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A protoplanetary nebula (PPN) is neutral gas surrounding an asymptotic giant branch (AGB) star that is not yet sufficiently hot to ionize the gas. The name refers to the fact that it is a step in the development of a planetary nebula (PN). The neutral gas is from the AGB star's high stellar wind. After the PPN develops, the central star shrinks (further), eventually reaching sufficient temperature to emit enough ionizing radiation to make the PPN glow, at which point it is termed a planetary nebula.
The term protoplanetary nebula can confuse because the words "planetary", "protoplanetary" and "nebula" suggest a relation to planets, planet formation, the nebular hypothesis and protoplanetary disks. The term protoplanetary nebula has indeed occasionally been used in that respect. The term preplanetary nebula has been used for the (above) AGB phenomenon, for the purposes of reducing confusion.