February 2014 - Double Star of the Month
STF 1126 (07 04 06.99 31 51.9) is easily to locate - it lies in the same low power field as Procyon, preceding the bright star. Since discovery by Herschel in 1781 the stars widened slightly but at the mid-point of the 19th century they slowly began to close. Motion is very slow and at present the companion can be found at 174° and 0".9. With magnitudes of 6.6 and 7.0 this is a relatively easy object for 20-cm although being fairly low in the sky it is not often seen to full advantage. Observations by the writer with the 20-cm at Cambridge were made in 1992, 2002 and 2012 and over that period the position angle increased by 10 degrees with no change in separation. Either A or B is a spectroscopic binary according to the WDS. A third star of magnitude 11.4 can be found at 251°, 43". The primary star is an AO giant, and the catalogue value for the parallax is given as 12.6 13.7 milliarcseconds.
STF1146 (07 47 56.71 -12 11 33.8) is also known as 5 Pup. It lies near the extreme northern border of Puppis about 3 degrees north following M46 and M47. During a winter evening of very good seeing in early January the writer made a rare foray around the stars of Puppis and saw this star well resolved with the 20-inch Thorrowgood OG. After discovery by F. G. W. Struve when separated by 3".3, the pair started to slowly close. In the 1960s Ernst Hartung noted that it was 'a fine object in a starry field and 75-mm shows it well. In recent years, however, it has been closing more quickly and now is separated by barely 1". This is a long period binary with a highly inclined orbit and the stars will reach a separation below 0".5 before widening again. Thomas Lewis gave the colours as yellowish and blue whilst Webb thought the fainter star to be ruddy in 1851. The primary star is a F5 dwarf, and this pair lies 93 light years away according to Hipparcos.
Bob Argyle - Double Star Section Director