Double Star of the Month - March 2009

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.

Gamma Leo = STF1424 (10 19 58.1 +19 50 30.7) is one of the finest double stars in the sky in any telescope, with its components of visual brightness 2.37 and 3.64. It was found on Feb 11, 1782 by William Herschel using a 7-foot reflector with magnifications ranging from 227 to 6652 when, not surprisingly, he says `I had but a single glimpse of the star quite disfigured'. Herschel thought the brighter star white whilst the smaller was `..white inclining a little to pale red'. The WDS catalogue gives the spectral types as KOIII and G5III so many modern observers see yellow in both stars (viz. Hartung). Smyth found bright orange and greenish yellow whilst Webb noted gold and greenish-red. The early micrometer measures seemed to indicate relative rectilinear motion but Burnham pointed out that the considerable annual proper motion of about 0".4 per year was shared by both stars and therefore the motion was definitely orbital. Since Herschel's first measure of 84° 3".0 in 1782 the position angle has increased by some 42 degrees and the separation is now near 4".6. Orbits are bound to be preliminary and the one currently in the catalogue gives a period of 510 years, predicting closest separation of 1".1 in 1724, too close for any instruments of the time to resolve, and increasing to 4".6 in 2030. The revised Hipparcos parallax is 25 mas, which translates to 130 light years with an uncertainty of about 2.5 light years. In the same low-power field, some 6' distant, is AD Leo, a flare star which is a close binary. The companion was first noted in 1943 thanks to the astrometric perturbation on the primary star and has been detected at a wavelength of 750 nm. The period is about 27 years.

Near the south pole, delta1 Cha (10 45 16.38 -80 28 10.3) is a slow-moving binary, which is a good test for 12-cm aperture and was discovered by Robert Innes in 1898. Motion is direct with the 1898 position of 60° 0".6 increasing to 85° 0".8 when it was last measured in 1996. According to the WDS, the stars are of mags 6.15 and 6.49. There is interest here too for the binocular observer with the presence of the mag 4.5 delta1 some 6 arc min distant. Interestingly, Hipparcos gives the same parallax within the errors for both delta1 and delta2 (about 9 mas or 360 light years) but they have significantly different proper motions. Hartung makes delta1 and delta2 white and deep yellow whilst Sissy Haas notes that Ross Gould makes them pale yellow and deep yellow respectively.

Bob Argyle - Double Star Section Director