Question

Astronomers claim that objects throughout the universe are made of the same chemical elements that exist here on Earth. Given that most of these objects are so far away that we can never hope to visit them, why are astronomers so confident that the objects are made from the same set of chemical elements, rather than completely different substances?

Answer 1

every element and compound has a unique energy spectra, in other words measuring the energies of the photons coming from a single source can tell you the chemicals you are looking at