The amount of thermal energy inside a protostar increases with time, even though the protostar is losing radiative energy from its surface. How can we tell how much radiative energy the protostar is losing and how much thermal energy remains in the star? Which type of energy can we measure and which type do we infer from the law of conservation of energy?

a. We can measure thermal energy directly and radiative thermal energy from models. Protostars do not lose all their gravitational potential energy via radiation, so we can derive the amount left for thermal energy.
b. We can measure radiative energy directly and infer thermal energy from models. Protostars do not lose all their gravitational potential energy to thermal energy, so we can derive the amount left for radiative energy.
c. We can measure radiative energy directly and infer thermal energy from models. Protostars do not lose all their gravitational potential energy via radiation, so we can derive the amount left for thermal energy.
d. We can measure thermal energy directly and infer radiative energy from models. Protostars do not lose all their gravitational potential energy to thermal energy, so we can derive the amount left for radiative energy.