As well as the pinpoints of the stars, there are about a couple of dozen dim, fuzzy patches of light, like little glowing clouds, visible to the naked eye in a clear night sky. Long-exposure photographs taken through large telescopes reveal that there are thousands upon thousands of these blurs of light in the heavens. We now know that these patches of brightness include faint clusters of stars and distant galaxies, as well as wisps of glowing gases and banks of dust glistening in starlight. But until it was shown in the 1920s that many are really galaxies, all of them were called nebulae, Latin for "clouds". Only shining gas and dust deserve the name nebulae, however, as they are truly clouds.
Also present in the heavens are many invisible clouds that can be detected only by the radiation they emit or by their effect on light passing through them. These, along with the visible clouds, are known as the interstellar medium, and tend to be concentrated in the spiral arms of galaxies. Constantly changing as clouds drift apart or clump together, the interstellar medium is 99% hydrogen and helium gas with a tiny amount of other gases and even less icy cosmic dust.
There are three types of nebulae. Dark nebulae are dark because dust within them almost totally absorbs visible and ultraviolet light, making them appear as dark smudges against the background.
Reflection nebulae glow faintly because dust in them scatters enough starlight to make them visible. They tend to scatter blue light more, so they look bluish.
In glowing nebulae (emission nebulae) gas, which consists of three-quarters hydrogen and nearly all the rest helium, glows because it is heated to temperatures close to 10000 K by ultraviolet radiation from nearby hot stars. The energy is then re-emitted in the form of visible light.
Below, we take a closer look at one of the most beautiful nebulae known to man, the Horsehead nebula.
The Horsehead nebula
The Horsehead nebula in Orion appears as a distinctive silhouette against the brighter emission nebula behind. It stands out from the uniform background because it is an area with particularly dense dust concentrations. These absorb the light emitted from the regions beyond, giving the cloud its dark appearance.
A wider view of the Horsehead nebula reveals the amount of dust found in the gas clouds in this region of the sky. The photograph shows that the "horse's head" is merely a part of a much larger cloud beneath. A comparison of the number of stars visible in the upper and lower halves of the picture shows that fewer stars appear in the lower half. This is because the dust in this large dark nebula blocks out light from behind. Only stars that are in front of the cloud can be seen.
The horse's head protrudes into a bright red strjp of ionzied hydrogen gas. The gas has been made to glow its typical pink colour by light from Sigma Orionis, the bright star at the top of the picture. The "neck" of the horse is about 0.25 pc across. The nebular region is roughly 1500 pc from the Earth.
The bright patch just below and to the left of the horse's head. NGC 2023, is a reflection nebula, reflecting the light from stars within the dusty cloud. The bright star to the left of centre is Zeta Orionis, or Alnitak. This region actually forms part of the enormous cloud of gas which also includes the much brighter Orion nebula.