1. P. P. Parenago, Astron. Zh. 13, 3 (1940).
2. G. de Vaucouleurs, A. de Vaucouleurs, and J. R. Corwin, Second Reference Catalogue of Bright Galaxies (Univ. Texas Press, Austin, 1976).
3. A. Sandage, Astrophys J. 178, 1 (1972).
4. L. Spitzer, Physical Processes in the Interstellar Medium (Wiley-Interscience, New York, 1978).
5. P. P. Parenago, Popular Astron. 53, 441 (1945).