What diagnostic tool allows astronomers to map out chemical evolution across the galaxy by comparing nebulae?

Answer

Accurately measuring differences in elemental composition between one nebula and the next

The ability to accurately measure the differences in elemental composition between various nebulae allows astronomers to track chemical evolution, distinguishing older, less enriched material from newer, heavily processed material.

What diagnostic tool allows astronomers to map out chemical evolution across the galaxy by comparing nebulae?
SpaceastronomycompositionstarsElementsstellar nebula