How long does it take a protostar with one solar mass to become a main sequence star? 1 or 3 or 10 or 30 or 100 million years?

The process of a protostar evolving into a main sequence star takes approximately 30 million years.

To determine how long it takes a protostar with one solar mass to become a main sequence star, we can refer to the standard model of stellar evolution.

During the early stages of a star's life, a protostar forms from a collapsing cloud of gas and dust. As it gathers mass, it undergoes gravitational contraction and eventually reaches a point of sufficient density and temperature to initiate nuclear fusion in its core. This marks the beginning of its main sequence phase.

The time it takes for this process to occur is dependent on the mass of the protostar. In the case of a star with one solar mass, it typically takes around 10 to 30 million years for it to become a main sequence star.

This estimation is based on various factors, including the rate at which the protostar accretes mass from its surrounding envelope, the internal structure of the protostar, and its ability to radiate away energy.

Therefore, the correct answer would be either 10 or 30 million years.