Final answer:
Option (C), The Hertzsprung-Russell diagram, or HR diagram, is a crucial tool in astronomy used to plot stars based on luminosity and surface temperature, revealing relationships such as the main sequence where most stars are found.
Step-by-step explanation:
The Hertzsprung-Russell diagram, or HR diagram, is a foundational tool in astronomy that was independently developed by Ejnar Hertzsprung and Henry Norris Russell. This diagram plots stars according to their luminosity against their surface temperature or spectral type. The HR diagram reveals various patterns and relationships in the characteristics of stars and is instrumental for understanding stellar evolution, types of stars, and their life cycles.
Main sequence stars, which make up about 90% of all stars, appear on the diagram in a continuous band that extends diagonally from high temperature and high luminosity to low temperature and low luminosity. The mass of a star determines its position along this main sequence. The HR diagram also includes regions for giants, supergiants, and white dwarfs. As stars age and evolve, their luminosity and temperature change, leading to movement on the HR diagram. Astronomers use this diagram to trace the life of a star from its formation to its eventual end state.