* It is a massive ball of hot gas. The Sun is primarily composed of hydrogen and helium, held together by its own gravity.
* It generates its own energy through nuclear fusion. The immense pressure and temperature in the Sun's core allow hydrogen atoms to fuse together to form helium, releasing enormous amounts of energy in the process. This is what makes the Sun shine.
* It emits light and heat. The energy generated by fusion is radiated outward as light and heat, making the Sun a bright and warm object.
* It has a gravitational pull that holds its own matter together. The Sun's gravity is strong enough to overcome the outward pressure from the nuclear fusion reactions, keeping the Sun from expanding and dispersing into space.
In essence, the Sun's ability to generate its own light and heat through nuclear fusion is what defines it as a star. While there are other celestial objects that emit light (like planets reflecting sunlight), only stars possess the internal mechanism to generate their own energy and light through fusion.
