Here's why:
* Temperature: The horizontal axis of an H-R diagram represents the star's surface temperature, with hotter stars on the left and cooler stars on the right.
* Luminosity: The vertical axis represents the star's luminosity, which is the amount of energy it emits per unit time. Brighter stars are at the top, and dimmer stars are at the bottom.
Main-sequence stars are those that are fusing hydrogen into helium in their cores. This process defines a specific relationship between temperature and luminosity. The main sequence forms a diagonal band on the H-R diagram, with hotter, more massive stars being more luminous.
Let me know if you'd like to know more about the Hertzsprung-Russell diagram or any other aspects of stellar evolution!
