Final answer:
Gas circulation from disks to stars and back again is a continuous process where stars are formed from molecular clouds and over time, expel mass and enriched elements back into the interstellar medium, altering its composition and participating in the ongoing cycle of stellar evolution.
Step-by-step explanation:
Circulation of Gas and Stellar Evolution
The interstellar medium is a dynamic environment where gas is continuously exchanged between stars and their surroundings. Stars form from giant molecular clouds where gravity causes the gas to collapse. These protostars are surrounded by a rotating disk, which may lead to the formation of planets. Once born, stars evolve and lose mass through stellar winds. This mass loss can create complex structures, which are ejected back into the interstellar medium, enriching it with new chemical elements created via nuclear reactions.
The composition of the interstellar gas is altered by this process as stars contribute heavier elements and dust grains to it. Over time, these materials can be incorporated into new stars and thus continue the cycle of stellar birth and death. It's pertinent to consider that the interstellar gas also includes material accreted from extragalactic space, adding an external component to the galactic ecosystem. Ultimately, this cyclical exchange shapes the composition of stars and influences the evolution of the galaxy.