August 2023 - Double Star of the Month

The constellation of Sagitta is one of the few on the sky that vaguely resemble what they are purporting to represent (Another in my opinion is Leo). The bright stars of Sagitta trace the shape of an elongated `Y' with eta and gamma in the `shaft' which then divides at delta to form the `fletching'.

Just 45 arc-minutes NE is zeta Sge (19 48 58.65 +19 08 31.1). A small telescope will reveal the duplicity of the star - the primary is magnitude 5.0 whilst the companion is 9.0 and is currently about 8".3 distant, a system first observed by William Herschel and catalogued H 2 30. Alvan G. Clark used this star to test one of his firm's famous refractor objectives and in the course of doing so found that the primary was a very close, but equally bright pair, which subsequently turned out to have a period of 23 years.

Image of a finder chart for the double star zeta Sge in Sagitta
A finder chart for the double star zeta Sge in Sagitta created with Cartes du Ciel.

Maximum separation of 0".25 occurs in 2038, and there is some evidence that the bright star (but which component?) is also a spectroscopic binary. It is this complex structure that prevents the star appearing in the Gaia DR3 catalogue, although the magnitude 9 star is in there and is 328 light-years away.

The double stars of James Dunlop include some of the most beautiful pairs in the southern hemisphere. No exception is DUN 227 (19 52 37.76 -54 58 15.7) which is a lovely pair for the small aperture.

The stars are magnitudes 5.8 and 6.4 and in 2013 when I measured them they were at 148 degrees and 22".8, although there has been little change since they were first noted in 1826. There is a fine colour contrast. The primary is a K0 giant whilst the secondary has spectral class A2V. Hartung noted colours of orange-yellow and white whilst more recently Ross Gould found deep yellow and cream.

Image of a finder chart for the double star DUN 227 in Telescopium
A finder chart for the double star DUN 227 in Telescopium created with Cartes du Ciel.

Gaia DR3 places the primary at 407.50 light-years and its companion at 414.38, and when the errors on each distance are considered the two stars are formally at different distances, but if they form a a binary system then the period is likely to be very long. Its worth looking out for - find it about 5 degrees WNW of the 1.9 magnitude star alpha Pavonis.

Bob Argyle - Double Star Section Director