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PostPosted: Tue Mar 28, 2017 1:41 pm 
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Location: Near Frankfurt, Germany
I´ve been looking over my various old attempts at star system generation rules lately, and revisiting the threads on the topic. For some reason, my mind just can´t let go of the topic in the long term. Nor can I ever be happy with the strongly mainworld-centric approach of Traveller and its derivatives (such as Cepheus).

For some reason, I can feel the beginning of that fabled "wisdom of old age" thing setting in - so rather than dive head-first into building yet another rule set, I decided to first figure out what I want out of such a rule set, and what sort of rule set can do what I want it to do.

Firstly, I want the rules to generate a system from the ground (i.e. the star) up, not from a mainworld of whatever kind. Call it the heliocentric rather than geocentric model if you want. ;)

Secondly, I want the rules to be simple enough that the average Referee or other worldbuilder (reasonably educated, but neither a scientist or engineer nor a mathematician) can do any required math in their head, that any random elements can be determined with commonly available dice (ideally, spectral subclass is the only part where you use dice other than d6), and that even without an Excel spreadsheet full of formulae, it is possible to build a complete star system in a reasonable amount of time.

Thirdly, while I don´t need this to be ultrarealistic (I´ll leave that to the experts), I want the results the rule set produces to look at least semi-realistic, which (among other things) means:
- reasonably realistic distribution of spectral and luminosity classes among stars
- planets are not strewn haphazardly around a star, but follow a reasonably internally consistent logic in their placementt (even if that logic is not the cutting edge of planetary science)
- only a fairly limited range of stars can actually harbor life-bearing worlds
- life develops only where it makes sense for life to develop (How did Arrakis or Tatooine or Hoth get an oxygen/nitrogen atmosphere? How did the titular Planet of the Apes have time to develop in orbit of Betelgeuse which is less than 10 million years old?)
- not all life-bearing worlds are habitable to humans
- most worlds that are habitable to humans are fairly unpleasant places for one reason or the other; those that are not are therefore quite prized

In addition, I think the litmus test for a star system generation rule set should be that, at least in theory, with the right dice rolls, generate something that bears fairly close resemblance to our own solar system, at least to the layman.



...okay, gotta deal with some other stuff first. I´ll post more later, starting with generating the stars themselves. Hope we´ll have some good discussion on that.

_________________
Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards. Sir Frederick Hoyle
Earth is the cradle of humanity, but one cannot live in a cradle forever. Konstantin Tsiolkovsky
Man has earned the right to hold this planet against all comers, by virtue of occasionally producing someone completely bat**** insane. xkcd #556
Just like people, stars can be very important without being terribly bright. Phil Plait, "Bad Astronomy"


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PostPosted: Tue Mar 28, 2017 3:20 pm 
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Location: Near Frankfurt, Germany
All right... stars.

Wikipedia (last refuge of the layman) gives us a distribution of main sequence stars as follows:
M: 76.45%
K: 12.1%
G: 7.6%
F: 3%
A: 0.6%
B: 0.13%
O: 0.00003%

I couldn´t find any numbers on the prevalence of white dwarves, except that 4 are listed among the list of 76 closest stars.


In the vein of being reasonably realistic without getting too complex, I feel the table I had in my old system works fairly well with some adaptation:

Spectral class (2d6):
2-8: M (72.2%)
9: K (11.1%)
10: G (8.3%)
11: F (5.6%)
12: Rare (O/B/A or white dwarf) (2.8%)

Rare (2d6):
2-8: White dwarf (2.1%)
9-10: A (0.55%)
11: B (0.15%)
12: O (0.08%)

(FWIW, the main change to the old version is paring down the number of O/B/A-class stars by filling the Rare table with white dwarves instead)

This somewhat inflates the number of F- and especially G-class stars, which is fine by me, since they´re the most interesting ones to begin with - in that they´re the best shot for life-bearing worlds. O-class stars are vastly overrepresented, but I don´t mind that, since they´re still very rare (you can have entire Traveller sectors without a single O- or B-class star).

_________________
Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards. Sir Frederick Hoyle
Earth is the cradle of humanity, but one cannot live in a cradle forever. Konstantin Tsiolkovsky
Man has earned the right to hold this planet against all comers, by virtue of occasionally producing someone completely bat**** insane. xkcd #556
Just like people, stars can be very important without being terribly bright. Phil Plait, "Bad Astronomy"


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PostPosted: Thu Mar 30, 2017 11:41 am 
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Posts: 316
Location: Near Frankfurt, Germany
Now... size. From what I have been able to figure out, the VAST majority of stars (not counting white dwarves or brown dwarves) are size V, as larger stars simply do not exist long enough for very many of them to be around at any given time.

