1. Aller, L.H., Czyzak, S.J.: In D. E. Osterbrock and C.R. O’Dell (eds.), Planetary Nebulae, IAU Symp. 34, 209 (1968).
2. Aller, L.H., Greenstein, J.L.: Astrophys. J. Suppl. 5, 139 (1960).
3. Ambartsumian, V. A.: In D. J. K. O’Connell (ed.), Nuclei of Galaxies, Pont. Acad. Sci. Scripta Varia 35, North Holland Publ. Co., Amsterdam (1971).
4. Amiet, J.H., Zeh, H.D.: Z. Phys. 217, 485 (1968).
5. Anders, E.: Geochim. Cosmochim. Acta 35, 516 (1971).