Here's why:

* Planets are celestial bodies that orbit a star. They do not produce their own light and heat, instead reflecting the light of the star they orbit.

* Nebulae are giant clouds of gas and dust in space. They are often the birthplaces of stars.

* Stars are massive, luminous spheres of plasma that produce their own light and heat through nuclear fusion.

The Sun is a star because it generates its own energy through nuclear fusion, which is a process that combines hydrogen atoms into helium atoms, releasing a tremendous amount of energy in the process. This energy is what makes the Sun shine and provides the light and warmth that sustains life on Earth.