  Globular clusters (GCs) are dense, gravitationally bound systems of thousands
to millions of stars. They are preferentially associated with the oldest
components of galaxies, and measurements of their composition can therefore
provide insight into the build-up of the chemical elements in galaxies in the
early Universe. We report a massive GC in the Andromeda Galaxy (M31) that is
extremely depleted in heavy elements. Its iron abundance is about 800 times
lower than that of the Sun, and about three times lower than in the most
iron-poor GCs previously known. It is also strongly depleted in magnesium.
These measurements challenge the notion of a metallicity floor for GCs and
theoretical expectations that massive GCs could not have formed at such low
metallicities.

Astrophysics of Galaxies;Solar and Stellar Astrophysics