A protostar is any star in the very earliest stage of development, when interstellar gas is still undergoing gravimetric collapse, and nuclear fusion at the core has just begun. Protostars are often marked by high levels of magnestascopic interference. (TNG: "Preemptive Strike") The Enterprise NX-01 surveyed a protostar one week before the incident at the Vulcan temple of P'Jem in 2151. (ENT: "The Andorian Incident")
Collapsing protostars sometimes emit E-band bursts. No protostars were located in the vicinity of Krios. (TNG: "The Mind's Eye")
Worf surveyed a cluster of protostars in the Gamma Quadrant while commanding the Defiant during a survey mission in 2373. (DS9: "Let He Who Is Without Sin...")