What is the life cycle stage of a massive star where the core reaches extremely high temperatures and heavy elements form by fusion?

The life cycle stage of a massive star where the core reaches extremely high temperatures and heavy elements form by fusion is indeed the supergiant stage. During this phase, the star has exhausted its hydrogen fuel and has begun fusing heavier elements in its core, such as helium, carbon, oxygen, and even heavier elements up to iron.

As the energy from these fusion reactions increases, the star expands significantly, becoming a supergiant. The core temperature in this stage can exceed millions of degrees Celsius, enabling the fusion of elements heavier than hydrogen and helium, which are crucial for the evolution of stars and the chemical enrichment of the universe.

In contrast, other stages such as the red giant primarily involve the fusion of hydrogen into helium and oxygen, and do not reach the extreme temperatures needed to fuse heavier elements. Meanwhile, a white dwarf is the remnant core of a star that has shed its outer layers, where fusion has ceased entirely, and a neutron star is the end stage of a supernova explosion that follows the supergiant phase, primarily composed of neutrons and no longer undergoing fusion of elements.