Thermal radiation. This is heat that radiates from near (relatively speaking) the surface of the star. All stars produce a continuous spectrum, from gamma rays to radio waves. They differ, however, in the relatively contributions of the spectrum they emit, as well as in where they peak. The peak of the spectrum is roughly the "color" of the start, which in turn, gives a pretty accurate measurement of its surface temperature. The hottest stars peak in the x-ray and gamma ray areas of the spectrum, and appear to us as super blue/white, because we can't see in those very short wavelenghts. The blues and violets we may see are just a relatively small portion of the entire emitted spectrum that we able to see.
