de Vaucouleurs، Gérard (أبريل 1963). "Revised Classification of 1500 Bright Galaxies". Astrophysical Journal Supplement. ج. 8: 31. Bibcode:1963ApJS....8...31D. DOI:10.1086/190084.
Hubble، E. P. (1926). "Extra-galactic nebulae". Contributions from the Mount Wilson Observatory / Carnegie Institution of Washington. ج. 324: 1–49. Bibcode:1926CMWCI.324....1H.