Double Star of the Month - July 2007
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 double stars selected for this month have a number of common features. Both are relatively close-by, both have short periods, and both are visible in small to medium-sized telescopes at the present time.
zeta Herculis (16 41 17.48 +31 36 06.8) is a famous binary found by the elder Herschel on 1782 Jul 18. Although the separations at the closest and widest points in the apparent orbit are respectively 0".6 and 1".6, the large difference in magnitude, and the glare of the 3rd magnitude primary makes the system a difficult one to measure accurately.
In his 1906 volume, Thomas Lewis devotes five pages to the orbit of this pair which he found could only be explained if the orbital period was changing with time. This seemed to imply the presence of a third component but no such star has been found and there is no evidence from the extensive radial velocity history of this star. Recent direct measurements of the diameter of star A, a G7 giant, show that it is some 2.5 times the diameter of the Sun. The distance from Hipparcos measurements is 35.2 light years and the V magnitudes are 2.9 and 5.6. The pair, whose orbit has a period of 34.45 years, is now widening and in 2007.5 the position will be 200°.2, 1".08.
MLO 4 = BU 416 = R 298 (17 18 56.36 -34 59 22.5) is just visible to the naked-eye on a clear night and can be found sitting some 3 degrees north-west of lambda Sco in the tail of the Scorpion. It was found to be double by Burnham with the 6-inch Clark in 1876 and it appears as BU 416 in his 1906 General Catalogue. His estimates of the magnitudes were 6.0 and 8.5. The modern measures of the magnitude difference by Hipparcos are nearer 0.9. It was also found independently by Russell using the 11-inch refractor at Sydney. According to R. T. A. Innes however, the pair was `first noted at the Melbourne Observatory in 1867', hence its catalogue name.
The distance to MLO 4 is 22.7 light-years, although, strangely, the quoted Hipparcos parallax error is almost 12%, with the annual proper motion exceeding 1 arc second. The period of the binary is 42.15 years, according to Soderhjelm in 1999 and for 2007.5 the position is 206°.0, 1".47.
The orange hues of each component betray the late spectral types; they are dwarf stars of class K3 and K5. There is some evidence that a more distant member of the system (31") is a M-type star. It has similar proper motion to AB so is physically connected.
Bob Argyle - Double Star Section Director