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Old 07-28-2015, 09:32 AM   #1
tolcreator
 
Join Date: Aug 2004
Default Gurps Space 4e and Astrophysics

Several times I've started, then got bored of, implementing the Gurps Space system generation rules in a program (and a few stabs before that of the very similar Gurps Traveller: First In).
But... I finally did it! Well nearly. Right now it does pretty much everything up until you start putting people in, because at the moment that requires some choice not clearly given in the rules. For instance, how old is this colony? Is this a homeworld? Etc. Rather than moving on to that, I'm putting spit and polish on what I have. Also I want to add a way to save to a file, and read back from the save.

But with what I've got, I'm able to say, generate 1000 systems and pick out the worlds with highest affinity.

Now, I want to represent *every* star. And I'm using the optional rules for brown dwarfs: roughly half the stars are brown dwarfs. I did this mostly so that a traveller type jump ship will have more options, and for stepping stones across gaps.

I've run into some issues:

1) Hot brown dwarfs. The rules don't give a way to work out the temperature of a Brown Dwarf, but I extrapolated from the rule for star radius. That is:

R= 155,000 x sqrt(L)/T^2
Where R is radius in AU
Where L is luminosity (in units of The Sun's luminosity)
And T is temperature in Kelvin.

With a brown dwarf, we know radius (actually we know diameter in earths) and we know luminosity, so we can rearrange this to give temperature:

T = sqrt( 155,000 x sqrt(L)/R)
R is radius in AU, with is diameter in earths * 8.52699302 10-5 / 2

So taking a young, big brown dwarf: mass 0.07, age 1 billion years: luminosity 0.042, diameter 14.2
So that's a radius of 0.000605417 in AU

Plugging in above, we get:
T = sqrt( 155,000 x sqrt(0.042) / 0.000605417 )
= sqrt( 155,000 x 0.204939015/0.000605417 )
= sqrt( 155,000 x 338.508854228 )
= sqrt( 52468872.40534 )
= 7243
Ouch! That's hot. That makes it the colour of an F1 V!

Looking at it, it seems the radius is about right... slightly smaller than an M7 star. However, the luminosity is way higher: 0.042 puts it as luminous as an M2.5. Its much smaller diameter makes it much hotter.

I've been doing some googling, and I did see a reference here:
http://www.scholarpedia.org/article/Brown_dwarfs

That indicates that a very young (millions of years) brown dwarf, still burning dueterium, will have a luminosity up to 0.04, but at a temperature of about 3,600k. By 1 billion years (the base age for the table), it should be much less luminous. I'd say the luminosity is right for say a 1-5 million year old brown dwarf, but them the diameter is wrong. The diameter might be right for a 1 billion year old brown dwarf, but then the luminosity is wrong.

Mostly I've been writing this to check my math, that I didn't make a mistake. I guess it's OK for the rules to be wrong: I can "fix" them. At the moment I'm seeing mostly L type brown dwarfs, and only a smattering of Ts, and no Ys (going by above article, L = temperature >= 1400, T >= 600, Y < 600).

2) Lovely Brown Dwarves
I had thought that the brown dwarves would be just stepping stones, allowing low jump ships to cross otherwise big gaps between the real stars. However, in printing out the highest affinity world in every system, I'm seeing loads of brown dwarf systems more attractive to settlement than "real" star systems. Almost always these are belt systems, with affinities as high as 5 due to the high resource value of the belt.

The rules say to generate a star system for the brown dwarf as normal, so I did just that. Reading articles on the subject, it seems that brown dwarves form systems and planets just like normal stars, albeit closer in (which is what I get). The only thing articles say is that water might be rare in such systems, because planets are likely to be "Carbon planets". Gas Giants may also be rare (which is represented in the rules). So it looks like I'm stuck with my attractive brown dwarf systems, and my vast sea of "Belt" systems. I may put in a modifier for water presence on planets in brown dwarf systems.

Yes I realise my players won't give a crap, but this is the kind of stuff that keeps me up at night
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Old 07-28-2015, 10:09 AM   #2
weby
 
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Default Re: Gurps Space 4e and Astrophysics

Brown dwarves are.. interestingly boring stellar objects.

Young Brown dwarves can indeed have high luminosity and temperature, but once they have stopped fusing hydrogen that is likely not the case any more.

And as to how many there are in the universe compared to other stars.. no one knows as the detection distances for them are so much lower.

As for attractiveness of systems: I would definitely give a lower modifier for low water/low gas giant type systems. How much lower depends a lot of your choice of technologies. If you have Traveller jump drives that require hydrogen or your normal space drives require such then the lack/rarity of such will be a big modifier, if not then a small one.

Further the total mass of a system with just a brown dwarf is likely a lot less, meaning that it would likely not attract as many other planets/other objects. It is much closer to something like the Jupiter surroundings than the full Solar system. Thus I would reduce the objects around such compared to a normal star system. (With the exception things like "almost binary star systems" where one of the objects made it to brown dwarf and the other is just a gas giant type object... )
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Old 07-28-2015, 10:20 AM   #3
tolcreator
 
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Default Re: Gurps Space 4e and Astrophysics

Quote:
Originally Posted by weby View Post
Further the total mass of a system with just a brown dwarf is likely a lot less, meaning that it would likely not attract as many other planets/other objects. It is much closer to something like the Jupiter surroundings than the full Solar system. Thus I would reduce the objects around such compared to a normal star system. (With the exception things like "almost binary star systems" where one of the objects made it to brown dwarf and the other is just a gas giant type object... )
I'm not seeing any evidence for that. Evidence shows baby brown dwarves with accretion disks, just like "real" stars.

There is a brown dwarf with a planet... albeit, that planet is orbiting at 40 AU and is on the cusp between a gas giant and a brown dwarf itself. So it's more like a companion star. I've read that you can have anything down to about 1 jupiter mass forming out of nebula collapse (i.e. like a star), so there's probably a smooth distribution of brown dwarfs, sub brown dwarfs, and free roaming gas giants that are almost impossible to detect.

Probably what I'll do is leave it as is, and then add an option for more sucky brown dwarf systems (with an eye towards eventually releasing it into the wild, not everyone might want that option). It's cool I think to have an odd one, or even a few, being mined by belters, but not every second star in the sector :p
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Old 07-28-2015, 10:31 AM   #4
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Default Re: Gurps Space 4e and Astrophysics

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Originally Posted by tolcreator View Post
I'm not seeing any evidence for that. Evidence shows baby brown dwarves with accretion disks, just like "real" stars.
I have not followed the latest developments so I could be wrong on that, but back when I last read about them the mass of brown dwarf disks was thought to be much less than that of disks around "real" stars. And if I remember correctly that was the reason/one of the reasons for the low count of gas giants around them.
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Old 07-28-2015, 10:32 AM   #5
tolcreator
 
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Default Re: Gurps Space 4e and Astrophysics

Quote:
Originally Posted by weby View Post
I have not followed the latest developments so I could be wrong on that, but back when I last read about them the mass of brown dwarf disks was thought to be much less than that of disks around "real" stars. And if I remember correctly that was the reason/one of the reasons for the low count of gas giants around them.
My "evidence" is mostly "a few hours with google/wikipedia", so take anything I say with a heavy dose of salt.
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