Double Star of the Month:
Porrima
AKA: Gamma Virginis; STF 1670
Position: 12 hr 41 min 40 sec -01 degrees 26 min 58 sec
Due south at 22:09 (BST) on 15 May 2022
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
On the whole, I avoid tight doubles for the Double Star of the Month. Some observers concentrate on tight doubles, regarding double star observing mostly as a challenge for their telescope and their eyesight. Each to their own taste, but I find splitting tight doubles a complete pain. A few years ago I was trying to split a number of tight doubles, having exhausted all the easy ones, and it gave me real trouble with floaters in my eye and with my eyesight more generally. If you are new to double star observing it is also disheartening to take all the trouble to find a star and then find it impossible to split. While light pollution is not a major problem for double star observing, the air above nearby houses is often unstable because of the heat given off by the buildings, either from internal heating or heat stored in the bricks from the daytime sun which can make difficult splits impossible.
So what exactly is a tight double? This is determined by your telescope. The theoretical limit for a given aperture is given by the Dawes’ limit, named after the famous double star observer William Rutter Dawes (1799-1868). The Dawes limit for a 100cm telescope is 1.2 arcseconds. There are three important points to be made here. The Dawes limit is “optimistic” and the similar Rayleigh limit is 1.4 arcseconds. The Dawes limit applies to stars of equal magnitude which hardly ever arises. Stars of unequal magnitude are more difficult to resolve. Finally the Dawes limit applies to a “notch” between the two stars and not a clean split. In practice, I would say that you should double the Dawes limit to get a realistic estimate of what is feasible, especially if you are not experienced. Incidentally there is an absolute limit of 0.5 arcseconds imposed by the unsteadiness of the atmosphere, but to reach this limit you need a telescope with an aperture of 9” (23cm).
Porrima is the classic example of a tight double. Furthermore the two stars have a short period of 169 years which means the separation changes significantly each year. Between 2002 and 2010 it was impossible to split Porrima in a small telescope. I found it impossible to resolve in the summer of 2011, but was able to do so in early 2012. Since then the separation has increased from 1.8 in 2012 to 3.2 in 2022. By 2030 it will be a fairly easy 3.9. Porrima is a rare example of two stars being the same brightness, as they are both magnitude 3.5 (3.48 vs. 3.53 to be precise). The two stars are spectral class F1 and F2; they are 39 light years away from us. They are seen as white, perhaps with a greenish tinge, which is a common feature of tight doubles. It is easy to locate in the sky with a combined magnitude of 2.7; it is the only bright star on a line between Denebola and Spica. It lies on the bottom of the so-called “bowl of Virgo” (see DSO of the Month for April 2022).