AstronomyTeacher

SPACE.com -- Andromeda Galaxy Five Time Bigger Than Thought: "According to current galactic formation theories, the halo is the first part of a galaxy that forms. Stars in the halo are predicted to be metal poor because they formed during a time when the universe had much less heavy metal content than it does now. Heavy metals are created as stars evolve and then spewed out into interstellar space when ancient stars either explode as supernovas or shed their outer layers to become white dwarfs.
'The first stars are expected to be chemically deficient, and as these other components such as the disk of the galaxy form later, it is contaminated by the products of those first stars, so those stars are more metal rich,' Kalirai said.
However, instead of being metal-poor, previous studies have found that Andromeda's halo stars were actually 10 times richer in metals than halo stars in our galaxy. This finding puzzled astronomers because both Andromeda and the Milky Way should have similar formation histories."