Double Star of the Month - December 2008

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.

zeta Per = STF 464 (03 54 07.92 +31 53 01.2) is a rare animal - a supergiant in a visual binary system. The star was thought to be a member of the Perseus OB2 association but the recently revised Hipparcos parallax puts it around 230 parsecs away, significantly closer than the association. The WDS notes 3 other companions two of which appear by dint of differing proper motions, to be optical but Kaler thinks that both B and E are physical stars. The early type of the primary (B1Ib) and star B (B8V) result in the system appearing white to the small telescope user but Smyth says `flushed white and small blue' whilst Webb says `green white and ash' and Haas has `banana-yellow and blue'. The position of B has changed little since it was measured by Struve in 1824 and is currently near 209 degrees and 12".9. The magnitudes of A and B are 2.85 and 9.16 making it not particularly easy for a small aperture. The WDS notes that A is a spectroscopic binary but no orbit appears to have been calculated for it and a paper in 2003 shows no variation in radial velocity. Both stars appear white to the writer.

f Eri = Dun 16 (03 48 35.82 -37 37 12.5) `Superb double star but ill defined' wrote John Herschel in 1847 commenting on his observations made 11 years earlier. Hartung gives both stars as pale yellow whilst the WDS spectral types of B9V and A1V seem at odds with this assessment. The stars are magnitudes 4.72 and 5.25 and the separation has increased from 7".0 in 1826 to 8".4 in 2002 with the angle increasing from 202 to 218 degrees in the same period. This system is some 185 light years distant and the proper motion is sufficiently large that over 200 years the stars would have separated by about 15 arc seconds if they were not binary in nature. `Showcase pair' says Sissie Haas in her book.

Bob Argyle - Double Star Section Director