What are Population III stars? In short, the alleged story is as follows:
The super-hot big bang fireball produced only hydrogen (~75%), helium (~25%) and tiny traces of lithium. So the first stars to form (given the name Population III stars) could only form from these gases. Astronomers label all elements heavier than helium as ‘metals.’ Thus they call these type of stars extremely metal-poor. But each successive generation of stars, being formed from the products of supernova explosions of the generation of stars before them, which produced all the heavier elements, became more and more metal rich. The nuclear fusion within stars during their life produced the heavier elements, the ‘metals,’ like carbon, oxygen, and nitrogen, which were released into space when the stars exploded. During the actual explosion it is theorized that the very heaviest elements were produced also. Population III stars allegedly were the first stars formed just shortly after the big bang.
Until now (as claimed) these original stars have never be observed, hence they were nothing more than hypothetical. But their existence is a big bang prediction.
Population I, II and III stars
Astronomers classify stars into three types: Population I, II and III. Population II are those generation of stars, which allegedly formed from the Population III stars and have only a low metal content. Population I stars were allegedly the last to form, hence are the youngest and hottest stars and those with high metal content. Population I and II stars were historically first identified in our Galaxy. Population I stars are found predominantly in the spiral disk of the Galaxy and Population II stars are found above and below the disk. They have other distinguishing features also but their metal content is the major distinguishing feature.
Those early-generation stars also first formed into small galaxies that later by merging with other galaxies grew larger, or so the story goes.1 Growth in galaxy size and in ‘metal’ content is called ‘galaxy evolution.’
The first generation of small galaxies was likely well in place 400 million years after the Big Bang. Following this initial phase of galaxy formation, galaxies then went through an extended phase of merging and coalescence with other galaxies, whereby they built up from masses of several thousand solar masses to billions of solar masses. This buildup process extended until the universe was roughly two billion years old. Then, due to some feedback process — now predominantly speculated to be AGN feedback — it is thought that this buildup process halted and gas accretion and star formation in the most massive galaxies halted and galaxies underwent a much different form of evolution. This later evolution continues to the present day.
This is the big bang evolution story, but it vitally needs those Population III stars or there is no story. Now it is claimed that Population III have been found in a very distant galaxy.