What defines a red giant star?

A red giant star is primarily defined by the almost complete depletion of hydrogen in its core. As the hydrogen fuel is consumed, the core contracts under gravity, causing an increase in temperature and pressure. This change triggers the fusion of helium and other heavier elements, leading to the expansion of the outer layers of the star. Consequently, the star swells and cools, giving it a reddish appearance while becoming significantly larger than its original size.

The transformation into a red giant marks a crucial phase in a star's life cycle and is the result of the hydrogen burning process coming to an end. In this state, the star has transitioned from being a stable main-sequence star, such as our Sun, to one that is larger and cooler, yet still capable of undergoing further nuclear fusion. This depletion of hydrogen differentiates red giants from stars in earlier stages of their development, which primarily fusion hydrogen into helium in their cores.

In contrast to other options, the initiation of supernova implosion represents a stage much further along in a massive star's life cycle, while the formation of a planetary nebula occurs after the red giant phase for stars of certain masses. Lastly, a constant energy output is not characteristic of red giants, as their energy levels vary significantly