Population III stars have recently been modelled as very massive and short-lived stars (80-300 solar masses) that formed early in the Universe's history. Only one arises from each pre-galactic halo, whereas many Population II stars would be formed in the same halo.

They are thought to end their short lives with "hypernova" events, which could be the origin of gamma-ray bursts. The hypernova completely obliterates the Population III star into interstellar space, the result being that we cannot find any remnants that would tell us where the star was. To my knowledge, no population III stars have been observed in any form other than a gamma-ray burst. This rarity makes sense, as they are few and short-lived.