1. Skilling, J., Nature, 218, 531 (1968).
2. Salpeter, E. E., Ap. J., 134, 669 (1961).
3. Tolman, R. C., Relativity, Thermodynamics, and Cosmology, 242 (Clarendon Press, Oxford, 1934).
4. Oppenheimer, J. R., and Volkoff, G., Phys. Rev., 55, 374 (1939).
5. Harrison, B. K., Thorne, K. S., Wakano, M., and Wheeler, J. A., Gravitation Theory and Gravitational Collapse, 50, 63 (Univ. Chicago Press, 1965).