Question

How does galaxy composition typically relate to the age of the stars in the galaxy?
Responses

Star aging emits gas and dust, so galaxies with the youngest stars typically have the lowest proportions of dust and gas.
Star aging emits gas and dust, so galaxies with the youngest stars typically have the lowest proportions of dust and gas. - no response given

Stars form from dust and gas, so galaxies with younger stars typically consist of higher proportions of dust and gas than other galaxies.
Stars form from dust and gas, so galaxies with younger stars typically consist of higher proportions of dust and gas than other galaxies. - no response given

Star formation and the presence of young stars is common across galaxies, regardless of the proportions of dust and gas present.
Star formation and the presence of young stars is common across galaxies, regardless of the proportions of dust and gas present. - no response given

Star formation consumes dust and gas, so galaxies where stars are oldest typically consist of higher proportions of dust and gas than other galaxies.

Answer 1

Correct choice:
- "Stars form from dust and gas, so galaxies with younger stars typically consist of higher proportions of dust and gas than other galaxies."

Why: Star formation requires cold gas (mainly hydrogen) and dust to cool and collapse into stars. Galaxies with recent or ongoing star formation (e.g., spiral and irregular galaxies) therefore tend to be gas- and dust-rich and show many young, blue stars. By contrast, galaxies dominated by old stars (e.g., elliptical galaxies) are generally gas- and dust-poor and appear redder.

Caveat: There are exceptions — gas can be removed by feedback, environmental stripping, or consumed in starbursts, and gas can be reaccreted or brought in by mergers, leading to renewed star formation in otherwise old galaxies.