Double Star of the Month:
Gamma Delphini
AKA: STF 2727
Position: 20 hr 46.7 min +16 degrees 07 min 27.4 sec
Due south at 22:08 (BST) on 15 September 2021.
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
Delphinus is a pretty little constellation below Cygnus. It is one of those constellations that do look like their name, in this case a dolphin (although personally I think it looks more like a tadpole). It is an easy constellation to spot and it is not hard to pick out the main stars, which was an advantage when Nova Delphini suddenly appeared in August 2013. The four main stars form a quadrilateral called Job’s Coffin. The nose of the dolphin (or tadpole) is Gamma Delphini. Given its prominence and duplicity, it is perhaps surprising Gamma Delphini does not have a name, unlike its notorious neighbours Rotanev (Beta Delphini) and Sualocin (Alpha Delphini). Gamma Delphini lies some 115 light years away from us and was listed as a double star by the pioneering double star observer Christian Mayer. William Herschel observed it on 27 September 1779 and surprisingly described the colours as being white. The main star is magnitude 4.4 and the secondary is magnitude 5. Given the difference in brightness and a separation of only 8.9 arcseconds, it is not the easiest of doubles to resolve, but it is not very difficult. They are a binary system with a period of 3.25 years. The main interest of this pair is their colours. The main star is a K spectral type subgiant which should be yellow-orange and the secondary is a main sequence F star which should be white-yellow. It would be reasonable to assume that they would both be seen as shades of yellow, and many modern observers see them as such. Observers in the Victorian period (and some modern observers) saw them as green or blue-green. How do you see them?