For 10 points, identify this quantity, often designated rho, that is extremely high for black holes, because matter has been squeezed together so tightly.
In astronomy, the Toomre Q parameter is inversely proportional to this property of a gas surface. The LaneÂ–Emden equation contains a theta term raised to some index, where the theta term measures this quantity. Polytropes are solutions to that equation in which pressure is dependent upon this quantity. Structures that are named after this quantity are driven by orbital resonances that are multiples of the forcing frequency, which is a Lindblad resonance. Lin and Shu proposed that those structures of this name solve the winding problem of the arms of spiral galaxies. The escape velocity for an object equals the speed of light when this quantity is equal to mass divided by the volume of a sphere given by the corresponding Schwarzschild radius. For 10 points, identify this quantity, often designated rho, that is extremely high for black holes, because matter has been squeezed together so tightly.
In astronomy, the Toomre Q parameter is inversely proportional to this property of a gas surface.
density [accept density waves; prompt on "pressure" before mention; prompt on "waves"]