Question

stars are composed mostly of hydrogen and helium. where do heavier elements come from?

supernovas
black holes
supergiants
main sequence stars

Explanation:

Supernovas are explosive events where stars reach the end of their life cycle. During a supernova, the extreme conditions (high temperatures and pressures) allow for the synthesis of heavier elements (elements with atomic numbers greater than that of iron) through processes like neutron - capture. Black holes are regions of spacetime with extremely strong gravitational fields and do not directly produce heavier elements. Supergiants are large stars in a particular stage of their evolution but do not create heavier elements in the way supernovas do. Main - sequence stars mainly fuse hydrogen into helium and do not produce heavier elements beyond a certain point (up to iron in some massive main - sequence stars, but not heavier than that in a significant amount for the origin of most heavier elements in the universe).

Answer:

Supernovas