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Neutron Stars

Neutron stars are highly compact astrophysical objects and therefore of utmost relevance to learn about theories of gravity. Whereas the proper equation of state of the nuclear matter inside neutron stars is not yet known, and a wide range of equations of state is still compatible with observations, this uncertainty can be overcome to a large extent, when dimensionless neutron star properties are considered. In this case universal relations between neutron star properties and for the gravitational radiation in the form of quasi-normal modes arise. These universal relations can be rather distinct for alternative theories of gravity as compared to General Relativity. Moreover, the presence of new degrees of freedom in alternative theories of gravity leads to further types of gravitational radiation, that may be revealed by pulsar observations and gravitational wave detectors. Here an introduction to neutron stars, their properties and universal relations is presented, followed by two examples of alternative theories of gravity featuring interesting effects for neutrons stars.