January 2014 - Double Star of the Month

The two stars to be highlighted this month are both systems of higher multiplicity although the closest visual components are at the limit of amateur instrumentation.

2 Cam (04 39 58.03 +53 28 23.7) is a Struve pair (No 566) which has somehow eluded me for more than 40 years. I first saw it in 2012 with the Cambridge 8-inch Cooke and the companion was quite well seen and measurable. Since it was first measured in 1828 at 312° and 1".5, the two stars have slowly closed and moved in a retrograde manner and last year I obtained 169.7° and 0".89. A preliminary orbit gives a period of 425 years for this unequal (5.6 and 7.5) pair of stars. In 1901, using the Yerkes 40-inch OG, Burnham noted that the primary was a close pair in itself with the new component (V = 7.4) being found at 317° and 0".2. This turned out to be a rapid binary and the currently accepted period is 26.89 years, whilst the separation of the stars never exceeds 0".3. Dembowski adds another pair in the field (D 4) and it can be found about 4 minutes preceding - mags 9.0, 10.3 and separation 5".8. Whilst in this area look at 1 Cam = STF 550, a fine bright pair.

eta Orionis (05 24 28.62 -02 23 49.7) This fine, bright pair of white stars is a good test for the 20-cm aperture. It was first resolved, when separated by 0".9, by W. R. Dawes who noted 'This close and beautiful object was discovered by me on Jan 15, 1848, with an aperture of only 4.25-in which I happened to be using on my 6.33-in refractor'. It is one of 13 pairs in the WDS under his discovery code. The current separation is around 1".8 and the stars have been slowly separating since discovery. Notwithstanding the fact that they are similarly hot, young and massive stars, the WDS notes that A and B form an optical system; the more distant mag 9.4 star at 114" is also believed to be unconnected with the bright stars. DA 5 was not known to Smyth but Webb notes colours of white and purplish (this may be the observation of Dawes) whilst others see only two white components. The A star is actually a massive triple system. Many years ago it was found to be an eclipsing spectroscopic binary of 7.88 days period with both stars possessing about 12 solar masses. More recently, speckle interferometer observations by H. A. McAlister and colleagues found a third companion with a period of 9.9 years, whose mass is about 1.5 solar.

Bob Argyle - Double Star Section Director