Introductory Astronomy: Planetary Nebulae

The term "Planetary Nebulae" is a bit of a misnomer. When first viewed in the 19th century, the ejected outer layers of stars looked like round disks, similar to planets. Astronomers therefore called them planetary nebulae. Actually, planetary nebulae have nothing to do with planets. They are gases expelled from the star when the core forms a carbon core. At this stage in a star's evolution, its outer layers are loosely bound, and are thus easily blown away by the strong stellar wind. A stellar wind can carry away up to 1 solar mass in about 100,000 years. A planetary nebula is spherical in shape because the gas is ejected in all directions by the wind. However, a planetary nebula appears ring shaped when viewed from Earth. This is because we are looking at a thicker layer of the nebula when we look at the edge of the sphere than when we look at the center of the sphere, so the nebula appears brighter on the edges than in the center.

As the planetary nebula expands outward, the small, hot stellar core that remains becomes apparent.

An observation of a planetary nebula.