  A photon sphere is known as the geometrical structure shaping a black hole
shadow. The mechanism is well understood for static or stationary black hole
spacetimes such as the Schwarzschild and the Kerr spacetimes. In this paper, we
investigate and explicitly specify a photon sphere that shapes a black hole
shadow in a dynamical spacetime while taking the global structure of the
spacetime into account. We consider dynamical and eternal black hole cases of
the Vaidya spacetime, which represents a spherically symmetric black hole with
accreting null dust. First, we numerically show that there are the dynamical
photon sphere and photon orbits corresponding to the shadow edge in a moderate
accretion case. Second, the photon spheres are derived analytically in special
cases. Finally, we discuss the relation between our photon sphere and the
several notions defined as a photon sphere generalization.
