1. Burbidge, G. R. and Burbidge, E. M.: 1969,Nature 224, 21.
2. Burbidge, G. R.: 1972, in D. E. Evans (ed.), ?External Galaxies and Quasi-Stellar Objects?,IAU Symp. 44, 492.
3. Graaf, T. de: 1969,Astron. Astrophys. 5, 335.
4. Hubble, E.: 1926,Astrophys. J. 64, 321.
5. Landau, L. D. and Lifshitz, E. M.: 1958,Statistical Physics, Addison-Wesley Publishing Co., Reading, Mass., p. 325.