An example is the gas around new stars like the Orion nebula. If a continuous spectrum passes through a gas at a lower temperature, the cooler gas produces dark absorption lines . An object like this whose light emission properties depend only upon temperature is called a blackbody . It is a perfect radiator of energy. As a blackbody heats up, it begins to radiate away its energy as light, first starting off with most of its emission in the infrared then in the visible and finally in the ultraviolet.
Stars are good approximations of blackbodies.
A blackbody emits continuous radiation and meets the conditions outlined by Kirchhoff. A hot blackbody will appear to be blue and a cool blackbody will appear to be red. The energy per area radiated by a blackbody will increase with temperature. In other words, for two stars at the same distance with the same radii, the star with the highest temperature will appear to be the brightest. The German physicist Max Planck proposed in the late 1800's that light can be viewed as both a wave and a massless particle.