Final answer:
The position of a protostar on an H-R diagram changes as the protostar evolves, which is indicated by variations in its luminosity and temperature. These changes are plotted on the diagram to show the star's evolutionary path, ultimately reflecting the star's age and the dynamic balance between gravity and pressure within it.
Step-by-step explanation:
The position of a protostar on an H-R diagram changes because its luminosity and temperature change as it evolves. During the earliest stages of a star's life, it generates energy through gravitational contraction, which affects its luminosity and temperature, leading to changes in its position on an H-R diagram. As the protostar contracts, it moves in a path on the diagram, which often appears vertical at first and then approaches the main sequence where the star stabilizes. The speed of these changes and the path they trace on the diagram are influenced by the protostar's mass, with more massive stars evolving more quickly.
To put it simply, a star is often described as "moving" on an H-R diagram, but this movement is not in physical space; rather, it is a visual representation of the star's changing properties, like luminosity and temperature, during different stages of its life cycle. This is an essential tool for astronomers to understand and determine the age of a protostar, as well as to explain the interplay between gravity and pressure in its evolution.