g, Z.; Parker, Q. A.; Frew, D. J. (November 2008), "AAOmega radial velocities rule out current membership of the planetary nebula NGC 2438 in the open cluster M46", Monthly Notices of the Royal Astronomical Society 391 (1):  399–404, doi:10.1111/j.1365-2966.2008.13899.x, Bibcode: 2008MNRAS.391..399K Krause, Arthur (1961), Astronomy, Oliver and Boyd, p. 187, http://books.google.com/?id=gDSsAAAAIAAJ Kwok, Sun (2000), The origin and evolution of planetary nebulae, Cambridge University Press, ISBN 0-521-62313-8, http://books.google.com/?id=7NfqpZxO_o0C  (Chapter 1 can be downloaded here.)Kwok, Sun (June 2005), "Planetary Nebulae: New Challenges in the 21st Century", Journal of the Korean Astronomical Society 38 (2):  271–8, doi:10.5303/JKAS.2005.38.2.271, Bibcode: 2005JKAS...38..271K Kwok, Sun; Su, Kate Y. L. (December 2005), "Discovery of Multiple Coaxial Rings in the Quadrupolar Planetary Nebula NGC 6881", The Astrophysical Journal 635 (1):  L49–52, doi:10.1086/499332, Bibcode: 2005A