Double Star of the Month - March 2010

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 subjects of this month's column have several things in common. Both are binaries with unequal components, both contain F stars and in each case both contain a variable component according to the 1989 paper by Baize and Petit.

sigma2 UMa = STF1306 (091023.53 +670803.3) lies in NW UMa not far from the galaxies M81 and M82. The apparent orbit of this 1140 year system shows that at closest approach the stars were about 1"1 apart in PA 153 degrees as happened in 1912. With the magnitudes of 4.87 and 8.85 this would have been a difficult object. Since then the pair has continued to widen and the current position is 350 degrees and 4.15 arc sec. Widest separation occurs around 2520 when the pair are 11.3 arc sec apart. Suspicion of variability of B was voiced by Webb who noted that both Sadler and Dembowski had recorded this, the latter giving the range of B as 8.0 to 10.0. Smyth gives colours of flushed white and sapphire blue whilst Webb noted greenish and orange. The distance to this system is 66 light years.

psi Velorum = Copeland 1 (093041.97 -402800.2) lies on the Vela/ Antlia border. A close and occasionally very difficult binary of short (33.95 years) period it escaped the attention of John Herschel (it was only 0".3 in 1835/6) and was discovered by Ralph Copeland, later Astronomer Royal for Scotland, in the early1880s whilst separated by 1". One of the brightest systems in the sky, with components of magnitudes 3.91 and 5.12, psi is now widening and offers a

chance, in the next few years, for those with small to medium apertures to see it divided. The ephemeris is as follows:

  • 2010.0 101.3 0.85
  • 2012.0 110.1 1.00
  • 2014.0 117.0 1.08
  • 2016.0 123.2 1.11
  • 2018.0 129.4 1.08

The stars are both subgiants of spectral type F0 and F3 respectively and the distance to this system is 61 light years.

Bob Argyle - Double Star Section Director