Updating the maps, or stars do move.

[tjoneslo]

I was somewhat curious about this, how often did the IISS need to update the star charts. I found this paper describing how fast, in general, stars in the local space were moving relative to each other. According to the paper stars have an average radial velocity of 13.1 ± 11.1 km/sec. Radial velocity means either toward or away from the Sol system. Smaller (M, G, K) stars are moving faster than larger (F, A) stars.

To move a star one hex closer or farther away requires covering about 1/2 a parsec. At this velocity it takes (on average) 0.5 parsecs * 3.08e13 km / parsec ÷ 13.1 km / sec ÷ 3.15e7 sec / year = 37,320 years to cross the distance.

Take the 50 closest stars to a given target, this is, on average stellar density, the stars in an area a little larger than a subsector. Given a random distribution, one star should change hex position relative to Sol every 746 years.

The one problem this this analysis is it considers only radial velocity (away or toward Sol). And the stars would have a transverse (side to side) component to their velocity as as well. I think, since the original calculation multiplied the velocity vector by the sine of the angle between the star's vector and the radial vector, we now need to divide the vector by the sine of the angle to return to the full velocity.

Since we have not measured the velocity (and angle) of each star relative to Reference we have to assume the average is 45°. This would give an average velocity of 18.5 km/sec. So 0.5 parsecs * 3.08e13 km /parsec ÷ 18.5 km/sec ÷ 3.15e7 sec/year = 26,426 years per parsec. Or about 528 years per change for the 50 stars in the local space.

To expand this larger area there are two ways to consider this. First would be how often stars move relative to Reference, the center of the Imperium. For 9000 stars this would be one star changing position every 3 years.

But the stars are not moving just relative to Reference, but to each other as well.

1225 pairs of stars in the local space (50 stars), each having a 1 in 26426 chance of changing position per year,for an overall 0.04529 chance of a star changing position every year or about 1 every 22 years. Across the (about) 300 subsectors in the Third Imperium, this ends up with 13.6 stars per year, or a little more than one per month.

This assumes a 1 to 1 mapping between real space and the destinations of jump space. This is strongly implied by the description of jump distances, but not by the astrography of the real stars.

 

[rancke]

Future star maps are likely to be dynamic, changing automatically as time goes by and always up to date.


Hans

 

[Blue Ghost]

I seem to recall some factoid that when the dinosaurs were roaming the Earth, they had "seen" the Earth orbit the galactic center something like ten or eleven times before their extinction. So yeah, it's pretty apparent that stars do change positions constantly, but do their positions change significantly over three-thousand years of the game's time frame?

 

[Timerover51]

If you want to get technical, the stars that we see in the sky at night are no longer in the same position as they were when the light left them on the way to us, by a matter of several years to several decades. You also have no guarantee that the star has not gone nova or supernova on you prior to jumping to it, based on the last data received.

Part of the Navigation and Pilot skill will be to adjust for the movement of the target star system prior to jump, based on known apparent movement from the system you are jumping from.

As for the Traveller hex charts, I cannot see any reason to change them in the least.

 

[aramis]

The highest speeds I can find documentation on in a quick search are under 2000km/s... 7.2 Gm/h, or about 1Pc per 500 years. The average stellar speeds are about 2 orders of magnitude lower (1Pc per 50,000 years), and relative stellar motions.

But also, keep in mind that most stars are moving roughly in the same direction and close to the same velocity - another 2 orders of magnitude less relative to their cluster-mates. So, in the long run, almost nothing is going to change enough to matter, except the roughly 1 per million "high velocity stars"...

Still, compensating for a cluster drift can be an interesting element to add to ones TU...

... and in a fast-forward adventure, a few stars will have crossed a single parsec. About 1 in a million.

 

[Blue Ghost]

It would be interesting to extrapolate the number of supernova through astronomer estimates, and apply that statistic to known Traveller space. A fast moving sun sweeping through a system, or a central star going nova doesn't make for much of an adventure though.

 

[JimMarn]

Barnard's Star is moving rather rapidly near our solar system. It is moving at 10.3 arc-seconds per year. So some do ziiiip by.

http://www.solstation.com/stars/barnards.htm

 

[Blue Ghost]

Jim; I must admit, it's been twenty years since I took astronomy with a lab. Can you translate that to mph for us lay astronomers (I tried searching for something online to do this, but no go).

 

[tjoneslo]

But also, keep in mind that most stars are moving roughly in the same direction and close to the same velocity - another 2 orders of magnitude less relative to their cluster-mates. So, in the long run, almost nothing is going to change enough to matter, except the roughly 1 per million "high velocity stars"...

In the paper I cited, the average radial velocity for stars relative to sol is 13 km/s, or about 50,000years per pc. Adding another 2 orders of magnitude decrease on top of that is really too small a relative difference.

Having one star shift position on the hex map in an average campaign would be a plausible, and interesting, event.

 

[salochin999]

In the paper I cited, the average radial velocity for stars relative to sol is 13 km/s, or about 50,000years per pc. Adding another 2 orders of magnitude decrease on top of that is really too small a relative difference.

Having one star shift position on the hex map in an average campaign would be a plausible, and interesting, event.

