Deathstar

Besides the geographical aspects of Kazh, the supercontinental empire and the alliance of maritime kingdoms, there is another detail about the planet that I am planning for. Besides Kazh’s midsize primary star it will have a distant companion star. This star is much larger than the star Kazh orbits and consequently much brighter. In spite of that, the two stars orbit their mutual barycenter such that, even at the closest approach of their long, highly eccentric orbit, the luminosity of the larger companion was insufficient, as a main sequence star, to affect the climate of Kazh. Thus, though life on Kazh faced all the difficulties of life on any other world, it was not troubled by any extraordinary astronomical difficulties. In fact the distant companion star may have aided in sweeping away much of the small debris associated with astrogenesis, so Kazh had remarkably few mass extinctions due to the impact of cometary and asteroidal material.

For simplicity, since I can’t seem to find any reliable indication of the evolution times and luminosities of a star like this, I’ve decided to use the Stellar Evolution Table from page 103 of GURPS Space 4th ed. to work out the evolutionary life for Kazh’s companion star, which, for reasons which should become obvious, I have given the working title of Deathstar. Deathstar needs to be as massive as possible, consistent with a timescale sufficient for life to evolve considerably while the star is yet in the main sequence and remain in the giant stage through the evolution of sentience to the present. For this reason, I have chosen for Deathstar to be a star of 1.45 Solar masses and the system to be about 3.85 billion years old.

During its Main Sequence life, Deathstar would have been an F3 class star, with an effective temperature of 6,900 Kelvins and a luminosity that varied from 4.3 times that of the Sun in its youth to 5.7 as it approached the end of its 3.3 billion year lifetime as a main sequence star. At this time, 550 million years before the present, the star began to expand and cool. According to GURPS Space 4th ed., the luminosity doesn’t change significantly during this, subgiant phase, so I will assume that Deathstar will still not affect the climate of Kazh during this period. Perhaps some early land dwelling creatures turned their eyes up to see the bright white star in the sky slowly turn into a bright red star, but their attention would soon turn back to the more immediate and concerning pursuits of survival. After about 500 million years the core of Deathstar will have become hot and compressed enough to begin its helium burning phase(it is one of those seeming paradoxes of astrophysics that even as an old star’s outer layers are expanding, cooling and becoming ever more tenuous, its core is compressing, becoming immensely hotter and increasingly dense). At this point, if page 104 of GURPS Space 4th ed. is to be trusted, its luminosity will increase by about 25 times with a 10% margin of variation. Using the upper range of that, I will set its new luminosity at about 156 times the luminosity of our Sun.

From this point forward as the two stars orbit around their mutual barycenter, most of the time they will still be too far apart to affect the climate of Kazh. Every 5,000 years or so, as their highly elliptical orbit approaches its closest separation of about 30 astronomical units(~4.5 billion kilometers), Deathstar will increase the insolation of Kazh by nearly 20%(this is assuming that Kazh is in the middle of its star’s habitable zone with an insolation roughly identical with Earth. A decent assumption as I haven’t yet determined the precise orbital configuration of Kazh, but I am intending for it to be habitable). This would equate to a temperature change of over 13 Kelvins, probably enough to catastrophically affect the global climate.

I owe thanks to Poul Anderson for this idea, which he uses in the novel, “Fire Time,” a great read if you can track a copy down. I was surprised at how difficult it was to track down information, especially timescales, for the evolution of stars different from the Sun. I don’t really like to have to fall back on game aids for technical information like this. On the other hand, the GURPS line has been a very good and reliable source for esoterica on a variety of subjects and the GURPS Space 4th ed planetbuilding rules come highly recommended. In Fire Time, Anderson gives the luminosity for a much smaller red giant of 1.22 Solar masses as 280 times the luminosity of the Sun. This much higher luminosity makes me a little suspect of the numbers I’m getting from the GURPS book. A higher luminosity would also be more convenient for my purposes. My star, with its lower luminosity and its necessarily much larger semimajor axis(to account for a 5,000 year period rather than Anderson’s mere 1,000) requires a much larger eccentricity. This will make for more sensitivity to gravitational perturbations from nearby stars.

I’m currently studying my copy of Stellar Interiors by Hansen and Kawaler, but it’s slow going as the book is highly technical and my understanding of nuclear astrophysics is a bit, shall we say, shallow. Any assistance would be greatly appreciated.

With all of that, though, I think it is time to continue with the development of my planet.

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