The term nebula comes from the Latin word for mist but astronomers use this word to refer to any gathering of dust and gas that has been herded by gravity in a vast cloud. Nebulae can become exceptionally enormous, by the way, and often span dozens or hundreds of light years from one end to the other!

The principal material comprising a nebular cloud is molecular hydrogen- the simplest and most abundant material in the Universe. In addition, other gases, such as helium, are also present as are great smatterings of atoms that have been ionized by stellar radiation. The final significant constituent in a nebula is copious amounts of dust formed from the remnants of previous stars that have either exploded or thrown off their outer layers during old age. Interestingly, the term dust is broadly applied astronomically- it's not your household variety but grains of material that are only fractions of a micron in diameter. Cigarette or cigar smoke, for example, contains boulder sized particles compared to the dust within these clouds.

Nebulae are found throughout the Universe and, other than stars, are the most prominent features visible in a galaxy, In fact, images of distant galaxies are essentially pictures of their nebula, often, arranged into a lovely spiral pattern. Individual stars in far away galaxies, except a handful that are located nearby, are simply too small and dim to be discerned by even our largest telescopic instruments therefore a picture of a galaxy is actually an image of its nebulae.

Although nebulae occur in an unlimited number of shapes and sizes, they can be grouped into three broad categories based upon their interaction with light.

For example, the dust in a nebula will scatter light shining from one or more nearby stars and cause the cloud to appear blue using the same process that causes our skies to take on a similar hue- dust reflects blue light while red wavelengths are too large and pass around it. These are therefore called reflection nebulae.

If the dust within the cloud is in great abundance, it will act like a sponge and prevent light from passing through. Known as absorption nebulae, these take on a dark and somewhat sinister appearance when seen from the outside however, quite often, their interior is filled with the light of hot, newly formed stars that have incubated within. Young, hot stars that release large amounts of ultraviolet radiation can cause the hydrogen gas within a nebular cloud to glow redthus giving rise to the term emission nebulae.

The picture features each type of nebula- in fact, it overflows with this stuff!

It depicts a scene that is about 5,000 light-years in the distance towards the heart of our galaxy in constellation of Sagittarius. It represents a three panel mosaic of images that were digitally stitched together to form a single picture of two very energetic star forming regions. In addition to nebulae, the image contains several thousand of the over 400 billion stars that fill our galaxy. So, to understand the scale of the cloud formations in the image, consider that each bright point represents a star, most likely, as large or considerably larger than our own sun. Of course, the actual size of the stars that are visible in the picture would appear as pin-points were it not for the bloating brought on by our planet's atmosphere and a photographic artifact that causes brighter stars to seem larger.