Before turning to the derivation of the virial theorem, it is appropriate to review the origin
of the fundamental structural equations of stellar astrophysics. This not only provides insight into
the basic conservation laws implicitly assumed in the description of physical systems, but by
their generality and completeness graphically illustrates the complexity of the complete
description that we seek to circumvent. Since lengthy and excellent texts already exist on this
subject, our review will of necessity be a sketch. Any description of a physical system begins
either implicitly or explicitly from certain general conservation principles. Such a system is
considered to be a collection of articles, each endowed with a spatial location and momentum
which move under the influence of known forces. If one regards the characteristics of spatial
position and momentum as being highly independent, then one can construct a multi-dimensional
space through which the particles will trace out unique paths describing their history.
This is essentially a statement of determinism, and in classical terms is formulated in a
six-dimensional space called phase-space consisting of three spatial dimensions and three
linearly independent momentum dimensions. If one considers an infinitesimal volume of this
space, he may formulate a very general conservation law which simply says that the divergence
of the flow of particles in that volume is equal to the number created or destroyed within that
volume.