The Toomre analysis, first studied by Viktor Safronov in the 1960s, considers not only gravity and pressure, but also shear forces from differential rotation. Conceptually, if a fluid is differentially rotating (such as in the keplerian motion of an astrophysical disk), gravity not only has to overcome the internal pressure of the gas, but also needs to halt the relative motion between two parcels of fluid, allowing them to collapse together.