It's very difficult to measure the mass of neutron stars because in order to measure the mass of a star you need two stars close together, a neutron star and another star in a binary, or two neutron stars together. And fortunately nature has been kind to us; there is one pair of neutron stars discovered by Taylor in Princeton for which he got the Nobel Prize. They are so close together that they emit gravitational waves by their interaction and due to the emission of gravitational waves they get closer and closer together in time. This is the first proof and the chief proof that Einstein's general relativity theory is correct in all detail to all orders, because to get gravitational waves you have to go way beyond Newtonian Theory and well, gravitational theory in the first order beyond Newton was proved by observations by Shapiro of the transit of electromagnetic waves close to the sun, which showed that the sun not only deflects these waves but also changes their frequency. Both of these are predictions of general relativity theory, but only the first order, so-called post-Newtonian theory, and so that post-Newtonian Einstein gravitation theory was proved very well by the Shapiro observations. But it was still an open question: is it just the first order after Newton, or is gravitational theory correct all the way? And the Taylor observation of the two neutron stars in the binary is the real clincher of... of this gravitational theory to all orders.