Final answer:
Stars that formed in the early universe were made up of mostly hydrogen and helium, while stars formed more recently have more heavy elements. Young, distant galaxies are smaller, bluer, and have a higher rate of star formation than those we see now. The composition and color of distant galaxies serve as evidence of early star formation.
Step-by-step explanation:
Stars formed in the early years of the universe differ from stars formed more recently in several key ways. Primarily, early stars would have a different chemical composition, as they would be made almost entirely of hydrogen and helium, the elements produced by the Big Bang, with very few heavier elements (metals) which are synthesized in stars and spread throughout the universe in supernova explosions. Over time, as stars have formed and died, they have enriched the interstellar medium with heavier elements. So, stars formed more recently would have a higher metallicity compared to their ancient counterparts.
When it comes to galaxies, younger, more distant galaxies observed in the universe tend to be smaller, bluer, and show a higher rate of star formation compared to the larger, older galaxies we see in the present-day universe. This color difference arises because younger galaxies have a higher proportion of young, massive, and hot stars which emit more blue light. The galaxies visible today have gone through multiple stages of evolution, resulting from processes such as galactic mergers and the aging of their stellar populations.
As for the evidence that star formation began when the universe was only a few hundred million years old, astronomers use the redshift of light from distant galaxies to infer their age. They have found that these galaxies, which we observe as they were billions of years ago, contain old red stars and are extremely blue, indicating significant early star formation activity within them.