On the origin of planetary nebulae.

The stellar wind from a red giant produces an extensive cool circumstellar nebula of appreciable mass. It is suggested that in some cases mass loss continues until the hot core of the star is exposed and that the stellar wind from the remnant star collides with the circumstellar nebula, producing a relatively dense shell of gas which increases in mass and radius at a constant rate. It is shown that such a shell, when ionized by radiation from the central star, has the characteristics of a planetary nebula. V1016 Cygni is used as an example of a star which has recently undergone the transition from a red giant to a young low-mass planetary nebula.