What is a Nebula?

Orion nebula

The word nebula means "cloud." A nebula, however, is a special kind of a cloud. It might be called an "outer space cloud." Very few nebulae can be seen with the naked eye. Through a telescope some of them look like hazy white clouds in the night sky. Some are dark and look like holes in the starry sky. These dark ones hide the stars that are behind them in space.

It is believed that most nebulae are clouds of gas left behind from stars that died ages ago, or perhaps from comets that were torn apart. Some shine by reflected light from nearby stars. The dark ones are not near enough to any stars to catch light from them. One of the shining gaseous nebulae can be seen in the constellation Orion. It is the hazy patch of light around the center star of Orion's sword. A dark nebula can be seen in this constellation, too. It is shaped like a horse's head.

Since astronomers have begun to study nebulae, they have discovered that some of the patches are not true nebulae. Some of them are really enormous systems of stars, or galaxies, very much like man's own galaxy, the Milky Way. One of these can be seen in the constellation Andromeda.

There is a theory that the dust and gases that make up nebulae are raw material for future stars and stellar systems.