The Enterprise-D approaches the McAllister C-5-Nebula, a protostellar nebula
A protostellar nebula was a type of nebula that contained high levels of particle flux. This flux prevented starships from staying inside for more than 72 hours, but it also hid them from sensor scans.
The McAllister C-5 Nebula was an example of a protostellar nebula. (TNG: "Chain Of Command, Part I")