Steffen, W.; Espíndola, M.; Martínez, S.; Koning, N. «The 3D velocity structure of the planetary nebula NGC 7009». Revista Mexicana de Astronomía y Astrofísica, 45, 10-2009, pàg. 143–54. arXiv: 0905.2148. Bibcode: 2009RMxAA..45..143S. «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.»
Steffen, W.; Espíndola, M.; Martínez, S.; Koning, N. «The 3D velocity structure of the planetary nebula NGC 7009». Revista Mexicana de Astronomía y Astrofísica, 45, 10-2009, pàg. 143–54. arXiv: 0905.2148. Bibcode: 2009RMxAA..45..143S. «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.»