Galaxies and Nebulae:

Nebulae

Planets

Galaxies and Nebulae

Space Phenomena

Space Fiction

A planetary nebula forms when a star can no longer support itself by fusion reactions in its center. The gravity from the material in the outer part of the star takes its inevitable toll on the structure of the star, and forces the inner parts to condense and heat up. The high temperature central regions drive the outer half of the star away in a brisk stellar wind, lasting a few thousand years. When the process is complete, the remaining core remnant is uncovered and heats the now distant gases and causes them to glow.
Despite the name, these objects have nothing in common with "planets". They may, however, represent the likely final episode of the Sun's existence as a star. It is estimated that there are about 10,000 planetary nebulae in our galaxy, so they are a relatively common, although short-lived phase (about 25,000 years) of the stellar life cycle.