Double Star of the Month:
95 Herculis
AKA: STF 2264.
Position: 18 hrs 01 min 30.4 sec +21 degrees 35 min 45 sec
Due south at 01.25 (BST) on 16 June 2021.
Image credit: Jeremy Perez (http://www.perezmedia.net/beltofvenus)
Used with permission
Let’s begin with the appearance of this very pleasing double. It is harder to split than many of the doubles I have showcased up to now, but at a reasonably high magnification (e.g. 100x) this one should be easy enough with a separation of 6.4 arcseconds. The magnitudes are nicely equal at 4.9 and 5.2. The colours are often described as white and yellow, but like some observers I see them as yellow and yellow. William Herschel observed 95 Herculis on 8 September 1780 and called them white and bluish white, and equal. Remarkably Admiral William Smyth in the 1840s saw the pair as apple-green and cherry-red, a veritable fruit salad! To find it go to the south-east of the Keystone of Hercules in the direction of Sagitta and then look halfway on a line between Alpha Ophiuchi (Rasalhague) and Vega (Alpha Lyrae) for a 4th magnitude star.
The two stars which make up this binary system are both elderly, but at different stages of their life. 95 Her A the primary is a spectral class A white giant whereas its companion 95 Her B is a G class yellow-white giant. Their masses are similar, they are both about 3 solar masses, and they are both about 500 million years old. However 95 Her A is much hotter (8000K) than 95 Her B (4900K). The reason for the difference between them is that 95 Her A is transitioning from hydrogen burning to helium burning and is expanding and cooling, whereas 95 Her B has already reached the helium burning stage, thus converting helium into carbon and oxygen. This difference should result in different colours for the two stars in the telescope, namely white and yellow, but as we have seen this is not the case for all observers.