A nebula is a region in interstellar space with higher than usual concentrations of hydrogen, helium and dust (in this context dust is just another word for a mixture of iron, oxygen, hydrogen, carbon, nitrogen and many other elements). There are different types of nebula:

1. An emission nebula (plural nebulae or sometimes nebulas) is a cloud of ionized gas. The different types of emission nebulae originate from the source that causes the ionization of the gas:
   
   
   • Many famous emission nebulae (such as, for example, the Eagle Nebula or the Orion Nebula) are the places where new stars are born. These emission nebulae are also called H II regions. When a cloud collapses under its own weight, parts of the cloud become denser and therefore attract further matter. This process continues until parts of the huge cloud are so dense that stars form. Our Solar System was formed 4.6 billion years ago in the same way, together with many other stars. The hottest and brightest of these young stars are the ones that excite the surrounding gas. This gas is mainly hydrogen and shines in red when excited by the nearby stars. More information about this type of nebula can be found in our Emission Nebula article.
   
   
   • Planetary nebulae have, despite their name, nothing at all to do with planets; they are a certain type of emission nebula. Planetary nebulae are expanding clouds of ionized gas surrounding red giant stars before they become white dwarfs. During the final stage of the life of these stars they eject their outer layers of gas which subsequently are excited by the star’s radiation and therefore emit light. A beautiful example of such a planetary nebula is the Ring Nebula.
   
   
   • Supernova remnants are the last type of emission nebula; they are the leftovers of supernovae, the most massive explosions that occur in the universe. Our supernova article covers many details of supernovae and supernova remnants. A shockwave heats up the gas to an initial temperature of well above one million K, causing the plasma to glow in many wavelengths. The Crab Nebula is a gorgeous example of such a nebula.


2. Reflection nebulae are gas clouds that reflect the light of very hot and bright nearby stars. M78 is a great example of such a nebula.


3. Finally, there are dark nebulae; interstellar clouds that are so dense that they obscure any visible light from background light sources such as stars or emission nebulae. Dust grains within the dark nebula are responsible for the extinction of the light. More information can be found in our encyclopedia article about molecular clouds and dark nebulae. The Horsehead Nebula is a famous example of a dark nebula; the image of IC 2944 is a particularly beautiful image with huge dark clouds in front of an emission nebula.