Given the information in EDG´s New Revised Stellar Generation Tables document, I´m settling for a simple 2d6 roll here:
2-11: Size V (dwarf)
12: Size IV (subgiant) if it is a F, G or A class star, otherwise it is a giant of some sort
(in other words, not too different from EDG´s table)

I´m thinking that distinguishing between various kinds of giants is a secondary concern here, probably best relegated to optional rules. They cannot have life-bearing worlds, and any they might have had earlier were destroyed or sterilized when they evolved away from the main sequence. As such, I expect them to exist mainly as waypoints between more hospitable places.



Now, for the number of stars in a system, EDG´s table already does everything one could ask for from a rule set. Seriously, it´s simple and it works well.

_________________
Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards. Sir Frederick Hoyle
Earth is the cradle of humanity, but one cannot live in a cradle forever. Konstantin Tsiolkovsky
Man has earned the right to hold this planet against all comers, by virtue of occasionally producing someone completely bat**** insane. xkcd #556
Just like people, stars can be very important without being terribly bright. Phil Plait, "Bad Astronomy"


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PostPosted: Thu Mar 30, 2017 3:41 pm 
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Joined: Sat Sep 01, 2012 2:44 pm
Posts: 187
Location: Colorado
Randomly generating the stars, planets (diameter, mass and density) and planetary orbits is the way to go. Once you have that I think the math can play an important part to detailing the system - black body temperature is a function of distance and stellar luminosity, atmosphere is a function of planetary mass, radius and BBT etc. It's true that you can band the options together and then assign them probabilities and at this time, our knowledge of what's "realistic" is very much in it's infancy and therefore your results are going to be fine but I think you get better detail if you automate it in a spreadsheet or python programme as many here have.


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PostPosted: Thu Mar 30, 2017 8:28 pm 
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Location: Near Frankfurt, Germany
hiro wrote:
Randomly generating the stars, planets (diameter, mass and density) and planetary orbits is the way to go. Once you have that I think the math can play an important part to detailing the system - black body temperature is a function of distance and stellar luminosity, atmosphere is a function of planetary mass, radius and BBT etc. It's true that you can band the options together and then assign them probabilities and at this time, our knowledge of what's "realistic" is very much in it's infancy and therefore your results are going to be fine but I think you get better detail if you automate it in a spreadsheet or python programme as many here have.


Yeah, I know. I´ve been using Excel to assist in random generation, especially in "mass-producing" random results, for a long time - I don´t know Python, otherwise I would probably have done that, too. Still, my ambition is that you should not NEED either.

For some reason, my worry is not so much about what we all do not know, but about what I, personally, do not know. I´d rather not make any perfectly avoidable beginners´ mistakes - like putting a habitable world around a red supergiant (Planet of the Apes, the original novel). Hence the hope for a discussion here.

_________________
Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards. Sir Frederick Hoyle
Earth is the cradle of humanity, but one cannot live in a cradle forever. Konstantin Tsiolkovsky
Man has earned the right to hold this planet against all comers, by virtue of occasionally producing someone completely bat**** insane. xkcd #556
Just like people, stars can be very important without being terribly bright. Phil Plait, "Bad Astronomy"


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PostPosted: Thu Mar 30, 2017 8:40 pm 
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Joined: Sat Sep 01, 2012 2:44 pm
Posts: 187
Location: Colorado
I'm with ya, I have the same sentiment. It's been fun putting the Excel sheet together, my take away has been there's more that we don't know than we know and so it's OK to fill the gaps to make a system the way I want it to fit in with the setting.

I think Python would have made it easier but I would have had to learn Python first and my Excel skills are passable.

It's interesting the paper that Thrash has just posted as one of the things I found was that most M stars are X-ray emitters, I haven't worked out what that will mean for native life, I'm figuring that a deep sea vent that's under enough water will be as good a place as any to start life and the mutations could be fun but I'm pretty sure humans aren't gonna like the place! That and that most planets in the habitable zone of M stars are going to be tide locked, I've seen a fair few gas giant moons in M star habitable zones but I've not found enough info online as to whether these would be viable for life or not.


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PostPosted: Fri Mar 31, 2017 10:17 am 
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Joined: Thu Aug 28, 2008 10:09 am
Posts: 501
hiro wrote:
I think Python would have made it easier but I would have had to learn Python first and my Excel skills are passable.


If you go the Python route, the Makhidkarun group on Github has plenty of Python scripts for world building you can use or look at for inspiration.

https://github.com/makhidkarun

Simon Hibbs

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PostPosted: Fri Mar 31, 2017 11:40 am 
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Location: Near Frankfurt, Germany
The first thing that really has me stumped is orbits.

