Double Star of the Month - February 2008

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.

This month the two binary systems in question both have late-type giant stars as primaries.

In 1882, using the 12-inch refractor on Mount Hamilton, Burnham found that eta Gem (06 15 52.70 +22 30 24.6) was double and it eventually became number 1008 in his catalogue. In several cases where he discovered very faint and close companions, Burnham tended to underestimate the brightness of the companion. When, for instance, he found alpha UMa to be double (BU 1077) he gave the magnitudes as 2.0 and 11.1 whilst the modern values for A and B, as found by Hipparcos are 1.95 and 4.87. In the case of eta Gem, he estimated A and B to be 3.0 and 8.8 - again Hipparcos notes that B is considerably less faint than Burnham's estimate and gives V equivalent magnitudes of 3.3 and 6.0 with the companion at a distance of 1".1. Since then the position angle has reduced by about 40 degrees and the separation has increased to 1".5. Even in 1961, Hartung was able to see it with 10.5-cm aperture and these days it is somewhat wider than that. The primary, spectral type M3.5I-II is orange but no colour estimate of B is noted.

In the 1880s, astronomers at Sydney Observatory were busy looking at the double stars, including re-observation of those of John Herschel using an 11.4-inch refractor. Under the directorship of H. C. Russell, R. P. Sellors was one of the observers. The WDS shows 24 pairs under his name, the first and brightest of which is beta Phoenicis - a bright, close binary. The second brightest pair is SLR 8, located in Vela at (08 32 04.97 -53 12 43.1). Consisting of stars of magnitudes 6.13 and 7.08 this pair was separated by 0".4 in 1892, widened to about 1" in 1925 and is now closing again. An aperture of 15-cm should show it but there have been no measures since 1991 - an indication of how the southern pairs continue to be neglected. The colours are orange-yellow and whitish, reflecting the spectral types of KOIII and A3. This system is 227 parsces distant according to the revised Hipparcos parallax.

Bob Argyle - Double Star Section Director