The sun, gas flames, and electric light filaments are visible because they are?

The sun, gas flames, and electric light filaments are considered self-luminous because they produce their own light through various processes. In the case of the sun, nuclear fusion generates extreme temperatures that emit light, while gas flames produce light through the combustion of fuel, causing excited atoms and molecules to release energy in the form of photons. Electric light filaments, such as those in incandescent bulbs, become luminous when electric current heats the filament to a point where it emits visible light.

Self-luminescence is the characteristic that sets these light sources apart from others, as they do not merely reflect light from an external source but generate it independently. This intrinsic ability to produce light is why these examples are classified as self-luminous.