Final answer:
A star shines due to nuclear fusion of light elements like hydrogen into helium in its core, releasing vast amounts of energy that we see as light. This process is key to a star's luminosity, unlike nuclear fission, radioactive decay, or reflected light which are not responsible for a star's shine.
Step-by-step explanation:
The primary factor that makes a star shine is nuclear fusion of light elements within its core. This process involves the combination of lighter atomic nuclei, such as hydrogen, to form heavier nuclei, like helium, releasing enormous amounts of energy. This energy is what we perceive as the star's brightness. The initiation of nuclear fusion requires extremely high temperatures and pressures, which are found at the core of stars.
Nuclear fusion is a stark contrast to nuclear fission, which is the splitting of heavy elements, and does not typically occur in stars until the very final stages of a massive star's life leading to a supernova. Additionally, a star's shine is not due to reflected light from the galaxy or radioactive decay of elements in its core. Planets, on the other hand, do not generate their own light and shine because of reflected light from a star, as is the case with Jupiter reflecting sunlight.