A Planetary Nebula, in essence, appears to be a roughly spheroidal or ellipsoidal shell of gas with a nuclear star in or close to its center. These nebulae (planetaries) received their name not due to any possible generic relationship to planets, but because early observers, while searching for planets with primitive telescopes, sometimes came across these disc-like objects and they at first glance looked like planets. The central star of a planetary is usually quite dim. It is not often brighter than eleventh magnitude and it is the exception that can be seen at all. The body or expanded shell-like ring of gas of the nebula is also faint and tends to blend into the sky background. The hot central stars in these nebulae seem to be evolving to the white-dwarf stage and radiate by far the greatest portion of their energy in the far ultraviolet portion of the electromagnetic spectrum. Since these objects are expanding, they are probably short-tem and will disappear as a result of expansion in something like 30,000 years. The Ring Nebula in Lyra is perhaps the best known of the planetaries. The Crab Nebula, while often considered as an example of this category, is not a true planetary, but rather the remains of a massive supernova, the ghost of a cosmic fireworks.

Copyright (c) 1997-99 Michael Erlewine