What stellar diagram plots luminosity against star type?

The correct choice is the H-R diagram, or Hertzsprung-Russell diagram, which is a fundamental tool in stellar astronomy. This diagram effectively plots stars based on their luminosity, which is a measure of brightness, against their temperature or spectral class, indicating different types of stars.

The H-R diagram reveals the relationships and characteristics of various stars, categorizing them into regions such as main sequence stars, giants, and white dwarfs. By analyzing the placement of a star on this diagram, astronomers can infer its age, evolutionary stage, and even its potential lifespan. The classification of stars along this diagram is crucial for understanding stellar evolution and the lifecycle of stars in the universe.

Other options like geological maps, phase diagrams, and temperature gradient charts pertain to completely different fields such as Earth science, materials science, and thermodynamics, and do not relate to the classification or analysis of stars in an astrophysical context. You can see how the focus on luminosity and star type makes the H-R diagram specifically suited for the study of stars.