* Nuclear Fusion: The Sun's energy comes from nuclear fusion reactions happening in its core. Hydrogen atoms fuse to form helium, releasing enormous amounts of energy in the process.

* Electromagnetic Radiation: This energy is released as electromagnetic radiation, including visible light.

* Self-Illumination: Unlike planets, which reflect sunlight, the Sun produces its own light. This makes it a luminous object.

In short, the Sun is luminous because it is a giant ball of hot gas undergoing nuclear fusion, which produces light and heat.