Nebulae

Galaxies  Globular Clusters  Open Clusters  Emission Nebulae  Reflection Nebulae  Dark Nebulae Supernova Remnants

Globular Clusters:
Globular clusters are gravitationally bound groups of many thousands of stars. They consist primarily of very old stars. Globular clusters are not concentrated in the plane of the galaxy but rather are randomly distributed throughout the halo. There are hundreds of globular clusters associated with our galaxy.
 
Open Clusters:
Open clusters are loose aggregations of dozens or hundreds of young stars. They are generally not gravitationally bound and will disperse in a relatively short period of time. They are often associated with more diffuse nebulosity, as well. Also called "galactic clusters" because they are usually found in the plane of the galaxy.

Emission Nebulae:
Emission nebulae are clouds of high temperature gas. The atoms in the cloud are energized by ultraviolet light from a nearby star and emit radiation as they fall back into lower energy states. These nebulae are usually red because the predominant emission line of hydrogen happens to be red. Emission nebulae are usually the sites of recent and ongoing star formation.

Reflection Nebulae:
Reflection nebulae are clouds of dust which are simply reflecting the light of a nearby star or stars. Reflection nebulae are also usually sites of star formation. They are usually blue because the scattering is more efficient for blue light. Reflection nebulae and emission nebulae are often seen together and are sometimes both referred to as diffuse nebulae.

Dark Nebulae:
Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind. They are physically very similar to reflection nebulae; they look different only because of the geometry of the light source, the cloud and the Earth. Dark nebulae are also often seen in conjunction with reflection and emission nebulae.

Supernova Remnants:
Supernovae occur when a massive star ends its life in an amazing explosion. For a few days a supernova emits as much energy as a whole galaxy. When it's all over, a large fraction of the star is blown into space as a supernova remnant.