The word "nebula" is derived from the Latin word for "clouds". A nebula is a cosmic cloud of gas and dust floating in space. The stars within nebulae cause them to glow with beautiful reds, greens and blues, which are the result of the different elements within the nebula. Most are composed of around 90% hydrogen, 10% helium and 0.1% heavy elements such as carbon, nitrogen, magnesium, potassium , calcium and iron. These clouds of matter are among the largest objects in the galaxy with some being hundreds of light years across.
There are five major categories:
Emission nebula - a cloud of high temperature gas. Within the nebula a star energies the atoms in the cloud with ultraviolet radiation. As the atoms fall back to lower energy states, they emit radiation, which causes the nebula to glow, much like a new light. These nebulae tend to be red because of the abundance of hydrogen. The Orion nebula (M42) is a great example.
Reflection nebula - does not emit radiation of its own. It is a cloud of dust and gas that reflects the light energy from a nearby star or group of stars, usually tending to be blue because blue light is scattered more efficiently. Reflection nebulae are frequently the sites of star formation. A good example is The Trifid Nebula (M20).
Dark nebula - a cloud of dust that blocks the light from objects behind it. They are of a similar composition to emission nebulae, but look different because of the placement of the light source. The Horsehead Nebula in Orion is the most famous example.
Planetary nebula - a shell of gas produced by a star as it nears the end of its cycle. They have nothing to do with planets, but were thus named because they look round. The outer shell of gas is illuminated by the remains of the star at its centre. The Ring Nebula (M57) is a great example.
Supernova remnant - created when a star ends its life in a massive explosion known as a supernova. This throws large amounts of matter into space, which glows with the remains of the star that created it. The Crab Nebula (M1) is one of the best examples.
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