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Planetary nebula

A planetary nebula is an astronomical object that usually appears nebulous and disk-like in low-resolution observations. Because of this appearance, similar to the appearance of planets in early observations, the "planetary" adjective was attached and has since been retained for historical consistency.

According to current observations and models, planetary nebulae in fact have little to do with planets. Instead, as a small star (less than a few times the mass of the Sun) grows older, it tends to throw off upper layers of stellar material in various episodes of fusion reactions. While the central progenitor star dwindles to a white dwarf, the thrown-off gases form a cloud of material around it. It is this cloud which is designated as a planetary nebula. The remains of the star which produced the nebula is also responsible for the energy which causes it to glow.

Some of the more famous examples of this type of object are the Ring Nebula, the Helix nebula and the Cat's Eye nebula.

(Chu et. al., left; NASA/HST, right; GDL-OK)

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