The source of light coming from stars in the sky is primarily nuclear fusion. Stars are massive celestial objects composed mainly of hydrogen gas. In their cores, under extreme temperatures and pressures, hydrogen atoms fuse together to form helium atoms. This fusion process releases an enormous amount of energy in the form of light and heat. This energy is emitted from the star's surface as electromagnetic radiation, which includes visible light. This is what makes stars visible to us from Earth.
The light emitted by stars is a result of the energy released during nuclear fusion reactions occurring in their cores. It is important to note that stars emit light across a wide range of wavelengths, not just visible light. However, our eyes are most sensitive to the visible light spectrum, which is why we primarily perceive stars as points of light in the night sky.