Final answer:
The key quantity that controls the evolution of a star is its mass. The luminosity of a star depends on its mass, and it can vary as the fourth power of the mass. During certain stages of a star's evolution, its luminosity, temperature, and radius can change.
Step-by-step explanation:
The key quantity that controls the evolution of a star is its mass. The luminosity of a star, which is the total energy emitted per second, depends on its mass. It's a good approximation to say that luminosity varies as the fourth power of the mass. For example, if two stars differ in mass by a factor of 2, the more massive one will be about 16 times brighter. On the other hand, if one star is 1/3 the mass of another, it will be approximately 81 times less luminous.
During the time when the evolutionary track for a star of 1 solar mass remains nearly vertical in the H-R diagram, its luminosity remains constant. However, its temperature may change as it undergoes nuclear fusion and different fusion reactions occur in the stellar core. The star's radius may also change as it evolves.