Bright nebulas generally only contain aetherial, empyreal and nebular worlds. Fleet visibility is reduced by 50%.
Dark nebulas can contain any class of worlds except aetherial and empyreal. Fleet visibility is reduced by 75%. In future eras, direct attack damage will reduced by 50% and missile attack range will be reduced by 25%.
Found only in dark nebulas, transpace rift nebula zones are totally impassable to fleets. Fleets may be able to navigate through gaps in the rift zone.