Final answer:
A planetary nebula is a shell of gas ejected by an extremely hot low-mass star nearing the end of its life. It can have different shapes and is believed to be surrounded by a doughnut-shaped disk of gas.
Step-by-step explanation:
A planetary nebula is a shell of gas that is ejected by and expands away from an extremely hot low-mass star that is nearing the end of its life. It glows because of the ultra-violet energy of the central star. These nebulae can have different shapes, such as a simple ring or multiple lobes of matter flowing in opposite directions. Astronomers believe that a considerable number of planetary nebulae consist of a dense, doughnut-shaped disk of gas surrounding the dying star.