Here's why:
* Main Sequence: This is where stars spend the majority of their lives. They are fusing hydrogen into helium in their cores, creating a stable balance between gravity and outward pressure. This stage is characterized by a specific relationship between a star's temperature and luminosity.
* Other Stages: While there are stars in other stages (giants, supergiants, white dwarfs), they represent a smaller proportion of the total stellar population. These stars are transitioning to different stages of their life cycles.
Therefore, the vast majority of stars plotted on the HR diagram will fall within the region known as the main sequence.
