I noticed that the Astrography post for April disappeared so here it is again!
I did an Astrography post already for 'A' and stupidly deleted it (long story short). Anyway here's the new uprated Astrography post with the same long windedness you've known me for. You will get an entry for F tomorrow (not another expletive).
Traveller set the industry standard for star mapping with 2d hex maps. There's been some attempt to palm this off as the hyperspatial relationship between stars that is equivalent to the flat surface of a three dimensional spherer rotated into jump space rooba-rooba-rooba. Ookay fine. It's easy to map. But when have I ever stuck with the easy way to do things (an astute reader of my posts will chime in with 'Never!')
One of the problems with flat maps was that unconsciously they remind you of a planetary map. Planetary maps of course have borders which are often based on geographic features that are defensible. But there no features in space that will make a multi-light year stretch easier to defend. So the idea of borders is a misconception. Forces can move fairly freely through your territory and hit you where you live. Space is frigging huge. You can't guard all of it.
Winchell Chung of course showed off another way to represent a space empire: the node map. There are problems with this too (sorry Winchell). Real space being what it is and paper being flat you can only produce distance relationships between a star and it's closer neighbors. You have to hit a few stars to get anywhere. There are no express routes, they're all local.
(Node map from Atomic rockets by Winchell Chung. Seriously check the man's site out.)
Now a node map will work fine for a game system like Traveller where you are limited to jumps of a few parsecs or a setting where you have wormholes to travel between stars. We could leave it at that by golly but we won't. Let's look at warp drive: something to make you go so fast the stars in the way want to jump up on chairs and get the heck out of your way (though that would imply sentient stars in your setting.)
Warp drives are hella fast taking you where no people have gone before. Let's look at a galaxy image.
(Credit: ESA/Hubble & NASA)
This galaxy is similar to our galaxy and on a large enough scale it is indeed flat. If you had jump-600 or so drives inTraveller you'd not be far off mapping a sector as flat. Indeed our galaxy is 100K light years across and measly 1000 light years thick in our locale.
But again moving that way doesn't say much for borders and no borders means no interstellar states. If you can't defend your people they aren't going to send you taxes.
But look at a more local region (800 light years radius or so):
(Map from The Guide to the Galaxy by Nigel Henbest and Heather Couper with thanks to Atomic Rockets.)
First this section of the Orion Arm is about 1000 light years thick. So the third dimension is a little more than half the width of the paper so we are approaching flatness. Earth is located in the very human oriented Local Bubble. Seen on this scale the Galaxy is a series of bubbles that are each remnants of the death of a mighty star (in the case of the Local Group that's Geminorum.) These supernova push interstellar gas and dust away forming shells of nebula.
How a warp drive will react to flying through this stuff is anyone's guess. Since this is my blog you get my guess which is 'not very well.' Flying too fast through a nebula can have a variety of effects to make your navigator wish he'd chosen the security branch. What they are are up to your setting. Trek style games have a long history of preposterous space phenomena. Maybe you'll hit a grazing space manatee.
That being said you now have borders. If your ships are impeded going through said nebula then defenders on the other side have a defense benefit. If it merely slows travel then the defenders have a chance to mass their forces at points of attack. If the attackers are damaged by the effects of nebulae then even better (and believe me it sucks when your Tactical Officer becomes a psionic sociopath right before your offensive starts!)
So we can have empires with borders with warp drives. We can extend these benefits to hyper and jump drives in several ways. A nebula might increase fuel consumption or increase the chance of a misjump. In Traveller a nebula might interfere with the jump limits increasing the minimum safe jump range from a planet. Increasing safe jump range is a two edged blade. Attackers take that much longer to reach your worlds, allowing you more time to mount a defense but this also means it will take your forces longer to evacuate or to jump to other worlds to help defend them. Optionally it could cause or magnify jump sickness resulting in incapacitated crew after a jump. Not the way you want to go into battle.
Given the nebula effect -whatever you decide that is- species inside a bubble will tend to develop ftl faster and expand quicker. Empires within a bubble will have no defensible borders from each other. They may ally or one could come out on top. In my space opera campaign I'm putting the Confederation inside the local bubble.
Wait there's more. If you want your campaign powered by some sort of unobtainium you could say said wonder element is produced only in the death of a massive star (although elements are produced there too). The nova could scatter the element through out the bubble denying most of it to the species evolving in the nebula region and further slowing their emergence on the stage of galactic politics so you have a physical reason for Traveller's major and minor races and all those advanced races in Trek that never seemed to go very far into the galaxy.
Races may actively seek or manufacture clear routes through bubble boundaries, posting beacons or other navigation aides. Stars along naturally clear routes will be strategically valuable. empires will rush to exploit them (and build defenses around them). They might be declared neutral zones or demilitarized zones and the game of empires will continue.