Friday, 20 July 2018

Kappa Herculis: Not what it seems

Did I already mention that appearances can be very deceiving when looking at the night's sky? Wait until I tell you about Kappa Herculis!

Kappa Herculis, also known as Marsic, is a fine double star that can easily be separated with small telescopes or even binoculars (separation 27"). Though their measured temperatures are almost identical (4990°K for K Her A and 4650°K for K Her B), most observers - including me - see them as bright yellow and orange. Their luminosities are 170 and 70 Suns respectively and they have radii of 3 and 2,5 times solar. This means that their respective ages are 400 and 700 million years. Nothing out of the ordinary so far... er...

What did I just say? Two components of a double star that don't share the same age? Highly suspicious! Now, if we measure their distances we find that A lies 388 light-years away from us, whereas B is almost 100 light-years further away! Okay, given the proximity of A, the measured distance to B has a large uncertainty so they still could be near to each other. On the other hand, in the 300 years that this double star has been observed, their separation has diminished from 57" to 27", much more than it should. 

Conclusion: This is no double star but merely an incredible line-of-sight coincidence!

Yet it does seem that A is a binary star with a small companion an arcminute away, which would be 7,500 times the distance from Earth to the Sun, and with an orbital period of 340,000 years. From A, this tiny companion would shine with the same brightness as Saturn.

To make things even more confusing, there's also 8 Her, the bright, white star above our "double" on my sketch. It appears that 8 Her is 367 light-years away, more or less the same measured distance as K Her A! Could they be a real pair? Unlikely, since in that case they would be 1.3 light-years apart, too far to maintain a gravitational bond. Their apparent motion across the sky also appears different. 

Even if 8 Her and K Her A are not related, the former doesn't seem to be ordinary as well because it's an extremely fast rotator. It spins at an amazing speed of 259 km/s, only slightly under the limit at which the star would tear itself apart, resulting in a rotational period of 10.3 hours. 


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