Final answer:
The Sun's luminosity is due to nuclear fusion, where hydrogen nuclei combine to form helium, releasing vast amounts of energy in the process.
Step-by-step explanation:
The ultimate source of energy that makes the Sun shine is nuclear fusion. This process occurs in the Sun's core where hydrogen atoms under immense pressure and temperature fuse to form helium atoms. During fusion, a small portion of the mass of hydrogen atoms is converted into energy, as described by Einstein's famous equation, E = mc², where E represents energy, m represents mass, and c stands for the speed of light. This energy radiates outward to the Sun's surface and is eventually emitted as sunlight, which provides the energy necessary for life on Earth and drives our climate system.