July 2024 - Double Star of the Month

The constellation of Draco is draped across 11 hours of Right Ascension and perhaps the most distinctive part of it are the four stars which form the head of the beast. The faintest of these, and the most north-westerly is nu Dra (17 32 16.04 +55 10 22.5). Even a small pair of binoculars will show that it is really two equally bright, white gems, both of visual magnitude 4.9. Discovered in 1690 (by Flamsteed) this beautiful pair is worth seeking out in any aperture.

Finder chart for the double star ν Draconis in Draco
A finder chart for the double star ν Draconis in Draco created with Cartes du Ciel.

The separation is slowly decreasing with time but at the time of writing is still a generous 62". Gaia EDR3 indicates that the stars are essentially at the same distance (98 light-years) and have very similar proper motions. This object forms one of the calibrating standard pairs which I use to convert the settings of the micrometer on the 8-inch Cooke telescope at Cambridge to position angles in degrees and separation in arc-seconds on the sky.

Meanwhile, the stars of Ara sit south of Scorpius and the brighter members resemble a `butterfly' shape. Beta and gamma Ara, at the head of the butterfly, sit a degree apart and have distinctly different hues, thus creating a fine view in binoculars. Beta is red and gamma is blue whilst 4 degrees due south is 3.6 magnitude delta Ara (17 31 05.96 -60 41 01.8).

Finder chart for the double star δ Arae in Ara
A finder chart for the double star δ Arae in Ara created with Cartes du Ciel.

John Herschel noted a distant and faint companion (V = 11) some 40" away and entered the pair as number 4951 in his catalogue. The companion should be visible in 10 or 15-cm and is now somewhat better placed having eased out to a distance of 50".4 in 2016. In 2010 a project involving deep infra-red imaging of stars which may have sub-stellar companions revealed 3 images within 12" whose K magnitudes were between 12 and 14.

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.