Galaxy
A galaxy is a great set of stars, systems, clusters, and star associations, gases and dust, linked together by the reciprocal force of gravity. The name comes from the Greek γαλαξίας, meaning "milk, milk"; is a clear allusion to the Milky Way, the galaxy, which is part of the solar system. The galaxies are vast objects, ranging from the smallest dwarf galaxies, containing tens of millions of stars, to the most impressive giant galaxies, who come to count inside a thousand billion stars, all orbiting around a common mass center. Historically, galaxies have been categorized according to their apparent form, that is, based on their visual morphology. A very widespread type is the elliptical one, which, as one may well argue with the name, has an ellipse profile. The spiral galaxies, on the other hand, have a discoid shape with spiral-shaped structures that depart from the nucleus.