= Toomre parameter
{c}
{title2=$Q=c\kappa/(\pi G\Sigma)$}
For a razor-thin fluid disk with <sound speed> or effective random speed $c$, <surface density> $\Sigma$ and radial epicyclic frequency $\kappa$, $Q=c\kappa/(\pi G\Sigma)$ compares pressure and rotational support with <self-gravity>. The local axisymmetric fluid <Toomre stability criterion> is $Q\geq1$. For a <Keplerian shearing sheet>, $\kappa=\Omega$. Collisionless stellar disks use a different numerical normalization; gas drag can permit secular growth even when this fluid parameter exceeds unity.
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