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An exact solution of Einstein's field equations for a static isentropic spherically symmetric perfect fluid is investigated in detail. The analysis yields a strong indication that the model is stable with respect to infinitesimal radial pulsations. We also find that the adiabatic speed of sound is smaller than the speed of light everywhere inside the fluid sphere if and only if the radius of the sphere is larger than 1. 46 times its Schwarzschild radius. The necessary and sufficient criterion for the sound speed to be decreasing outwards close to the centre is given, but if this criterion is fulfilled the fluid must necessarily be supraluminal somewhere. It is further found that the strong energy condition is fulfilled everywhere if it is fulfilled at the origin, and the ratio of the pressure p and the density ϱ is decreasing outwards. This necessarily yields that temperature is decreasing outwards. The relativistic adiabatic index γ is examined, and it is shown that we have | 2|. Demanding the fluid to be causal, and taking the values for the pressure and the density to be somewhere given by the maximum values from Baym et al. 's equation of state, i. e. | ₀ =5. 1 10^14\, g\, cm^-3\, and \, p₀= 7. 4 10^33\, dyne\, cm^-2|, we calculate the maximum mass of the fluid sphere to be 3 solar masses.
Henning Knutsen (Sun,) studied this question.