Neutrons which constitute a neutron star have a rest mass that is greater when separated from the star because they are bound with a certain potential energy. This potential energy causes the system to have less mass. So my question is how much less? Is it significant? Do we measure it?
Answer
The gravitational mass of a neutron star is quite a lot less than its baryonic rest mass (plus the mass associated with the kinetic energy of its contents), because a bound neutron star, by definition, must have a total energy (the sum of its internal energy and gravitational potential energy) that is less than zero.
In a “normal star” this is also true, bit the difference is that the gravitational potential energy of a neutron star can be comparable with its rest mass energy.
How significant is this? It depends on the baryonic mass of the neutron star and the equation of state of the dense matter. For a typical neutron star of 1.4 solar masses and 10km radius, the order of magnitude estimate for binding energy as a multiple of rest mass energy, $GM/Rc^2$, is about 0.2, suggesting a significant reduction in the gravitational mass compared with the baryonic mass.
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