Metallicity specifies the relative amount of elements heavier than helium (collectively called metals by astronomers) in a star, galaxy, or some part of the interstellar medium. The metallicity of the Sun is Z⊙ = 1.5 % by mass. The most metal rich stars in the inner Galaxy have two or three times that figure, whereas the most metal poor halo stars of the Milky Way have metallicities as low as 0.0001 Z⊙. Usually, the abundance of some key element (like Fe or O) is used as a proxy for metallicity. Host stars of extrasolar giant planets (observed in the solar neighborhood) are more metallic than non-planet hosting stars.