Stellar Size

Size some times really does matter.

One of the things that I pass the time with is Traveller, especially G:Traveller, and the new G:Traveller Interstellar Wars. ‘Hard’ Science fiction has always been a favorite read for me, and Traveller brought it to the gaming table. I think that I got my first Traveller book in ’82. It’s been a non-stop obsession ever since. I own a copy of the basic rules of every iteration of the game – Classic Traveller, Mega Traveller, Traveller: The New Era, Traveller 4, Gurps Traveller and Gurps Traveller 4th edition.

So, I guess that you could call me a traveller geek!

One of the basic technologies of Traveller is the Jump Drive. It allows you to leave ‘normal space’ at point A and emerge at point B in about a week. Depending on the ship, point B could be as much as 6 parsecs (18 light years) away. There are a few caveats, however. The most important is that you do not want to start or end your jump with in 100 diameters of a massive object. For some reason, stellar objects have been ignored in this calculation.

Enter my quick and dirty Stellar Data Tables! (v2:Stellar Data)

11 thoughts on “Stellar Size”

  1. Thanks – we know that. In traveller, it is assumed that they freaking rounded it off after a few hundred years of dealing with the decimals. :)

  2. Well, the sun is about 1.5 g/cm3 and the earth is about 5.5 g/gm3. All iron-nickel planets seem to be somewhere in this neighborhood with Earth being a little on the heavy side.

    Here is a page that gives some generalizations:

    Larger/brighter stars are less dense.

    I would guess that if you take this into account, the jump shadow of a star is irrellevant for the vast majority of destinations. Habitable worlds would always be outside of the jump shadow. Only weird destinations (mining colonies, perhaps a military installation purposefully placed deep in a jump shadow) would be effected by this.

  3. Would you have some data on stellar density as opposed to planetary density? It might be intresting to look at.

    Perhaps the ‘shadow’ is cast by a combination of energy and mass?

  4. Of course nothing is a ‘continguous massive object’ as everything is more ‘nothing’ than it is ‘something’

    The big problem I see with using the steller diameters without considering density, is it makes planetary diameters pretty much irrellevant for most earth-type worlds. The 100 diameter number for the earth doesn’t matter much if you need to be 2 AUs away from the sun anyway. I would imagine that the vast majority of habitable planets would have the same problem.

  5. The solar system and galaxy are not contiguous massive objects. What we do know about Jump Space is that entering Jump from within 100 diameters of an object more massive then the object entering jump causes problems.

    A few years ago there was a huge brouhaha about ‘jump masking’. It seems that some one interpreted Jump limits to mean that a ship in jumpspace would be precipitated into normal space when it’s vector in realspace intersected the 100 diameter limit of a massive object. This threw jump as we know it on it’s ear. See, if the target for a jump was on a path through the star of either system, you would have massive problems.

    Well, Traveller Cannon was consulted, Marc Miller and Loren Wiseman voiced their (powerful) opinions, and as I recall, jump masking was tossed out of the OTU (official traveller universe).

    Some feared that IW brought it back, but that is not the case. The jumpspace physics are simply not as well understood in the 2100’s (IW’s time period) as in the 51st century (traveller’s main time line). Go figure.

    In the 2100’s, in order to make a jump, the equations need to have a massive object at the end. You cannot jump into deep space, unless you know where there is a brown dwarf or rouge planet to ‘anchor’ your jump. This had a massive impact on history. There is a brown dwarf between Sol and Bernard’s Star. As Bernard’s Star is about 2 parsecs away, and we only had Jump-1 (one parsec at a time) we needed a ‘staging point’. So, the Star Leaper One was used to ferry fuel between Sol and the Brown Dwarf Jump Point, building up a reserve for the Star Leaper Two, which jumped onwards to Bernard’s Star, and found the Vilani Empire, thousands of years old, technically advanced, stagnate, and populated with Humans.

    I do not postulate jump masking at all – I see the trip through jumpspace as an arc of sorts. At the start and end, you are closer to realspace, and therefore interact with massive objects. The objects cast ‘shadows’ into jumpspace based on their realspace size, not blocking but making entry and exit much more difficult if with in the 100 diameter limit. it’s not a question of mass, but rather a question of past a certain mass per cubic centimeter, that shadow is cast. Jumpspace is one of the few ways that Traveller breaks rules, so I allow it a certain amount of ‘spooky action at a distance’. In the rules, it is stated that you can initiate jump from a planet’s surface, it’s just not advised!

  6. Volume without mass being considered doesn’t make much sense. Why not use the ‘volume’ of the solar system or the ‘volume’ of the galaxy?

    It has to be a gravity field thing.

  7. That’s based in the assumption that it’s mass that causes an object to intrude upon jumpspace. It could be volume of the object in realspace.


  8. I think you failed to be geeky enough (or perhaps you were way to geeky)

    The 100 diameters thing doesn’t really make a lot of sense from a scientific point of view. What would really matter is how much gravity distortion there is at what distance. 100 diameters is probably a good rule of thumb for terrestrial type worlds (which is mostly what Traveller is concerned with) but would be way off for objects of different densities.

    The sun is about 1/4 as dense as the earth, so using 100 solar diameters doesn’t make a lot of sense. It is unlikely that given these considerations a solar body would ever have much effect on a jumping ship, few destinations would be that close to a star. That is probably why they have been ignored. Similar adjustments should probably be made for gas giants.

    I can’t believe I bothered to reply to a post this geeky. That says really bad things about me.

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