Recent results
One of the big surprises to have come from recent studies is that there is a large difference in the metal content of the most primitive stars known and the material produced directly from the Big Bang. One possibility is that stars made purely from Big Bang material, so-called Population III, were so few in number that they are difficult to find. Such stars would indeed be very rare, because even a single supernova can produce enough metals to enrich a typical-sized star-forming region by 0.0002%. Population III stars could also have become supernovae so quickly that they polluted their more slowly forming neighbours which we now observe as Population II stars. Population III stars may also have formed before the Galaxy, so few would be found near the Sun. Another explanation is that perhaps only high mass stars could form from Big Bang gas, and these would have burnt out by now.

A second surprise is the huge amount of carbon that was produced in the earliest stages of the Galaxy; many of the primitive stars have ten or even one hundred times more carbon (relative to iron) than stars that formed later. One possibility is that primitive stars were better able to throw off their outer layers than more modern stars are; perhaps they rotated faster and this gave an extra boost to the loss of their outermost, carbon-rich layers. This would enrich surrounding gas clouds with carbon, but not iron. Another suggestion is that in Population III stars, freshly made carbon can be mixed to the outer layers much more readily than in Population II stars.