1 An early stage in the formation of a planetary nebula. In this phase the central star has shed its outer layers, exposing the hot stellar core. Ultraviolet light from the core starts to ionize the surrounding cloud of gas and dust, and for a brief phase the circumstellar envelope contains both hot ionized material near the star and cool molecular material far from the star.
2 A cloud from which planets are formed around a newborn star, as in the solar nebula.
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