Double Star of the Month:
Mu Draconis
AKA: Alrakis; STF 2130
Position: 17 hr 05 min 20 sec +54 degrees 28 min 14 sec
Due south at 22:31 (BST) on 15 July 2022 (and almost overhead)
Mu Draconis
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
As the night is still short in July in Havering, I have chosen a double star which is almost overhead in July, but can be seen all the year round. There are some excellent double stars in Draco, but as it is both a large and relatively dim constellation, it can be difficult to locate them. Mu Draconis is one of the best. The two stars are more or less equal and are the same colour, a pale yellowish-white. But they are relatively faint at magnitude 5.7 and hard to split with a separation of 2.6 arcseconds. Ideally you need a four inch telescope or larger and a high magnification. Staring at you in an area devoid of bright stars, they are sometimes described as a pair of car headlights coming at you in the dark. William Herschel who discovered the duplicity of this star on 19 October 1779 had an easier time of it as the separation was then a more reasonable 4.4 arcseconds. He described them as being in the tongue of the Dragon and white. The two stars form a physical system and are relatively close to us being 89 light years away. They are both spectral class F5 stars hence their yellow-white colour and they have an orbital period of 424 years. The separation is slowing increasing and will reach 2.7 by 2030. The star was fomerly known as Arrakis, which is also the name of the planet (not the star) in Frank Herbert’s novel Dune, but it has now been given the official name of Alrakis, perhaps to avoid any confusion. Alrakis is roughly halfway between Vega and Mizar, but the key to locating it is the quadrilateral known as dragon’s head which is made up of four stars ranging from the second to the fifth magnitude (Gamma, Beta, Nu and Xi Draconis; Nu Draconis is a good double itself), and Mu Draconis is currently just below the quadrilateral.