Double Star of the Month Archive 2023

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.

February 2023 - Double Star of the Month

One and a half degrees just east of the fine but unequal binary pair delta Gem (see this column for February 2013) is the pair STF 1108 (07 32 50.63 +22 53 15). The star is displayed on page 8 of the second edition of MacEvoy and Tirion's fine Double Star Atlas (CUP) but without a label. Having checked back over my file of measures I note that this pair does not appear in it, nor does it appear in my earlier visual survey carried out with a 21-cm in the late 1960s.

Image of a finder chart for the double star STF 1108 in Gemini
A finder chart for the double star STF 1108 in Gemini created with Cartes du Ciel.

The Washington Double Star catalogue gives the visual magnitudes as 6.6 and 8.2 whilst Gaia DR3 notes G mags of 6.3 and 8.8. There has been little motion since the pair was found by F. G. W. Struve. DR3 notes that the parallaxes are the same within the errors (and indicate a distance to the pair of 643 light-years) although these are some ten times larger than might be expected for stars of this brightness.

Half a degree south is the unequal, wide pair 63 Gem and about a degree to the south-west is STF 1081 (7.7, 8.5, 204 degrees, 1".9, slowly increasing)

2 Pup = STF 1138 (07 45 29.14 -14 41 25.7) lies in a string of naked eye stars which stretch about 30 minutes of RA along the line of south declination 15 degrees. Other objects in this area include M46 and M47 and the binary STF 1104 (see this column for February 2020). STF 1138 is about a degree due east of M46 and is a beautiful, easy pair for the small aperture.

Image of a finder chart for the double star STF 1138 in Puppis
A finder chart for the double star STF 1138 in Puppis created with Cartes du Ciel.

During a visual survey carried out in the late 1960s I found colours of yellow and lilac whereas Admiral Smyth records hues of silvery white and pale white for `2 Argo Navis'. The stars of magnitudes 6.0 and 6.7 were separated by 16".7 at PA 340 degrees when I measured them in 2015, having closed from 17".4 when measured by William Herschel in 1782.

A third star of magnitude 10.6 lies 100" away in PA 229 and is some five times further away than the stars in the pair. They both have very precisely determined parallaxes thanks to Gaia DR3 which puts them 279 light-years away with an uncertainty of about 1 light-year.

Bob Argyle - Double Star Section Director

If you'd like to try out the Clear Skies Observing Guides (CSOG), you can download observing guide for the current Double Stars of the Month without the need to register. CSOG are not associated with the Webb Deep-Sky Society but the work of Victor van Wulfen.

January 2023 - Double Star of the Month

STF 845 lies in Auriga (06 11 36.59 +48 42 39.6) and is also known as 41 Aurigae and it is also H 3 82, observed by William Herschel in November 1782 and recorded by him as a pretty double star. It is a beautiful pair for the small telescope. The writer first saw it in 1969 using the 12-inch reflector of a friend. At x208 the colours were noted as yellow and lilac. The Washington Double Star (WDS) catalogue gives the spectral types as A1V and A6V.

Image of a finder chart for the double star STF 845 in Auriga
A finder chart for the double star STF 845 in Auriga created with Cartes du Ciel.

James Dunlop found the 30th entry in his catalogue in 1826. It is located in Pictor at 06 29 40.03 -50 14 20.7 some 2.5 degrees NNE of Canopus. It offers a fine sight to the small telescope user.

The stars are magnitudes 6.0 and 8.0 and at present they are separated by 11".5, being 14" apart when observed by Dunlop. The position angle has hardly changed between these two epochs and is currently 312 degrees.

Image of a finder chart for the double star DUN 30 in Pictor
A finder chart for the double star DUN 30 in Pictor created with Cartes du Ciel.

In 1871, Russell, observing with an 11-inch refractor at Sydney Observatory, found the primary star to be a close double. R 65 has a period of 111 years according to Docobo and Ling in 2021. The stars are almost equally bright but according to the orbit, which is extremely eccentric, the separation never exceeds 0".7 and, at times, drops to 0".012 which was the case in mid-2021. By 2027 the stars will be at least 0".4 apart.

In the 1890s Harvard Observatory was site testing in the Peruvian Andes and was using a 13-inch refractor. This telescope discovered several hundred new double stars amongst which was the fainter companion of DUN 30. This has also turned out to be a binary pair of period 101 years, giving a predicted position of 224 degrees, 0".44 in early 2023. The stars have visual magnitudes 8.0 and 8.7.

Bob Argyle - Double Star Section Director