What characteristic indicates how hot a star is?

The characteristic that indicates how hot a star is its surface temperature, which can be determined by its color and spectrum. Hotter stars emit more blue light and have higher temperatures, while cooler stars emit more red light. This temperature is often measured in Kelvin, with hotter stars typically exceeding 10,000 K, while cooler stars may be around 3,000 K. The surface temperature also influences a star's brightness and the type of nuclear fusion occurring in its core.