Which phenomenon is characterized by the fusion of light elements into heavier elements in the cores of stars?

The phenomenon characterized by the fusion of light elements into heavier elements in the cores of stars is known as fusion. In stellar environments, particularly in the core where pressures and temperatures are extremely high, hydrogen nuclei (which are light elements) combine to form helium and subsequently heavier elements through nuclear fusion processes. This process releases a tremendous amount of energy, which is the source of a star's light and heat.

As stars evolve, they continue to fuse heavier elements, such as carbon and oxygen, until they can no longer sustain fusion at their cores, leading to different stages of stellar evolution and potentially resulting in supernova explosions. The fusion process is fundamental to nucleosynthesis, the mechanism by which the elements heavier than hydrogen and helium are produced in the universe.