PLANETARY-NEBULAE

Planetary nebulae (PN) are shells of low-density ionized gas that have been expelled by their central stars. Intermediate-mass stars, while leaving the red giant stage to become white dwarfs, undergo mass loss, producing PN. The main emphasis of the review is on the properties of the shell and their relation to other branches of astrophysics. Recent studies on the distance scale to galactic PN, the extragalactic distance scale, and the PN birth rate are described. The relevance of the PN chemical composition to the study of stellar evolution and the chemical evolution of galaxies is discussed.