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In the 1930s the concept of a neutron star was introduced as a theoretically possible state of an astrophysical compact object. The first observational evidence arose in 1967 when Jocelyn Bell, Antony Hewish, and Martin Ryle detected radio signals from a pulsar. Since now more than 2000 neutron stars have been detected, but only for a few of them, it was possible to measure the neutron star masses reliably.

Neutron stars can arise from supernova explosions of massive stars (between ~8 and 30 solar masses), from the merger of white dwarfs, or from the merger of light neutron stars.

Because of their extremely high densities, neutron stars are a perfect nuclear physics laboratory. Hence, measuring neutron star properties allows us to place constraints on the matter at supranuclear densities. Given that such densities are not reachable on Earth, we rely on theoretical models describing the neutron star interior, but those come with large uncertainties.

Neutron stars are surrounded by an `atmosphere' consisting of a few micrometers. , and its dynamics are fully controlled by the neutron star's magnetic field. Below the atmosphere, one expects that the material at the star's surface consists of ordinary atomic nuclei placed in a solid lattice with surrounding, freely moving electrons. In the case of young neutron stars with high temperatures exceeding 1 million Kelvin, the atomic nuclei are in a fluid state rather than a solid lattice. Once moving inward the neutron star, one expects nuclei with increasing numbers of neutrons, which are only stable due to the extreme pressure inside the star, up to a point where mostly free neutrons are present. Finally, the state of matter inside the core of the neutron star is unknown, e.g., there might be superfluid neutron-degenerate matter, degenerate strange matter, or ultra-dense quark matter. The following figure (from Dany Page).