DSO of the Month
Kappa Herculis
AKA: Marsic, Mirfak, STF 2010. Position: 16 hr 08.1 min 17 02 49.2
Due south at 23:30 (BST) on 15 June 2020.
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
Kappa Herculis is an easy double star to observe, it is bright (the primary is 5.1 and the secondary is 6.2) and the separation is a comfortable 27 arc-seconds; it is not dissimilar to Albireo in terms of ease of observation although dimmer. It is also not too hard to find, being at the very bottom right hand corner of the constellation of Hercules, facing the triangle of stars which is the “head” of Serpens Caput and to the east (or left) of Arcturus (Star of the Month for May 2020). In the list of double stars in the Celestron go-to handset it is mistakenly called “Maasym”. The main interest of Kappa Herculis lies in its colours. I have written a paper on star colours with my friend Bill Sheehan for the next issue of The Antiquarian Astronomer. We look at the colours of 23 bright doubles as reported by nineteenth century and modern observers. One key finding is that there is a fashion in star colours and modern observers tend not to report “unscientific” colours such as purple or green (or use so many different terms for star colours). We calculated the “scientific” colour for each of the 46 stars in question using a combination of stellar classification and the observed B-V values. We then compared these “scientific colours” with the observed colours. We found that modern observers (and especially women) tend to observe the scientifically “correct” colours in contrast to their earlier (invariably male) counterparts. This was particularly the case when the two stars were similar in terms of their stellar class or the primary was cooler than the secondary (Albireo being the classic example). The only untoward effect in the latter case was that the secondary was often bluer than the stellar class would indicate (the secondary in Albireo for example should be silvery-white rather than cobalt blue): this is the well-known contrasting effect. However when the primary is hotter than the secondary, the eye cannot cope with the colour contrast. In a nutshell, it refuses to see blue and yellow whereas it is happy to see yellow and blue. A few observers do sometimes see the “correct” colours (especially women), and sometimes they see them as both yellow, but usually the eye sees the secondary as another colour rather than the correct yellow, such as purple (as in the case of the “Easter egg” double, Eta Cassiopeiae), brown, red or green (or just a “muddy” colour). Kappa Herculis is such a double. It should be yellow (G7) and orange (K0). The nineteenth century observers saw it as white-yellow and red-white (or orange), but modern observers see it as both yellow (which is not unreasonable), orange and reddish-white or even (in my own case) yellow and blue.
This double was discovered by William Herschel on 20 September 1779 and he described the stars as a little unequal and both stars as red. The separation since his first observation has narrowed considerably from 40 arc-seconds to the present 27 arc-seconds. They are an optical double; the primary is only 370 light years away whereas the secondary is 2,000 light years distant.
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