The first generation of stars were not like the stars we see today. This is because the first stellar generation was born directly from pristine hydrogen and helium–the two lightest atomic elements in the familiar Periodic Table. Both hydrogen and helium were born in the Big Bang (Big Bang nucleosynthesis). It is believed that the first stars were both gigantic and extremely brilliant, and their existence changed our Universe from what it was to what it now is.
There are three generations of stars. Our Sun is a member of Population I, meaning that it is a member of the youngest stellar generation. Population III stars are the most ancient, and they formed out the pristine gas that lingered after the Big Bang. In the jargon of astronomers, all atomic elements heavier than helium are called metals. Therefore, the term metal, as used by astronomers, is different from the same term when it is used by chemists. Population II stars are stars that are sandwiched between Populations I and III. These stars are older than our Population I Sun, but younger than the first stars of Population III. The first stars were depleted of metals, but the Population II stars show trace quantities of the metals forged in the hot hearts of the Population III stars. Population I stars, like our Sun, have the greatest metal content. However, this neat classification is somewhat misleading. This is because all stars, regardless of their generation, are roiling balls composed primarily of hydrogen gas.
Because metals can only be produced by way of the process of stellar nucleosynthesis, the existence of even trace quantities of metals indicates that an earlier Population of stars had to exist before the Population II stars were born. There had tp have been a population of stars that existed before them in order to create these metals. The Population III stars, which no longer exist in the visible Universe, left their chemical “footprints” behind in the generation of stars that came after them, and these stellar “footprints” tell of that now-vanished primordial population of the most ancient generation of stars.
Astronomers roughly categorize stars as either Population I (high metal content) or Population II (low metal content). But, because even the most metal-poor Population II stars sport a small quantity of metals, they reveal that their composition is composed of more than only the pristine primordial gas that formed in the Big Bang birth of the Universe. The Population III stellar giants were made up of only the lightest of pristine gases: hydrogen, helium, and scant amounts lithium. Therefore, the gas that composes Population III stars was not “polluted” by the heavy metals forged in the hot hearts of earlier stars. Population III stars triggered the gradual increase in stellar metallicity in increasingly younger and younger generations of stars.
Population III stars are generally thought to have been born in pure cradles of unpolluted gas. Numerical computer simulations have shed light on the very ancient and mysterious star-forming process, and the extremely short life-span of the first stars. The gigantic Population III stars did not go gentle into that good night, and they noisily blasted themselves to pieces in brilliant supernova explosions, that hurled their supply of newly-formed metals howling noisily into the space between stars. This made the newborn heavier atomic elements available to be incorporated into the giant cold, dark molecular clouds of gas and dust that served as the strange nurseries for later generations of more metal-rich stars.
Because the first stars were so massive, they rapidly used up their necessary supply of pristine hydrogen gas–and then blasted themselves to shreds in what were likely extraordinarily powerful, brilliant, and violent supernovae. Population III stars burned out at a comparatively youthful age by star-standards. These ancient supernovae were largely responsible for triggering a remarkable sea-change in the Universe. These stellar dazzlers changed utterly the dynamics of the Universe by heating it up. This new heat ionized the ambient gas.