Which of the following is NOT part of the life cycle of a star?

The life cycle of a star typically includes several key stages, such as the protostar phase, the main sequence phase, and various end-of-life stages depending on the star's mass. The term "protosun" specifically refers to the early stage of the sun's formation, which is a type of protostar. However, in the context of a general star's life cycle, the terms used are usually "protostar" or "protostar phase," rather than naming a specific star like "protosun."

In contrast, the other options—red giant, white dwarf, and supernova—are all established phases that many stars go through:

• A red giant describes a late phase in a star's life when it has expanded and cooled after exhausting hydrogen in its core.
• A white dwarf is what remains after a low to medium-mass star has shed its outer layers and ceased nuclear fusion.
• A supernova is the dramatic explosion that occurs at the end of a massive star's life cycle, leading to the creation of neutron stars or black holes.

Each of these terms represents a defined phase in stellar evolution, while "protosun" as a specific descriptor deviates from the general terminology for the life cycle of stars.