Final answer:
The statement does not make sense; the Milky Way's brightness will not remain unchanged over 100 million years due to eventual depletion of gas for star formation, death of current stars, and varying star lifespans.
Step-by-step explanation:
The statement that the star-gas-star cycle will keep the Milky Way looking just as bright in 100 million years as it looks now does not make sense (b). While it is true that the Milky Way contains enormous amounts of gas and dust that can be turned into new stars, and the formation of stars at a rate of about 1 solar mass per year will continue, this does not mean the brightness of the galaxy will remain static. Eventually, the available gas and dust will be used up or dispersed, leading to a decline in star formation. Moreover, as current stars exhaust their nuclear fuel, they will fade away or explode as supernovae, removing their light from the galaxy.
Additionally, it is important to consider that stars have varying lifespans based on their masses. The ones that shine brightest tend to have shorter lives. As these massive stars die, their replacements may not be as luminous, contributing to a gradual dimming effect over time. The universe also contains a finite amount of matter for star formation, and as galaxies evolve, they may no longer be able to form new stars at a rate that maintains their current brightness.
In the context of the future of galaxies, the model of the universe predicts that there will come a time when all the stars within a galaxy like the Milky Way will burn out, leaving galaxies dark and lifeless as they continue to move apart in their lightless realm.