Double Star of the Month:
Delta Serpentis
AKA: STF 1954
Position: 15 hr 34 min 48 sec +10 degrees 32 min 20 sec
Due south at 23:00 (BST) on 15 June 2022
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
It is difficult to observe anything in the night sky during June even in Havering, but just as bright double stars manage to overcome light pollution, they can be observed during the night-long twilight in June. Last summer (August 2021) I discussed one of my favourite doubles, Alya or Theta Serpentis which is an easy split. By contrast, Delta Serpentis is on the western side of Ophiuchus and is best observed earlier in the summer. It is also a more difficult split with a separation of 4 arcseconds, which thus needs a higher magnification of 150 times or more. Fortunately the two stars are relatively close in magnitude, the primary being 4.2 and the secondary 5.2. Delta Serpentis is a true binary and there are in fact five stars in the system, but the others are all too dim to interest us. This system is 228 light years away from us and will only be 115 light years distant in 1.2 million years. The two stars are both F type subgiants and have an orbital period of 3,200 years. They were first observed by William Herschel on 3 September 1782 and he described them as white and greyish. They are generally both seen as being yellowish-white in keeping with their spectral type. To find Delta Serpentis, locate bright yellow Arcturus in the southern sky and then go lower and left (i.e. to the east) to Zeta Boötis (mag. 4.5). Now continue in the same direction twice as far and you should see Alpha Serpentis (Unukalhai) which is magnitude 2.6. Now go above Unukalhai in a slightly westerly direction and you will reach Delta Serpentis which has a combined magnitude of 3.8. Alternatively find the small triangle of fourth magnitude stars, made up of Beta Serpentis (mag. 3.7), Gamma Serpentis (3.9) and Kappa Serpentis (4.1) directly above Unukalhai. Delta Serpentis is half-way between the triangle and Unukalhai but slightly more to the west.