A nebula is a region of rarified gas at pseudo-vacuum density. Its effect will probably be limited to colouration of the background, but conceivably, it may affect astrogation; visually by a partial white-out of the background, or electronically by radiation interference, depending on how big and how dense the nebula is, and its type. The term 'nebula' can be a bit nebulous, covering everything from stellar cradles to supernova remnants.
Generally, scientists classify them into 2 types with regards to size:
In-system nebulas
and the much larger
Interstellar nebulas
The in-system nebulas are exactly what they sound like. They are a large gas cloud (as Icosahedron described) that may be the remnants of that solar system's creation process aka the stellar cradle (or also the "death process"). Once long long ago, our Earth's Solar System had this huge gas cloud, but over time, the huge cloud and various floating matter condensed.... leaving us with the planets, their satellites, the asteroid belt(s), and of course The Sun.
The interstellar ones are the nebulas that we see on the Traveller maps. They are huge. Some will blanket entire sectors, or many many sectors. One example that is known in Traveller is the Dark Nebula, which covers much of the Dark Nebula Sector, which is mostly Aslan territory.
IMTU, I treat them as a mild astrogation distraction. They inflict a mild electronic interference. Under Classic Traveller rules, I've done the following:
All starship operations and skills requiring the use of Sensors and Scanners are treated as receiving a -1 Penalty. As an example, a character using Sensors to detect something (maybe a pirate ship on the far side of a planet) will have his SensorOps-3 skill pretty much treated as SensorOps-2. Other skills that may be affected (per my GM discretion) include Navigation (Astrogation) skill.
Is this a crippling distraction? No. Does the nebula become a blinding hazard? No. Is it a lethal hazard? No. It is simply a mildly irritating astrogation barrier. That's all. None of these game effects are based on any science; after all, this is a fictional game universe about fictional science. Based on my treatment above, I have over-simplified nebulas, exactly in the same way that CLASSIC TRAVELLER over-simplifies the mechanics of the entire universe.
