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").
Worf surveyed a cluster of protostars while commanding the Defiant in the Gamma Quadrant, during a survey mission in 2373 (DS9: "Let He Who Is Without Sin...").