Abstract
The Helix Nebula (NGC 7293) is the nearest planetary nebula to the Sun, at a distance of about 650 light-years. The remnant of a dying, Sun-sized star, the nebula is composed of a series of concentric, expanding, shell-like structures. They are the surface layers of the star, material ejected during different phases of the star’s death throes. The inner part of the nebula shows a series of cometary knots arranged like bicycle spokes, pointing away from the central hot star. Cometary knots are seen in other planetary nebulae, and they each are several times the size of our Solar System, much too big to be comets.
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