PROXY MEASURES. “Only astrophysical observations of neutron stars can constrain the properties of the cold supranuclear matter in their cores. Because we cannot sample the matter directly, we need to infer its state by measurements of neutron star masses, radii, and cooling rates. For the last two of these, the method of measurement is highly indirect and thus subject to systematic errors.”
https://link.springer.com/chapter/10.1007/978-3-662-62110-3_1