This is a simulation of the continuous spectrum which is emitted by any hot or "incandescent" object. As an object is heated, the red colors are apparent first, and as heat is increased, the blue colors become stronger. This is because blue light contains more energy than the red light. In a very hot object, such as the filament in a light bulb, the blending of all the colors produces an appearance of white, and we say that such an object is "white hot".
We can't get the temperature of a light bulb hotter than about 3400 deg. K, when the tungsten filament melts. But stars can get much hotter than this. At higher temperatures, a star becomes brighter and brighter, but the color only moves slightly further toward a bluish or blue-white tint. The hottest stars emit most of their energy in the ultraviolet, which is not visible to the human eye.
See a simulation of stars of different colors
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