A 17th-magnitude dwarf star in Leo ly distant has the lowest metallicity of any star yet discovered. Stars with very low metallicity are designated as extremely metal-poor (EMP).
SDSS J102915+172927 (aka UCAC3 215-112497, UCAC4 538-051259, Gaia DR2 3890626773968983296, or just J1029+1729 for short) was identified by Elisabetta Caffau and her team in 2011 to have a global metallicity of Z ≤ 6.9 × 10-7 which means that the star is 99.999931% hydrogen and helium. Looking at this another way, the global metallicity of our Sun is 0.0134 (98.66% hydrogen and helium), so Caffau’s Star has only about 1/19,000th the abundance of elements heavier than helium in comparison to the Sun.
Metallicity is usually expressed as the abundance of iron relative to hydrogen. It is a logarithmic scale. [Fe/H] = 0.0 for the Sun; positive numbers mean iron is more abundant and negative numbers mean iron is less abundant than in the Sun.
[Fe/H] = +2.0 means iron is 100 times more abundant than in the Sun
[Fe/H] = +1.0 means iron is 10 times more abundant than in the Sun
[Fe/H] = -1.0 means iron is 1/10 as abundant as in the Sun
[Fe/H] = -2.0 means iron is 1/100 as abundant as in the Sun
And so on. Caffau’s Star has an iron abundance [Fe/H] = -5.0, or 1/100,000th that of the Sun. Caffau’s Star is the only EMP star with [Fe/H] < -4.5 thus far detected that is not a carbon-enhanced metal-poor star (CEMP). In fact, Caffau’s Star has no detectable carbon! Nor nitrogen. Nor lithium.
Caffau’s Star is probably almost as old as our Milky Way galaxy. In order to have survived for 13 Gyr, its mass cannot be any larger than 0.8 M☉.
Aguado, D.S., Prieto, C.A., Hernandez, J.I.G., et al. 2018 ApJL, 854, L34
Aguado, D.S., Prieto, C.A., et al. 2018 ApJL, 852, L20
Aguado, D. S., González Hernández, J. I., et al. 2017, A&A, 605, A40
Bonifacio, P., Caffau, E., Spite, M., Spite, F., François, P., et al. 2018
Caffau, E., Bonifacio, P., François, P., et al. 2011, NAT, 477, 67