the major source of light in the expanding shell of gas in a planetary nebula is
Planetary nebula A mechanism for formation of most planetary nebulae is thought to be the following: at the end of the star's life, during the red-giant phase, the outer layers of the star are expelled by strong stellar winds. After most of the red giant's atmosphere is dissipated, the ultraviolet radiation of the hot luminous core, called a planetary nebula nucleus (PNN), ionizes the outer layers earlier ejected from the star.[2] Absorbed ultraviolet light energises the shell of nebulous gas around the central star, causing it to appear as a brightly coloured planetary nebula.
Nebular hypothesis The nebular hypothesis is the most widely accepted model in the field of cosmogony to explain the formation and evolution of the Solar System (as well as other planetary systems). It suggests that the Solar System formed from nebulous material. The theory was developed by Immanuel Kant and published in his Allgemeine Naturgeschichte und Theorie des Himmels ("Universal Natural History and Theory of the Heavens"), published in 1755. Originally applied to the Solar System, the process of planetary system formation is now thought to be at work throughout the Universe.[1] The widely accepted modern variant of the nebular hypothesis is the solar nebular disk model (SNDM) or solar nebular model.[2] It offered explanations for a variety of properties of the Solar System, including the nearly circular and coplanar orbits of the planets, and their motion in the same direction as the Sun's rotation. Some elements of the nebular hypothesis are echoed in modern theories of planetary formation, but most elements have been superseded.