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
