where N F e and N H are the number of iron and hydrogen atoms per unit of volume respectively, ⊙ ⊙ is the standard symbol for the Sun, and ⋆ ⋆ for a star (often omitted below). The unit often used for metallicity is the dex, contraction of "decimal exponent". By this formulation, stars with a higher metallicity than the Sun have a positive common logarithm, whereas those more dominated by hydrogen have a corresponding negative value. For example, stars with a [ F e H ] ⋆ ⋆ value of +1 have 10 times the metallicity of the Sun (10); conversely, those with a [ F e H ] ⋆ ⋆ value of −1 have ⁠ 1 / 10 ⁠, while those with a [ F e H ] ⋆ ⋆ value of 0 have the same metallicity as the Sun, and so on.