↑They are created after the red giant phase, when most of the outer layers of the star have been expelled by strong stellar windsFrew & Parker 2010, pp. 129–148.
↑Aller, Lawrence H.; Hyung, Siek (2003), "Historical Remarks on the Spectroscopic Analysis of Planetary Nebulae (invited review)", in Kwok, Sun; Dopita, Michael; Sutherland, Ralph (eds.), Planetary Nebulae: Their Evolution and Role in the Universe, Proceedings of the 209th Symposium of the International Astronomical Union held at Canberra, Australia, 19-23 November, 2001, 209,Astronomical Society of the Pacific.
↑Chaisson, E.; McMillan, S. (1995).Astronomy: a beginner's guide to the universe (2nd ed.). Upper Saddle River, New Jersey: Prentice-Hall.
