Stars observed in galaxies were originally divided into two populations by Walter Baade in the 1940s. Although a more refined means of classifying stellar populations has since been established (according to whether they are found in the thin disk, thick disk, halo or bulge of the galaxy), astronomers have continued to coarsely classify stars as either Population I (Pop I, metal-rich) or Population II (Pop II, metal-poor). However, even the most metal-poor Pop II stars have metallicities (commonly denoted [Z/H]) far above that of the gas left over from the Big Bang.

For this reason, astronomers have introduced a third class of star. Population III (Pop III) stars are composed entirely of primordial gas – hydrogen, helium and very small amounts of lithium and beryllium. This means that the gas from which Pop III stars formed had not been ‘recycled’ (incorporated into, and then expelled) from previous generations of stars, but was pristine material left over from the Big Bang. As such, these stars would have a [Z/H] ~ -10 and would constitute the very first generation of stars formed within a galaxy. These Pop III stars would then produce the metals observed in Pop II stars and initiate the gradual increase in metallicity across subsequent generations of stars.

The only problem is that Pop III stars are entirely hypothetical at present. Despite intense searches, no Pop III star has ever been observed. A number of explanations have been put forward to explain this:

• As the oldest population of stars, the majority of Pop III stars would have exhausted their fuel supplies long ago and would now be observed as remnants (white dwarfs, neutron stars or black holes), the original composition of which is nearly impossible to determine. However, this alone cannot explain the absence of Pop III stars, as those with the lowest masses should still be present (albeit difficult to observe due to their extremely low luminosities) in the Galaxy population today.
• Another explanation is that stars sweep up gas from the interstellar medium as they move through it, and this may contaminate the outer layers of Pop III stars. This would give Pop III stars the appearance of metal-poor Pop II stars.
• A more plausible explanation is that the metals produced in the cores of the Pop III stars have been dredged up to the surface by convection. Such ‘self-contaminated’ Pop III stars would also most likely be misclassified as metal-poor Pop II stars.
• The currently favoured explanation for the lack of observed Pop III stars, is that the Pop III generation of stars were all high mass stars, with masses ranging from 60 to 300 times that of the Sun. In other words, no low mass Pop III stars were ever formed. This is supported by recent theoretical models which show that primordial stars possessed much higher masses than the stars we see in the Universe today. If this bias in the mass distribution of primordial stars is the case, then all Pop III stars would have exhausted their fuel supplies long ago and would now be present only as remnants.

Whatever the reason, it is extremely unlikely that we will ever observe a Pop III star and they will remain hypothetical entities. Nevertheless, there are still some astronomers out there on the hunt for them.