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Kilonovae are electromagnetic transients observable in the infrared, optical, and ultraviolet bands. They originate from the neutron-rich outflows that are created during and after the merger of a binary neutron star or black hole – neutron star systems.

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.