When I first started working on the science fiction setting, I created several maps covering the Orion Spur and even the Milky Way galaxy. I quickly worked out that these settings were much too big for me to handle, even if I only worked on selected “important” systems.
I was in the process of working out the “correct” size out when Realmwright finally convinced me to watch Firefly – I had been hesitant because I knew that it had been cancelled. I was surprised how much Firefly “felt” like a Traveller game. More importantly, it made me think how important recurring characters and locations are, and I finally decided to bring the setting all the way down to the very small scale of a Traveller sector. (This was, incidentally, when I began using Traveller to model the setting.)
I ended up creating an entire sector, but I did not really think about the process first and found out, later, that it was flawed – one of the key lessons that led me to the creation of Contact Light.
How “big” is a sector really?
Obviously, a Traveller sector consists of 4×4 subsectors, each measuring 8×10 parsecs. But 36×40 parsecs doesn’t actually tell us much about the scale of a setting.
Travel times: Once I had decided on using a Traveller sector for my setting, I worked out that a constant-speed hyperdrive with a velocity of 0.5 parsecs would be ideal. A subsector is 14 hexes across (diagonally), so that means 28 days – four weeks. For comparison, it took Colonial Age settlers four to five weeks to cross the Atlantic ocean.
Number of systems: As it turns out, a Traveller sector is quite a crowded place. The Mongoose basic rules book recommends a 50% system density (and if memory serves me right this has always been more or less standard). Since a subsector has 80 hexes, this means forty worlds per subsector or 640 worlds per sector. That’s a lot. Incidentally, my current Sector contains 424 worlds – some subsectors are more sparse – and that feels overwhelming.
The stellar density near our solar system is 0.004 stars per cubic lightyear, or about 0.14 stars per cubic parsec. The nearest we can get with one or two dice is a 1-in-6, or 16.67%, chance. This would result in ~ 13 systems for a subsector, or 208 in a sector. (Anecdotal data: The Sol sector in Traveller contains 18 systems.)
What can we do with 2d6?
In reality, I can’t imagine why anybody would want to roll even 80 hexes manually, but let’s still stick to the dice conventions. So what can we actually do with 2d6? This quite easily calculated, too:
2 or less
3 or less
4 or less
5 or less
6 or less
7 or less
In other words, my sector worked out spot on at the 33% mark, or twice the stellar density near our Earth.
And I frankly can’t imagine what a sector at 58% or higher will look like. Messy, for sure.
I don’t think there’s a single number you can cite and say “this is what you should use”, since it’s too intimately tied to other assumptions in your setting.
Looking at these numbers and at my first sector, I have a gut feeling that 27.78% is the sweet spot for what I have in mind. But as we noted, it’s not only the number of systems that matters, but how interesting they are; let’s keep the numbers in mind and revisit them when we’re further along in the world generation system.