In the H-R diagram, color is used as a proxy for which property?

• A. Age
• B. Mass
• C. Distance
• D. Temperature

Answer: (D)

Color on the H-R diagram tracks the star’s surface temperature because the color you observe comes from the star’s spectrum, which shifts with temperature. Hotter stars emit more blue/UV light and peak at shorter wavelengths, while cooler stars emit more red light and peak at longer wavelengths. This relationship is described by blackbody radiation and Wien’s law, so the color index serves as a practical proxy for temperature. Since distance only changes how bright a star appears (not its color) and age or mass don’t directly set color in a simple way, color becomes the natural indicator of temperature used to place stars on the diagram. (Interstellar dust can alter color by reddening light, but with corrections the underlying temperature remains the key link.)