During which phase does a star lose its outer layers, creating a nebula?

The phase in which a star loses its outer layers, creating a nebula, is the red giant phase. During this stage, the star has exhausted the hydrogen fuel in its core and begins to fuse heavier elements. As it expands, the outer layers become increasingly unstable and are eventually ejected into space due to stellar winds and other processes.

These expelled outer layers form a nebula, which can eventually give rise to new stars and planets. This stage is significant because it contributes to the recycling of stellar material in the galaxy, enriching the interstellar medium with heavier elements produced in the star's core during its lifecycle. The nebula can take on various shapes and forms depending on the mass and characteristics of the original star, leading to beautiful structures observable by astronomers.

In contrast, during the main sequence phase, a star remains stable and does not lose its layers, while the protostar phase involves star formation and accumulation of material rather than loss. The white dwarf stage is the end state of many stars after going through the red giant phase, where the outer layers have already been shed, and it generally does not produce a nebula.