Open Clusters

A loose group of a few dozen to a few thousand stars, usually associated with clouds of gas and dust.  All known open clusters are, like our Sun, located relatively far from the galactic center, in the galaxy's spiral arms; they consist of young stars, with ages ranging from a few million to a few hundred million years

Globular Clusters

A densely packed sphere of several thousand to several hundred thousand stars, held together by their mutual gravitation.  Nearly all globular clusters are relatively close to the center of our galaxy.  They have also been observed in other galaxies.  In all cases their stars seem to be billions of years old.

Nebulae

Nebulae are objects composed primarily of dust and gas.  Classes of Nebulae include Emission Nebulae, Reflection Nebulae, Planetary Nebulae, supernova Remnants, and Dark Nebulae.

Emission Nebulae

Reflection Nebulae

Supernova Remnants

Dark Nebulae

Planetary Nebulae

Galaxies

Any very large collection of stars outside of the Milky Way.  Galaxies are classified as spherical, Elliptical, Spiral, Barred Spiral, and Irregular.

Double Stars

Any two stars whose positions (as seen from Earth) are very close.  Doubles are classified as visual doubles if the closeness is only apparent due to the stars lying in almost the same direction from Earth and true doubles or Binary Stars