Steffen, W., M. Espíndola, S. Martínez, and N. Koning. “The 3D Velocity Structure of the Planetary Nebula NGC 7009”. Revista Mexicana de Astronomía y Astrofísica 45, no. 2 (November 22, 2010). Accessed May 19, 2024. https://journals.unam.mx/index.php/revmexaa/article/view/20594.