Protostellar nebula

From Memory Alpha, the free Star Trek reference

A protostellar nebula is a type of nebula that contains high levels of particle flux. This flux prevents starships from staying inside for more than 72 hours, but it also hides them from sensor scans.

The McAllister C-5 Nebula is an example of a protostellar nebula. (TNG: "Chain of Command, Part I")