The early Universe presented a star formation environment that was almost devoid of heavy elements .
The lowest metallicity stars thus provide a unique window into the earliest Galactic stages , but are exceedingly rare and difficult to find .
Here we present the discovery of an ultra-metal-poor star , Pristine_221.8781+9.7844 , using narrow-band Ca H & K photometry from the Pristine survey .
Follow-up medium and high-resolution spectroscopy confirms the ultra-metal-poor nature of Pristine_221.8781+9.7844Â ( [ Fe/H ] = - 4.66 \pm 0.13 in 1D LTE ) with an enhancement of 0.3 - 0.4 dex in \alpha -elements relative to Fe , and an unusually low carbon abundance .
We derive an upper limit of A ( C ) = 5.6 , well below typical A ( C ) values for such ultra metal-poor stars .
This makes Pristine_221.8781+9.7844Â one of the most metal-poor stars ; in fact , it is very similar to the most metal-poor star known ( SDSS J102915+172927 ) .
The existence of a class of ultra metal-poor stars with low ( er ) carbon abundances suggest that there must have been several formation channels in the early Universe through which long-lived , low-mass stars were formed .