Only a handful of stars have been identified with an iron abundance [ Fe/H ] < -5 , and only one at [ Fe/H ] < -7 .
These stars have very large carbon-to-iron ratios , with A ( C ) \sim 7.0 , most likely due to fallback in core-collapse supernovae , which makes their total metallicity Z much higher than their iron abundances .
The failure to find population III stars , those with no metals , has been interpreted , with support from theoretical modeling , as the result of a top-heavy initial mass function .
With zero or very low metal abundance limiting radiative cooling , the formation of low-mass stars could be inhibited .
Currently , the star SDSS J1029 + 1729 sets the potential metallicity threshold for the formation of low-mass stars at \log Z / Z _ { \odot } \sim - 5 .
We have identified SDSS J0023+0307 , a primitive star with T _ { eff } = 6188 \pm 84 K , and \log g =4.9 \pm 0.5 , an upper limit [ Fe/H ] < -6.6 , and a carbon abundance A ( C ) < 6.3 .
In our quest to push down the metallicity threshold we find J0023+0307 to be one of the two most iron-poor stars known , and it exhibits less carbon that most of stars at [ Fe/H ] < -5 .