January 2015 - Double Star of the Month

STF 644 in Auriga (05 10 18.81 +37 18 06.7) is a pair noted for the contrasting colours of its components. The spectral types of the two stars are respectively B2II and K3 and when W. H. van den Bos observed the pair with the Lick observatory 12-inch refractor in 1962 he noted the colours were white and reddish. E. J. Hartung noticed this colour disparity too - he found orange amd white with 10.5-cm aperture adding if merely elongated the appearance of the ellipse is striking with one end orange and the other white. Sissy Haas on the other hand with 125-mm at x200 found both stars to be yellow-white.

This is a close pair which has remained virtually motionless since it was discovered by F. G .W. Struve. The magnitudes are 7.0 and 6.8 with the orange component being slightly brighter visually. G. van Biesbroeck added a distant mag 10.5, K3 dwarf at 192 degs, 72". To find STF 644 look about 1.5 degree south of mu Aurigae. It is the preceding of two mag 6 stars - the other being the unequal wide pair SEI 105 (6.5, 11, 27 degs, 35"), about 1 degree following.

HJ 3683 (04 40 17.72 -58 56 39.5). Very fine noted John Herschel from South Africa in 1836 when he first laid eyes on this pair of stars and gave them both magnitude 8. The WDS catalogue says they are 7.3 and 7.5 but the main interest lies in the nature of the apparent orbit of this 326 year binary. The high inclination means that motion is restricted to a narrow range of position angles and so mainly manifests itself in separation, whilst the very high eccentricity (0.95) means that the maximum separation is 4".3 whilst the minimum is only 0".03 which, given the Hipparcos parallax of 32.77 mas, indicates that the two stars are only 3.7 AU apart at periastron but 144 AU apart at apastron. When at their nearest separation the position angle changes by 1 degree per day, almost 5 times faster than that managed by the two components of gamma Virginis at periastron.

The pair is currently at 3".7 and so is an easy object for the small telescope. Gould with 175-mm calls both stars yellow and the pair can be found about three degrees south and slightly following alpha Doradus.

Bob Argyle - Double Star Section Director