  Theories that attempt to explain cosmic acceleration by modifying gravity
typically introduces a long-range scalar force that needs to be screened on
small scales. One common screening mechanism is the chameleon, where the scalar
force is screened in environments with a sufficiently deep gravitational
potential, but acts unimpeded in regions with a shallow gravitational
potential. This leads to a variation in the overall gravitational G with
environment. We show such a variation can occur within a star itself,
significantly affecting its evolution and structure, provided that the host
galaxy is unscreened. The effect is most pronounced for red giants, which would
be smaller by a factor of tens of percent and thus hotter by 100's of K,
depending on the parameters of the underlying scalar-tensor theory. Careful
measurements of these stars in suitable environments (nearby dwarf galaxies not
associated with groups or clusters) would provide constraints on the chameleon
mechanism that are four orders of magnitude better than current large scale
structure limits, and two orders of magnitude better than present solar system
tests.
