How does the color of a star indicate is age?

The color of a star is primarily determined by its surface temperature, which correlates with its age. Hotter stars, appearing blue or white, tend to be younger and burn through their nuclear fuel more quickly, while cooler stars, appearing red, are older and burn fuel more slowly. As stars evolve, they change color; for example, a star like our Sun will eventually expand and cool, shifting from yellow to red as it ages. Thus, by observing a star's color, astronomers can infer its age and stage in the stellar lifecycle.