What is a Nebula?

If you have seen pictures of nebulae in books, great spirals and whirlpools and clouds-don't expect to find anything like that in the sky. Most of the nebulae are so faint that they cannot be seen without a telescope. The word "nebulae", by the way, is really the Latin word for "mists" because they looked like mists when first observed through small telescopes.

There are two chief classes of nebulae, the galactic and the extra-galactic. The galactic nebulae are found in our own galaxy (the Milky Way) and are composed of dust and gas. Since "extra" means "outside", the extra-galactic nebulae are nebulae outside our own galaxy. They are made up largely of stars.

The galactic nebulae number less than 2,000. This means that most of the nebulae known to man are outside our own galaxy. How many are there? For all we know, there may be millions of them out there in the vast space beyond the Milky Way.

The extragalactic nebulae are sometimes called "island universes" or "galaxies". This means that if someone were looking at our own galaxy from out there he might well see it as a nebula.

The extragalactic nebulae have various forms. Some are irregular or elliptical. The most numerous are the spirals. The spirals, like our own galaxy, are made up of large number of stars, big gaseous clouds, and vast tracts of dust. These nebulae usually have a nucleus in the center and from this, arms extend in spiral fashion. The spiral nebula Andromeda is the nearest to earth and the largest and brighest nebula known. It gives out about 1,500,000,000 times as much light as our sun!