Final answer:
Stars have different colors that indicate their temperature. Blue or blue-white stars are the hottest, while red stars are the coolest. Astronomers use color index to measure and express the temperature of stars.
Step-by-step explanation:
Stars have different colors, which are indicators of temperature. The hottest stars tend to appear blue or blue-white, whereas the coolest stars are red. A color index of a star is the difference in the magnitudes measured at any two wavelengths and is one way that astronomers measure and express the temperature of stars.