  Neutron stars with extremely high central energy density are natural
laboratories to investigate the appearance and the properties of compactified
extra dimensions with small compactification radius, if they exist. Using the
same formalism, these exotic hybrid stars can be described as neutron stars
with quark core, where the high energy density allows the presence of heavy
quarks (c, b, t). We compare the two scenarios for hybrid stars and display
their characteristic features.
