What is a nebula?

One of the most striking objects in the sky is the Great Nebula in the constellation of Orion. It can be seen as a faint greenish patch with the naked eye just below Orion’s Belt, but through binoculars or a telescope it is seen as a mass of glowing gas. Nebulae are large clouds of dust and gas in space. Many produce their own light, and others are illuminated by stars. Some are dark, and we see them as black clouds or bands against the stars. Many luminescent nebulae are formed when stars explode into supernovae. The remains of the star move out through space as an expanding shell of glowing gas. Nebulae are among the most striking sights in the heavens. Photographs show them to have beautiful colours, but these colours may not show in a telescope.