In astronomy, the mass and size of a galaxy (or general overdensity) is often defined in terms of the " virial mass " and " virial radius " respectively. Because galaxies and overdensities in continuous fluids can be highly extended (even to infinity in some models, such as an isothermal sphere), it can be hard to define specific, finite measures of their mass and size. The virial theorem, and related concepts, provide an often convenient means by which to quantify these properties.