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Bolton, J. G.; Stanley, G. J.; Slee, O. B. (1949). „Positions of Three Discrete Sources of Galactic Radio-Frequency Radiation”. Nature164 (4159): 101. Bibcode1949Natur.164..101B. DOI:10.1038/164101b0.
Baade, W.; Minkowski, R. (1954). „Identification of the Radio Sources in Cassiopeia, Cygnus A, and Puppis A.”. The Astrophysical Journal119: 206. Bibcode1954ApJ...119..206B. DOI:10.1086/145812.
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Slipher, V. (1917). „The spectrum and velocity of the nebula N.G.C. 1068 (M 77)”. Lowell Observatory Bulletin3: 59. Bibcode1917LowOB...3...59S.
Humason, M. L. (1932). „The Emission Spectrum of the Extra-Galactic Nebula N. G. C. 1275”. Publications of the Astronomical Society of the Pacific44 (260): 267. Bibcode1932PASP...44..267H. DOI:10.1086/124242.
Mayall, N. U. (1934). „The Spectrum of the Spiral Nebula NGC 4151”. Publications of the Astronomical Society of the Pacific46 (271): 134. Bibcode1934PASP...46..134M. DOI:10.1086/124429.
Bolton, J. G.; Stanley, G. J.; Slee, O. B. (1949). „Positions of Three Discrete Sources of Galactic Radio-Frequency Radiation”. Nature164 (4159): 101. Bibcode1949Natur.164..101B. DOI:10.1038/164101b0.
Baade, W.; Minkowski, R. (1954). „Identification of the Radio Sources in Cassiopeia, Cygnus A, and Puppis A.”. The Astrophysical Journal119: 206. Bibcode1954ApJ...119..206B. DOI:10.1086/145812.