[dmm]

Check out the references of that Wiki article, esp. 38, 39, and 43.

There are 2 cases: Planets orbit one star of the pair (S type) or orbit the center of mass of both stars (P type).

S type: If the stars are closer than about 7 AU, then planetary formation is severely compromised. Note that most binaries have separations similar to the distance from the Sun to Neptune (~30 AU).

P type: (This is the type currently depicted in SE.) For this type, you need stars closer than about 0.2 AU or so, plus the eccentricity of their mutual orbits has to be close to zero. Otherwise, again, planetary formation is severely compromised. But at this distance, they are close enough to steal matter from each other as they form. So one companion almost always ends up significantly more massive than the other, and you wind up with a red giant / white dwarf combo. I admit, though, that I don't know how long this would take. It might take a long time. So I take back my original criticism. It's not (obviously) silly to have several main sequence stars in a close binary or trinary.

There's another important point. Although 0.2 AU sounds small, the Sun's diameter is only a bit less than 0.01 AU. I was rather shocked at that info. It turns out that SEIV depicts star+planet systems more accurately than SEV, in which the stars are shown WAY too big. Perhaps a "realism mod" for SEV should greatly reduce the sizes of main sequence stars. (Of course the planets are ridiculously large, but that's just a graphical depiction. Regardless of the planet size, it only takes up one hex.) Alternatively, the star size shown could be taken to be the diameter inside of which ships burn to a crisp. So the star isn't actually that large, but the non-navigable region is. Unfortunately, this is always the same distance regardless of the empire's tech level. A better way would be to make the star much smaller but surround it with rings of damaging storms that diminish in intensity as you get further from the star. Then high tech ships could take short cuts and use hiding places unavailable to low tech ships.