  The paper confirms the existence of a special configuration (among the
infinite number of virial states) which a B stellar(Baryonic) component may
assume inside a given D dark halo potential well.This satisfies the d'Alembert
Principle of virtual works and its typical dimension works as a scale length
(tidal radius)induced on the gravitational field of the bright component by the
dark one.Its dynamic and thermodynamic properties are here analyzed in
connection with the physical reason for the existence of the Fundamental Plane
(FP) for ellipticals.The analysis is performed by using 2-component models with
two power-law density profiles and two homogeneous cores and compared with some
observable scaling relations for pressure supported ellipticals.The virial
equilibrium stages of the 2-component system have to occur after a previous
violent relaxation phase. If the stellar B component is allowed to cool slowly
its virial evolution consists of a sequence of contractions with enough time to
rearrange the virial equilibrium after any step. The thermodynamic process
during the dynamical evolution is so divided into a sequence of transformations
which are irreversible but occur between two quasi-equilibrium stages.The
analysis allows the conclusion that the induced scale length is a real
confinement for the stellar system. The presence of this specific border on the
space of the baryonic luminous component has to be regarded as the physical
reason why a stellar galaxy belongs to the FP and why astrophysical objects,
with a completely different history and formation, but characterized by a tidal
radius (as the globular clusters are) lie on the same FP. An other problem
addressed is how this special configuration may be reached and why an
elliptical is not completely relaxed in respect to its dark halo.
