The first stars

Population III (or Pop. III) stars, the first stellar generation built up from metal-free primordial gas, first started to form at redshifts z ~ 30. They formed primarily in small dark matter halos with masses of a few million solar masses. The cooling of the gas in these halos was dominated on all scales by molecular hydrogen. Current theoretical models indicate that Pop. III stars typically formed in small clusters with a logarithmically flat mass function due to widespread fragmentation in the protostellar accretion disks around these primordial stars. Massive Pop. III stars are thought to have played a pivotal role in shaping the early Universe, as their feedback regulates subsequent star formation, although the immediate effects of this feedback remain uncertain. Direct detection of Pop. III stars is challenging, but our chances of detecting at least a few Pop. III supernovae within the next decade are brighter. Indirect approaches based on stellar archaeology or gravitational wave detections offer promising constraints. Current observations suggest that most massive Pop. III stars ended their lives as core-collapse supernovae rather than pair-instability supernovae, offering insight into the initial mass function and evolutionary pathways of these primordial stars.