Why do we think the first stars were different from stars today?

The first generation of stars, called Population III stars, formed from primordial gas created after the Big Bang. Because the early universe had almost no carbon, oxygen, or other heavy elements, gas cooling worked differently than in modern star-forming regions. Many of those stars may have been very massive, perhaps tens or hundreds of times the Sun’s mass, and they would have lived brief lives. Their supernovae then forged and spread the heavier elements needed for later generations of stars and planets. Although we have not directly observed a Population III star, they are a major prediction of cosmology.