Deep sky objects

Planetary nebula: An expanding shell of gases ejected from a dying star. Many of these are round in appearance, resembling a planet, hence the term planetary nebula.
Diffuse nebula: A cloud of interstellar gas and dust, often associated with star formation. This includes emission nebula, where the gas has been excited to shine from the intense radiation of nearby stars, and reflective nebula, where the dust reflects the light from nearby stars, and combinations of the two. Also included in this category are supernova remnants, the remains of stars that have exploded.
Dark nebula: A dark cloud of interstellar gas and dust that obscures the light from more distant objects, leaving what appears to be a dark void in space.
Globular cluster: A collection of tightly packed stars in concentrations from approximately ten thousand to over one million stars. Globulars are not associated with the disk of the Milky Way but rather move about in highly eccentric elliptical orbits that take them far outside the Milky Way.
Open cluster: A collection of stars held together by mutual gravitation, usually originating from the same gas and dust cloud. Thus typically the stars have similar ages. Unlike globulars they are associated with the disk of the Milky Way.
Galaxy: A huge agglomeration of stars, clusters, gas, and dust, such as our own Milky Way galaxy. There are several types (and subtypes) including spiral galaxies, elliptical galaxies, and irregular galaxies.
Double star: Two stars in close proximity to each other (along our line of sight), usually bound together gravitationally. Not usually associated with deep sky objects, this type is listed solely because Messier object M40 is a double star.