### mass shell

(conceptual volume of mass shaped like a hollow sphere)

In mathematically modeling stellar structure (and in 1D models
of other spherically-symmetric bodies), the term **mass shell** refers
to "all the mass within a certain distance interval from the center",
a concept useful in constructing differential equations for the
model. The shell may be considered "infinitely thin"
for the purposes of differential equations,
and "thin enough that its thickness doesn't appreciably affect the outcome"
for difference equations used in computation.

The term **mass shell** is also used with an entirely different
meaning in quantum mechanics: a phenomena or concept is referred
to as **on the shell** or **off the shell** depending upon whether
it follows certain classical ("every day") rules regarding
the relationship of energy and momentum. This "shell" is
the ellipsoid shape of the 3D graph of an equation that
shows the possibilities, i.e., the graph of quantities
when the rules are followed.

(*stellar structure,computation*)
http://en.wikipedia.org/wiki/Stellar_structure

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