a (very luminous) hot star, this can compete with gravity…but note the 1/R2dependence,
if arad> agrav, a star would blow itself completely apart.
note, free electron opacity, and the associated Thompson scattering, can be significantly
augmented by absorption
of photons in spectral lines - atoms act like a resonance chamber for electrons: a bound electron
can be ‘driven’ much
more efficiently by light than a free one (i.e. it has a much larger cross section), but it
can only be driven by light
with a very specific frequency.