PPN

PPN commonly refers to proto-planetary nebula, a brief transitional stage in the evolution of low- to intermediate-mass stars. During the asymptotic giant branch (AGB) phase, the star loses mass through slow winds, creating an expanding envelope of gas and dust. After the core contracts and heats up, it becomes a hot, luminous source but has not yet ionized the surrounding envelope. When ionization begins, a planetary nebula forms; the proto-planetary nebula stage ends as the central star becomes hot enough to ionize the gas in the nebula.

Proto-planetary nebulae are typically identified by strong infrared emission from dust and by scattered light that reveals axisymmetric or multipolar shapes, often bipolar. The timescale is short, on the order of a few hundred to a few thousand years, making PPNs relatively rare in the sky. Carbon-rich chemistry is common in many examples, though oxygen-rich dust features are also seen in some objects. Well-known examples include the Egg Nebula (AFGL 2688) and the Red Rectangle (HD 44179), both studied for their distinctive shapes and dust properties.

In astronomy, PPN denotes this specific evolutionary phase; the term may appear in other contexts as an acronym in different disciplines. For readers seeking alternatives meanings of PPN, a disambiguation may be helpful.