Steffen, W.; Espíndola, M.; Martínez, S.; Koning, N. (octubre de 2009). «The 3D velocity structure of the planetary nebula NGC 7009». Revista Mexicana de Astronomía y Astrofísica45: 143-54. Bibcode:2009RMxAA..45..143S. arXiv:0905.2148. «NGC 7009 is a planetary nebula with several morphological and kinematical sub-systems with multiple shells, a halo, jet-like streams, ansae and small-scale filaments and knots.»
harvard.edu
adsabs.harvard.edu
Steffen, W.; Espíndola, M.; Martínez, S.; Koning, N. (octubre de 2009). «The 3D velocity structure of the planetary nebula NGC 7009». Revista Mexicana de Astronomía y Astrofísica45: 143-54. Bibcode:2009RMxAA..45..143S. arXiv:0905.2148. «NGC 7009 is a planetary nebula with several morphological and kinematical sub-systems with multiple shells, a halo, jet-like streams, ansae and small-scale filaments and knots.»