure in 2008 by the writer put the fainter star at 13° and 2".0. For the larger aperture, the primary is the binary See 170 - discovered by Thomas Jefferson See and which is only 0".22 apart in mid-2012. This 27 year pair has components of 5.3 and 6.0. Hartung notes that the primary star is yellowish but it has the spectrum of a B5 dwarf. Some distance from AB-C are two faint companions of magnitudes 12.6 and 14.9. About 8' NE is CorO152 - a 25" pair with colours of orange and reddish according to Hartung. R213 is a pair of white stars of magnitude 6.6 and 7.0 which has moved little in PA since discovery by Russell but the separation has tripled to 0".9 and this makes it an excellent test object for a 12-cm aperture.

Bob Argyle - Double Star Section Director