Planetary nebula

An expanding shell of ionized gas expelled by a dying low- to intermediate-mass star and illuminated by its hot central remnant. Despite the name, it is unrelated to planets.

Asymptotic giant branch: A late stellar-evolution phase in which a star develops an inert core surrounded by hydrogen- and helium-burning shells. The progenitor expels much of its envelope near the end of this phase.
Ring Nebula: A bright planetary nebula in the constellation Lyra, catalogued as Messier 57. Its nearly ring-shaped appearance is a familiar example of projected nebular structure.
Stellar evolution: The sequence of physical changes a star undergoes from formation through its remnant stage. Planetary nebulae mark a short transition between giant-star evolution and a compact remnant.
Emission nebula: A cloud of ionized gas that emits light at characteristic wavelengths. Planetary nebulae are a distinctive stellar-evolution class within this broader kind of glowing cloud.
Binary star: A stellar system in which two stars orbit their common center of mass. Companion stars can shape ejecta and may explain structures a solitary star struggles to produce.
Stellar wind: A flow of gas continuously escaping from a star's atmosphere. A slow, dense wind builds the material later shaped into the nebula.
Helix Nebula: A nearby planetary nebula in Aquarius, catalogued as NGC 7293. Its broad, close-up structure makes the expanding shell and outer material especially evident.
Red giant: A luminous, expanded star that has exhausted hydrogen in its core and burns fuel in surrounding layers. Low- and intermediate-mass progenitors pass through giant phases before shedding their envelopes.
Spectroscopy: The study of matter through its interaction with electromagnetic radiation at different wavelengths. Nebular spectra reveal elemental abundances, temperature, and gas motion.
Common-envelope evolution: A binary-star phase in which one star's expanded envelope engulfs both stellar cores. This process can eject gas and influence the nebula's geometry and central system.

Linked from 32 pages