- A cloud of gas and dust from which planets are formed around a newborn
star, as in the solar nebula.
- An early stage in the development of a planetary
nebula in which the central star has shed its red
giant envelope, exposing its hot core. Ultraviolet light from the
core has begun to ionize and make visible the surrounding cloudl of
gas. Examples include the Calabash
Nebula and the Egg Nebula.