SDSS_J102915+172927
SDSS J102915+172927
Population II star in the constellation Leo
SDSS J102915+172927 or Caffau's Star is a population II star in the galactic halo, seen in the constellation Leo. It is about 13 billion years old, making it one of the oldest stars in the Galaxy.[3] At the time of its discovery, it had the lowest metallicity of any known star.[4] It is small (less than 0.8 solar masses),[5] deficient in carbon, nitrogen, oxygen, and completely devoid of lithium. Because carbon and oxygen provide a fine structure cooling mechanism that is critical in the formation of low-mass stars, the origins of Caffau's Star are somewhat mysterious. It has been suggested, both for theoretical and observational reasons, that the formation of low-mass stars in the interstellar medium requires a critical metallicity somewhere between 1.5×10−8 and 1.5×10−6.[6]
The metallicity of Caffau's Star is less than 6.9×10−7.[6] According to Schneider et al., cooling by dust rather than the fine structure lines of CII and OI may have enabled the creation of such low-mass, metal-poor stars in the early universe.[5][7] The absence of lithium implies past temperatures of at least two million kelvins.[6]
Data from Gaia's DR2 released in 2018 confirmed that SDSS J102915+172927 is a dwarf star.[2]
The star was described by Elisabetta Caffau et al. in an article published by the journal Nature in September 2011. Caffau had been searching for extremely metal-poor stars for the past ten years.[8] It was identified by automated software which analyzed data from the Sloan Digital Sky Survey. This was followed up by observations with the X-shooter and UVES instruments on the Very Large Telescope in Chile.[4] As of 2011[update] Caffau and her team expect to find between five and fifty similar stars with the telescope in the future.[7][needs update]