The Sun is a star because it produces its own light and heat through nuclear fusion. Here's the breakdown:

* The Sun:

* Fusion: The Sun's core is incredibly hot and dense, allowing hydrogen atoms to fuse together to form helium. This process releases immense energy, which is what makes the Sun shine and radiate heat.

* Self-Luminous: The Sun's light comes from the energy released during fusion.

* The Moon:

* No Fusion: The Moon doesn't have the necessary conditions for nuclear fusion (lacks sufficient mass, density, and internal temperature).

* Reflected Light: The Moon shines because it reflects sunlight. It's a giant, rocky body orbiting the Earth.

In short, the Sun is a star because it creates its own light through fusion, while the Moon is simply a large celestial body that reflects sunlight.