4th ed. (John Wiley and Sons, Hoboken NJ, 2004) ISBN 0-471-52229-5 Hibbett, D. S., M. Binder, J. F. Bischoff, M. Blackwell, P. F. Cannon, O. E. Eriksson
Karachentsev et al. 2004 Karachentsev & Kashibadze 2006 Cannon et al. 2006 環狀星雲 (NGC 6822) NGC 6822 Cannon, John M.; Walter, Fabian; Armus, Lee; Bendo, George J