A galaxy is a large collection of stars, star systems, clusters and stellar associations, gas and dust (forming the interstellar medium), bound together by the mutual force of gravity. The name comes from the Greek γαλαξίας (galaxìas), meaning “of milk, milky”; it is a clear allusion to the Milky Way, the galaxy par excellence, of which the Solar System is a part.
Galaxies are vast objects of extremely variable size, ranging from the smallest dwarf galaxies, containing a few hundred million stars, to giant galaxies, containing a number of stars on the order of a hundred trillion, orbiting a common center of mass.
Not all massive, self-gravitating systems of stars are defined as galaxies; the lower dimensional limit conventionally used to define a galaxy is a mass order of 106 solar masses, a criterion for which globular clusters and other star clusters are not galaxies. No upper limit is defined, but all observed galaxies do not exceed a maximum size of about 1013 solar masses.
Galaxies have been classified into three main types based on their apparent shape (or visual morphology): elliptical, spiral, and irregular (or peculiar). Elliptical galaxies are the simplest form, lacking structure except for the gradient of brightness from the center to the periphery, appearing visually as ellipses with blurred contours of variable ellipticity, and are in fact ellipsoids. Spiral galaxies, on the other hand, have a disk-like shape in which spiral structures develop from a central bulge called the nucleus. They can be observed from any angle. Galaxies of irregular or unusual shape are called peculiar galaxies. These morphological categories are in turn divided into further sub-categories, and there are also discoidal galaxies with some intermediate characteristics between elliptical and spiral galaxies.
The shape of galaxies is influenced by external factors, namely the presence of other galaxies. Irregular galaxies usually result from deformations caused by tidal interactions with neighboring galaxies or collisions. When the interactions are particularly intense, such as between galactic structures that are very close to each other, a merger of two galaxies can occur, resulting in the formation of an irregular galaxy. The collision of two galaxies can lead to intense star formation (starburst) phenomena.
Galaxies also differ in color, which is related to the dominant population of stars, and their apparent shape may differ depending on the wavelength of the electromagnetic spectrum in which they are observed: for example, an irregular galaxy may have regions and structures that appear in the infrared or ultraviolet, making it appear different when observed at these wavelengths than its shape in visible light.
Galaxies are the most numerous objects in the observable universe. The most recent calculation estimates their number at about 200 billion (2×1011).