Makes me think of this connected to an ancient treasure map.

 

[aramis]

In the paper I cited, the average radial velocity for stars relative to sol is 13 km/s, or about 50,000years per pc. Adding another 2 orders of magnitude decrease on top of that is really too small a relative difference.

Having one star shift position on the hex map in an average campaign would be a plausible, and interesting, event.

I noted 2000km/s which is 153x faster - which is in fact two orders of magnitude.

Sol is the outlier in the local area; most everything whithin 10Pc is toing 10-15km/s. They wont' change hex throughout the timeline of the 2nd-3rd imperiums.

 

[tjoneslo]

I noted 2000km/s which is 153x faster - which is in fact two orders of magnitude.

Sol is the outlier in the local area; most everything whithin 10Pc is toing 10-15km/s. They wont' change hex throughout the timeline of the 2nd-3rd imperiums.

Sorry. I read your "2 orders of magnitude" twice, and concluded you meant 4 orders of magnitude slower.

It also depends on at what distance between two stars J-1 no longer works and you need use a J-2. I assume it was just over 1.5 parsecs, which is where I get the 500 to 700 years. If you have J-1 cover distances closer to 2pc, the time gets longer.

This also goes to how you (the general you) think Jump Space relates to real space.

 

[aramis]

Sorry. I read your "2 orders of magnitude" twice, and concluded you meant 4 orders of magnitude slower.

It also depends on at what distance between two stars J-1 no longer works and you need use a J-2. I assume it was just over 1.5 parsecs, which is where I get the 500 to 700 years. If you have J-1 cover distances closer to 2pc, the time gets longer.

This also goes to how you (the general you) think Jump Space relates to real space.

The fastest are just under 2000km/s
A given cluster is about 20km/s relative to most other clusers
Within a cluster, most seem to be under 0.5km/s relative to the other members of the same cluster.

So, total, about 4 orders of magnitude overall. (We are in the middle of to but not in the same cluster as, most of our surrounding stars. The stellar chemistry and direction of movement don't match.)


Likewise, the issue with Alpha Centauri AB vs Proxima Centauri ... the chemistry matches between the three, and even if Proxima isn't orbiting AB, it is moving generally the same direction and speed.

Our own system's motion is very different from the local stellar group. But none of them close enough to matter in the 5000 years of imperial histories, and only a minor issue given the OTU 300,000 years on the timelines. And the closest High Velocity object is around 19kPc... until skip drives, a bit out of reach.

 

[Aramais]

This discussion is interesting...but I have to agree with those who suggest that it won't make enough difference to worry about in the average campaign. A supernova would be worse, as I understand it -- it would kill off life for parsecs, right? X-Rays? At the speed of light, fortunately, so there's time to pull a Heinlein/Robinson trick and jump around warning people to get into their lead pajamas...

As the big old computers WOULD be watching the relative shifts, I suspect that they would not be game-changers. They would be small and incremental, unless you decided to have your PCs time-jump in some way, and that would have to be a LARGE jump.

Let's not forget Patsy's Observation Regarding Camelot; it is applicable to role-playing games ("It's only a model").

 

[Blue Ghost]

What I didn't understand about Traveller when I was younger was that Traveller's leaning towards "hard science" means that you really can't have extraordinary tropes like tackling a disaster like supernovae or X-ray or gamma-ray bursts, unless your setting is outside standard TL 15 high Imperial (what is it now, TL17?). And even at TL 15 or TL 17 there's not much chance of doing much.

Having said that, related, but not directly involving, stellar movement, are rogue planet bodies. The meteor that wiped out the dinosaurs is said to be about the size of Manhattan. Ah, but here's the thing, it was moving ~30 Km/s. So fast that had you blinked you would have missed it moving by and impacting the Earth.

So, maybe stars don't move significantly on the map, but there is a danger for a world from rogue bodies.

 

[tjoneslo]

So, maybe stars don't move significantly on the map, but there is a danger for a world from rogue bodies.

This implies the second part of this discussion. If JumpSpace doesn't actually cover all the stars (which the 2D map seems to say), when would stars move into or out of jump space. Is there a point where you could add (or remove) a new star.

 

[Blue Ghost]

I'm not getting what you're trying to say. Stars entering jump space?

 

[whulorigan]

It also depends on at what distance between two stars J-1 no longer works and you need use a J-2. I assume it was just over 1.5 parsecs . . .

Prometheus (Alpha Centauri) is 4.3 ly from Terra/Sol, which translates to a little more than ~ 1.3 pc. It requires J-2 for the jump transition.

 

[mike wightman]

A jump 1 engine can move you 1 parsec maximum, for anything in the range 1-2 parsecs you need a jump 2 drive.

 

[Blue Ghost]

This implies the second part of this discussion. If JumpSpace doesn't actually cover all the stars (which the 2D map seems to say), when would stars move into or out of jump space. Is there a point where you could add (or remove) a new star.

I don't think anyone's suggested that there are stars in jumpspace (though that is a thought ... hmm ... stars in jumpspace? Adventuring in jumpspace? That is a major one of these ---> :CoW: ).

I think the proposition is that stars move incredibly fast relative to other stars, but the distances seem to preclude them from adjusting things much in Traveller.