A protostar is a star in the very earliest stage of development, when interstellar gas is still undergoing gravitational collapse, and nuclear fusion at the core has just begun. In Star Trek, the Argolis Cluster is an example of a protostar cluster. During the Dominion War, Jadzia Dax commanded the Defiant on a mission in the Argolis Cluster to destroy a Dominion sensor array. She had studied protostar clusters and would be able to recognize the gravimetric distortions from them. (DS9: "Behind the Lines") . The USS Defiant surveyed a cluster of protostars in the Gamma Quadrant in 2373. (DS9: "Let He Who Is Without Sin...").

The Enterprise NX-01 surveyed a protostar in 2151, one week before the incident at the Vulcan temple of P'Jem. (ENT: "The Andorian Incident").

Collapsing protostars sometimes emit E-band bursts. Protostars were ruled out as a cause for an E-band emission detected on the USS Enterprise-D in 2367, because there were no protostars within the sector. (TNG: "The Mind's Eye"). Protostars are often marked by high levels of magnetascopic interference, which impair sensor function. While undercover with the Maquis in 2370, Ro Laren used a protostar in the Topin system as cover to steal medical supplies from the Enterprise-D. (TNG: "Preemptive Strike")

In 2371, the USS Voyager discovered a protostar with unusually intense photonic activity. They collected samples of photonic matter in the hopes of improving their systems, but found that they had in fact captured photonic lifeforms native to the star. (VOY: "Heroes and Demons").

A propto-star is the stage in the evolution of a young star after it has fragmented from a interstellar gas cloud but before it has collapsed sufficiently for nuclear fusion reactions to begin. It may last from 100,000 to 10 million years, depending on the mass of the star. A protostar is surrounded by a dense cocoon of gas and dust that blocks visible light, but allows through large amounts of far infrared and microwave radiation.

It is formed by contraction out of the gas of a giant molecular cloud in the interstellar medium. For a solar-mass star it lasts about 100,000 years. It starts with a core of increased density in a molecular cloud and ends with the formation of a T Tauri star, which then develops into a main sequence star. This is heralded by the T Tauri wind, a type of super solar wind that marks the change from the star accreting mass into radiating energy.