The characteristic that is NOT associated with a protostar is:
"It has reached the main sequence on the Hertzsprung-Russell diagram."
1. A protostar is a stage in the formation of a star where it is still in the process of accumulating mass and gravitational contraction.
2. Protostars are typically found in molecular clouds, which are dense regions in space where star formation occurs.
3. During the protostar phase, the star is surrounded by a protoplanetary disk, which consists of gas and dust orbiting the protostar and may eventually form planets.
4. When a star reaches the main sequence on the Hertzsprung-Russell diagram, it means that it has stabilized and begun its main phase of hydrogen fusion, transitioning from a protostar to a fully-fledged star. This characteristic does not apply to a protostar, as it is still in the early stages of formation.
