A collection of luminous stars, non-luminous [[dark matter]], and in the case of spiral and [[irregular galaxies]] some amount of gas and [[dust]] that are gravitational bound to one another, and are separated from other similar structures usually by distances of tens of kpc s or more. Various categories of galaxies may be defined based on their appearance, such as [[spiral galaxies]] (barred or normal), [[elliptical galaxies]], [[lenticular galaxies]] and [[irregular galaxies]]. [[Docent/S284 Astronomy]]