This glossary is to provide short definitions of terms related to Deep Sky astronomy. Brief Introduction to Astronomy . Special type of a Spiral Galaxy with a bar structure in the central region of the galactic disk of the galaxy . Besides diffuse nebulae , Planetary Nebulae and Supernova Remnants are special types of bright emission nebulae. Group of physically neighbored and gravitationally bound galaxies.
Clusters of galaxies tend to form superclusters .
In a closer sense, the term applies to nonstellar objects only, i.e., star clusters , nebulae , and galaxies . Two stars situated close together in the sky, so that they may appear as one star with the naked eye, or under bad viewing conditions. These may be physically related binary stars or optical chance alignments of unrelated stars with different distances. Star Cluster of several dozen to several hundred, rarely few thousand stars filling a volume of several light years diameter. Mostly rather loose and composed of rather young stars.