  Non-minimal hidden sectors are an important generic possibility and arise in
highly motivated theories like Neutral Naturalness. A fraction of dark matter
could therefore have hidden interactions analogous to Standard Matter (SM)
electromagnetism and nuclear physics. This leads to the formation of
\emph{Mirror Stars}: dark-sector analogues of regular stars that shine in dark
photons. We examine the visible signatures of Mirror Stars in observations for
the first time. If the dark and visible photon have a small kinetic mixing, SM
matter is captured in Mirror Star cores, giving rise to an optical signal
similar to but much fainter than white dwarfs, as well as a separate X-ray
signal that represents a direct window into the Mirror Star core. This robust
and highly distinctive signature is a smoking gun of Mirror Stars and could be
discovered in optical and X-ray searches.
