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protoplanetary nebula
A protoplanetary nebula or preplanetary nebula (PPN) is an astronomical object which is at the short-lived episode during a star's rapid stellar evolution between the late asymptotic giant branch (LAGB) phase and the subsequent planetary nebula (PN) phase. A PPN emits strongly in infrared radiation, and is a kind of reflection nebula. It is the second-from-the-last high-luminosity evolution phase in the life cycle of intermediate-mass stars (1–8 ).
==Naming==
The name protoplanetary nebula is an unfortunate choice due to the possibility of confusion with the same term being sometimes employed when discussing the unrelated concept of protoplanetary disks. The name protoplanetary nebula is a consequence of the older term planetary nebula, which was chosen due to early astronomers looking through telescopes and finding a similarity in appearance of planetary nebula to the gas giants such as Neptune and Uranus. To avoid any possible confusion, suggests employing a new term preplanetary nebula which does not overlap with any other disciplines of astronomy. They are often referred to as post-AGB stars, although that category also includes stars that will never ionize their ejected matter.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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