Final answer:
Asymptotic giant branch (AGB) stars have high-mass loss rates due to their large radii and low escape velocities as red giants. Factors like radiation pressure, stellar pulsations, and events like the helium flash cause these stars to lose a substantial fraction of their mass into space. Aging AGB stars can lose even more mass as they evolve, resulting in expanding shells of gas surrounding them.
Step-by-step explanation:
Asymptotic giant branch (AGB) stars have high-mass loss rates because of several factors. Firstly, these stars swell up and become red giants, which have large radii and low escape velocities. This makes it easier for atoms in the outer atmosphere of the stars to be driven away by radiation pressure, stellar pulsations, and violent events like the helium flash. As a result, AGB stars lose a substantial fraction of their mass into space.
Secondly, as AGB stars evolve, they can lose even more mass when they ascend the red-giant branch for the second time. Astronomers estimate that by the time a star like the Sun reaches the point of the helium flash, it can lose as much as 25% of its mass, and it can lose even more mass during subsequent evolution. As a result, aging AGB stars are surrounded by expanding shells of gas, each containing a significant fraction of the star's mass.