When iron is hot, you can feel the energy radiating from it. That radiation is in the form of infrared waves leaving the iron. When iron gets extremely hot, it produces a red glow—and when it gets even warmer, it can take on a whitish glow. The temperature of iron and other objects can be measured by the amount of radiation flowing from it as well as by the light it emits.

Scientists measure the temperature of stars and the sun by analyzing the color and brightness of the stars. From such measurements, astronomers have determined that the surface of the sun is approximately 5,500°C (9,900°F).