Double Star of the Month - February 2008
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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