  In this work we analyze the observational properties of relativistic fluid
spheres when orbited by isotropically emitting sources, known as hot spots. We
consider fluid star configurations in four different regimes of compacticity,
from the Buchdahl limit to non-ultra compact solutions, thus obtaining fluid
stars with qualitatively different geodesic structures and observational
properties. We show that the observational properties for fluid stars at the
Buchdahl limit are qualitatively similar to the ones for the Schwarzschild
black hole, whereas for more dilute configurations one can find observational
properties similar to other fundamentally different models e.g. bosonic star
configurations with and without self interactions. For solutions with a radius
in the range $2.25M<R<3M$, where $M$ is the total mass of the fluid star, the
presence of a light-ring (LR) pair, which becomes degenerate at $R=3M$, leads
to the appearance of additional observational signatures e.g. secondary images
and LR contributions, which allow one to distinguish these models from their
black-hole counterparts. Fluid star configurations supported by thin shells are
also analyzed and it is proven that the stability of the inner LR is increased
for these solutions, resulting in a non-differentiable extremum in the
effective potential of the photons. Our results suggest that compact fluid star
configurations provide a suitable and physically relevant alternative to the
black-hole scenario in accordance with the current generation of observational
experiments.
