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When two neutron stars spiral into each other, the object that results from their merger has an inner density that can be several times the density of an atomic nucleus (itself around 10 trillion times that of solid gold) and a temperature of around 100 billion degrees Celsius. Such events can therefore offer key information about the properties of matter at extreme densities and temperatures. However, the equations that describe ultradense matter cannot be solved exactly, and current numerical techniques cannot yet approximate the dense environment of a neutron star1. Progress in understanding these properties therefore requires collaboration between scientists in different fields, and assimilation of different types of data. This ‘big science’ approach is showcased to great effect in a paper in Nature, where Huth et al.2 report a comprehensive analysis of neutron-star matter using astrophysical observations, experiments on heavy-ion collisions and nuclear-physics models.