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Originally Posted by GWJ
1. The most massive star in my solar system happened to be white dwarf. Should I still treat it as the main star, or replace stars and make it just a companion (it's 4-solar system) if it's not most massive anymore?
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That's a bit of a tricky one. Everything's orbit around the former main star will have changed, moving outwards as the former main star's mass diminished. Distant companions might even have escaped. But I can't say what will have happened because it all depends on details that aren't available.
Furthermore, "main star" isn't really a physics thing or an astronomy thing, it's just a starting point for the random generator.
What I suggest that you do is make the surviving star that is now most massive the main star, and set the distance of the white dwarf's orbit around it to the old value of its orbit around the former main star times the mass of the former main star divided by the remaining mass of the white dwarf. Then increase all other orbits around the white dwarf by the same factor.
That's kind of arbitrary, but then it's a rather random process.
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2. Should white dwarf have any planets?
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Planets have been detected orbiting pulsars, which are a kind of supernova remnant, so there is no reason to suppose that white dwarfs — which are merely red giant remnants — can't have planets of some sort. On the other hand the
GURPS Space star system generation sequence is not going to be suitable for generating the planets in such a system. The problem is that a white dwarf is the low-luminosity remnant of a star that was once much more luminous¹ — a red giant or perhaps a supernova. The system that the
GURPS Space star system generation sequence uses for working out what atmosphere a planet has is designed on the assumption that the blackbody temperature has been constant or steadily rising. In any system that has a white dwarf in it all the planets will once have been much more brightly illuminated and much hotter. The envelope of the red giant will have occupied the inner system presented drag that will have caused the inner planets to spiral in to their destruction, and the outer system will have been swept by expanding planetary nebulas. Gas giants might survive, but any terrestrial planets and moons will have been stripped of their atmospheres and probably most of their ices.
The system will most likely contain only gas giants and airless rocks. If it has been through a supernova perhaps even the gas giants will be reducing to their rocky cores.
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¹ Theoretically a red dwarf less that 0.5 M[sub]☉[/sub] will develop into a white dwarf without becoming a red giant, but (a) it will become a luminous blue dwarf first, and (b) that will take a very long time: the universe is too young to have any such any such white dwarfs in it.