# Sci-Fi / Astrophysics - Plausibility Check



## Ixarku (Mar 30, 2014)

Hi, all.  Once upon a time, I was a physics student.  Then reality sunk in, there was some wailing and gnashing of teeth, and 20 years later, I can barely remember how to spell physics.  Nevertheless, now I'm working on a sci-fi story, and, after doing some of my own research, I'll like to get some feedback from the knowledgeable about how plausible the setting of my story is.

The story isn't intended to be hard sci-fi, and the actual challenges of living in my setting won't be a primary focus.  Technobabble and technological problems will be mostly limited to new technologies (ie, the junk that I invent solely as a plot device).  It's also likely that much of the hard details about the setting itself won't be communicated to the reader; this is mostly for my own knowledge, and to ensure that what I'm creating isn't completely the product of fantasy.  That said, here it is:


The story is set on a vast, derelict city floating in space.  Eight miles long, three miles wide, and vaguely rectangular in shape, the city is nearly 10,000 years old.  It was built originally to serve as a center of trade, space-based mining, and manufacturing.  Once, it was capable of supporting a population between 500,000 and 1 million humanoids.  Now, the original builders are long gone, and the city is in ruins, inhabited by the descendants of spacefarers who were marooned on the city in centuries past, a population of no more than 15,000.

The city orbits a white dwarf star, some 0.5 light years away, in the midst of a cooling planetary nebula.  The star was originally a main sequence blue star (B6 III) of about 8.25 solar masses that formed some 50 million years ago.  It aged to become a red giant, passing through the asymptotic giant branch of stellar evolution, and finally went nova 8,000 years ago, forming the nebula.  (The space city was not present in the system at the time of the nova; it was built elsewhere in another star system, and moved into this system via super technology 6,000 years ago.)  The star originally had a planetary system that included 3 gas giants, 5 rocky worlds, and an unknown number of moons or other smaller bodies.  The vast majority of these bodies were within 25 AUs of the star when it went nova; all were destroyed, seeding the planetary nebula with rich mineral debris.  The city was originally moved to this neck of the stellar woods to take advantage of this mineral bounty.

At the time that the space city was moved into orbit, the nebula was about 0.4 light years in radius, meaning the city was beyond the edge of the nebula.  The nebula would continue to expand, however, and at present time would be about 1.2 light years in radius.  The majority of the heavy planetary debris would still be within 0.5 light years of the star.  For the sake of simplicity, I'm assuming that the orbital plane of each of the planets didn't have much eccentricity (or if any did, it doesn't matter much now).  Likewise, I'm assuming that the orbit of the city isn't inclined to the ecliptic and doesn't take it on a path out of the nebula, or if it does, the distances involved would mean a great deal of time would have to pass before it did pass out of the nebula.  And of course, the occasional rocky debris still threatens the space city to some degree or the another.

This stuff is important mainly because some events that occurred when the star went nova and sometime later after the city was moved into the star system have bearing on the present-day events in the story.




So, ignoring the stuff that requires super technology (ie building, moving, and maintaining a space city capable of supporting humanoid life, and all that this entails), do the general details of the star system seem plausible?  My facts seem broadly consistent with the information I've been able to find, but if anyone knowledgeable sees any specific flaws, please let me know.  Any feedback is appreciated.


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## Winston (Mar 30, 2014)

As I semi-literate consumer of science fiction, my take is that your science is sound.  Unless you plan on some kind of niche' marketing, only the hardest of hardcore will be nit-picking your science.  I wouldn't worry about them.

The setting seems fascinating, and ripe for detailed and exotic imagery .  My only advice would be first, have fun with the writing, build the characters, then work your knowledge into your physical world. You certainly have the tools at your disposal.  Good luck.


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## Cran (Mar 30, 2014)

The only sticking point I can see is around the starting mass of the star -


> The star was originally a main  sequence blue star (B6 III) of about *8.25 solar masses *that formed some  50 million years ago.


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as it is just above the accepted mass limit to form a white dwarf (< 8[SUB][SUP]sol masses[/SUP][/SUB]), and sufficiently massive to form an iron core >> neutron star or stellar black hole.
The estimated age is OK for that mass (slightly over 51 million years).


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## Ixarku (Mar 31, 2014)

Cool, thanks for the feedback guys.  Yeah, I wasn't exactly sure about the mass of the star.  Some sources seemed to suggest around 8 solar masses was the limit, others under 10.  I can adjust the star to a lesser value (say, 7.75 solar masses, which would give it a lifespan of around 59.8 million years) and it will still fit my requirements.  I want to make sure I've got my figures approximately correct in case I decide to include any of this as exposition.

Incidentally, I got some of my information from here:  http://hyperphysics.phy-astr.gsu.edu/hbase/astro/startime.html


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