The apparent absence of stars in the Milky Way halo with -5 \lesssim [ \mathrm { Fe } / \mathrm { H } ] \lesssim - 4 suggests that the gas out of which the halo stars were born experienced a period of low or delayed star formation after the local universe was lit up by the first , metal-free generation of stars ( Pop III ) .
Negative feedback owed to the Pop III stars could initially have prevented the pre-Galactic halo from cooling , which thereby delayed the collapse and inhibited further star formation .
During this period , however , the nucleosynthesis products of the first supernovae ( SNe ) had time to mix with the halo gas .
As a result , the initially primordial gas was already weakly enriched in heavy elements , in particular iron , at the time of formation of the Galactic halo .
The very high , observed \mathrm { C } / \mathrm { Fe } ratios in the two recently discovered hyper metal-poor stars ( [ \mathrm { Fe } / \mathrm { H } ] < -5 ) HE 0107 - 5240 and HE 1327 - 2326 as well as the diversity of \mathrm { C } / \mathrm { Fe } ratios in the population of extremely metal-poor stars ( [ \mathrm { Fe } / \mathrm { H } ] < -3 ) are then naturally explained by a combination of pre-enrichment by Pop III stars and local enrichment by subsequent generations of massive , rotating stars , for which the most massive ones end their lives as black hole-forming SNe , only ejecting their outer ( carbon-rich ) layers .
The possible existence of populations of mega metal-poor/iron-free stars ( [ \mathrm { Fe } / \mathrm { H } ] < -6 ) is also discussed .