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Neutron stars and black holes are possible end states of the stellar evolution for massive stars. While the gravitational forces within black holes are so large that the entire material gets compressed within the central singularity, nuclear forces enable neutron stars, which have slightly smaller masses than black holes, to be stable without a spacetime singularity. However, the density inside a neutron star exceeds any kind of density that we can realize here on earth and can reaches a few times nuclear saturation density, i.e., up to a few hundred tons per cubic centimeter.

Numerous compact objects are bound inside binary systems, some of these, so-called compact binary systems are either binary neutron star systems (often also called double neutron star systems), black hole - neutron star systems, and binary black hole systems. On top of these, well-established scenarios, there is also the possibility for the existence of exotic compact objects that also could be bound inside compact binaries. These exotic compact objects are, e.g., boson stars, i.e., stars consisting of bosonic fields, proca stars, gravastars. Up to now, there is no evidence for the existence of such exotic compact objects.