= Gas fraction of a galaxy
{title2=$\mu$}
The gas fraction of a <galaxy> is commonly $\mu=M_g/(M_g+M_s)$, using the gas and stellar masses remaining in the system. In a <leaky-box model of galactic chemical evolution>, it differs from the fraction $g=M_g/M_{g,0}$ of the initial gas reservoir. For an initially star-free box with outflow rate $\alpha\dot M_s$, $\mu=(1+\alpha)g/(1+\alpha g)$. These different fractions give different-looking metallicity formulae and <effective yields>.
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