For one thing, there´s the minimum orbital radius around a star. We have discovered exoplanets orbiting their stars VERY closely, even at less than 0.01 AU in some cases. But on the other hand, our own solar system has no object closer than 0.4 AU to the central star.
The Roche limit is a hard inner limit for orbital radius, and I wouldn´t be surprised if some of those observed exoplanets are pretty darn close to their star´s Roche limit. But for our solar system, if it was just the Roche limit, there should be space for another planet or two or three inside Mercury´s orbit.
For all I can tell, though, it seems that very close planets are to be found only on very dim stars (M-class red dwarves mainly), so apparently there is something about higher luminosity that prevents planets from forming very close to a star, or staying there if they migrate to a very close distance.

So my hypothesis, using solitary, minimally variable stars as the simplest case is this: Planets can only form in orbits that are both outside the parent star´s Roche limit, and in which the planet´s black body temperature is below a certain point. Most likely, this "certain point" is located somewhere not too far above Mercury´s black body temperature.

What do the more knowledgeable people here say about that?


The other thing is the distribution of occupied orbits.
I had an old formula according to which orbital radius would double for every two orbits. I.e. 0.25 AU, 0.5 AU, 1 AU, 2 AU, with the "odd" orbits in between at (square root of two) times (rounded to 1.4 times for the sake of simplicity) the radius of the neigboring inner orbit.
Working in and out from Earth´s orbit, that´d give us orbits of 0.125 AU, 0.175 AU, 0.25 AU, 0.35 AU, 0.5 AU, 0.7 AU, 1 AU, 1.4 AU, 2 AU and so on.
That holds up pretty well for Venus and Mars, but Mercury is more in the 0.35 AU orbit than in the 0.5 AU where the formula would place it.
If we place the asteroid belt at the 2.8 AU orbit, that´d leave the 2 AU and 4 AU orbits open, with Jupiter at the 5.6 AU orbit (instead of 5.2 AU), Saturn at either 8 AU or 11.2 AU (instead of 9.5 AU), Uranus at either 16 AU or 22.4 AU (instead of 19.2 AU) and Neptune at 32 AU (instead of 30 AU).

So the that simple method would break down completely unless we allow for empty orbits in between occupied orbits, even then especially Saturn´s and Neptune´s orbit would be poor matches. Still, I can´t think of a better system.

_________________
Space isn't remote at all. It's only an hour's drive away if your car could go straight upwards. Sir Frederick Hoyle
Earth is the cradle of humanity, but one cannot live in a cradle forever. Konstantin Tsiolkovsky
Man has earned the right to hold this planet against all comers, by virtue of occasionally producing someone completely bat**** insane. xkcd #556
Just like people, stars can be very important without being terribly bright. Phil Plait, "Bad Astronomy"


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PostPosted: Fri Mar 31, 2017 4:42 pm 
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Orbits are complicated, but they don't seem to follow any particular rule aside from ratios that don't mess each other up.

Something like 1.3 + 1d8/10 as your multiplier for next orbit out would work well enough for most cases. Someone else suggested 2d6/7 * previous orbit.

A GURPS supplement for Traveller suggests inner limit of 0.2 * the mass of the star, and GURPS space suggests 0.1 * stellar mass. If you worked outward from the innermost planet and rolling randomly where to start, you could roll 1d6 + 0.1AU * stellar mass, maybe.


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PostPosted: Fri Mar 31, 2017 5:03 pm 
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Posts: 187
Location: Colorado
As a caveat, I'm a gamer with an interest in astronomy, I have no qualifications or claim to expertise!

I use a version of 2300 to set the initial orbit. It's been changed to reflect that different star classes have different minimum orbits and that we know stars have planets much closer than originally thought. Looking at the HZ of an M star, it can be tiny in comparison to a G star, get too close to a A star and things will melt. I set the orbits of subsequent planets just as 2300 does with a 1d10 table that sets a multiplier of the initial orbit. There's speculation that there is a planet further out in our solar system than Pluto, I get a wide range, systems smaller than 5au around an M star are not unusual. I can get systems as big as 170 au. With planetary migration a pretty well accepted theory, (have a read on the Grand Tack theory) I am happy that my level of randomness is as accurate as I need.

I set the orbit of any asteroid belts based on the largest gas giant in system, our belt here in Sol forms between 4:1 and 2:1 resonances of Jupiter's orbit. There are more variables than I want to figure into this so I'm happy with this granularity.

I add the belt(s) as an afterthought tho and there are times that the belt will conflict with the orbit of a planet. If it's a small planet, I leave it there, if it's larger, I choose to loose the planet or belt.


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