A galaxy is a vast system of stars, stellar remnants, interstellar gas, dust, and dark matter held together by gravity. These cosmic structures vary widely in shape and size, ranging from small dwarf galaxies to giant spirals. Astronomers estimate there are hundreds of billions of galaxies in the observable universe, each containing billions of stars. Because of this overwhelming number, galaxies are identified and named in two distinct ways: a few bright ones receive common, descriptive names, while the majority are designated by systematic scientific catalogs.
The Most Famous Named Galaxies
The handful of galaxies known by common names are those visible to the naked eye or hold historical significance. Our own galaxy, the Milky Way, is the most recognized, named for its appearance as a milky band of light across the night sky. The ancient Greeks called it galaxias kyklos, or “milky circle,” which the Romans translated to Via Lactea, meaning “Road of Milk.” The word “galaxy” itself stems from the Greek word for milk, gala.
The nearest major spiral galaxy is Andromeda, also known as Messier 31 (M31). Its common name comes directly from the constellation in which it appears, which is named after the mythical Greek princess Andromeda. Another prominent neighbor, the Triangulum Galaxy (M33), is similarly named for the constellation Triangulum where it is observed. These historical names often reflect their mythological associations or their visual appearance to early observers.
Scientific Designation Systems
Most galaxies lack a common, memorable name and are instead logged within systematic astronomical catalogs. These catalog systems are necessary to organize the millions of deep-sky objects discovered by astronomers over centuries. The objects in these catalogs are assigned alphanumeric designations based on their order of discovery or their precise location in the sky, creating a universal reference system.
The oldest system still in common use is the Messier Catalog, denoted by the prefix “M”. French astronomer Charles Messier compiled this list in the late 18th century as a “non-comet” catalog. Messier, a dedicated comet hunter, created his list of 110 fixed celestial objects, such as the elliptical galaxy M87, to avoid confusing them with new comets.
The most extensive and widely used modern system is the New General Catalogue (NGC). Compiled by John Louis Emil Dreyer in 1888, it initially listed 7,840 objects, including galaxies and star clusters. The numbers in the NGC are assigned sequentially based on the object’s right ascension, or celestial longitude.
The NGC was later supplemented by the Index Catalogue (IC), which added over 5,000 more objects discovered between 1888 and 1907. These catalog numbers provide professional and amateur astronomers with a precise, standardized way to refer to faint, distant celestial bodies.
Our Immediate Galactic Neighbors
The Milky Way exists within a gravitationally bound collection of approximately 50 galaxies known as the Local Group. This group is dominated by the two largest spiral galaxies, the Milky Way and Andromeda, with the remaining members being mostly smaller dwarf and irregular galaxies. Defining our immediate neighborhood provides an important context for studying galactic evolution and interactions.
Two of the most prominent members of the Local Group are the Large and Small Magellanic Clouds (LMC and SMC). These irregular dwarf galaxies are satellites of the Milky Way, orbiting our larger system. They are easily visible to the naked eye from the Southern Hemisphere, appearing as hazy, detached pieces of the Milky Way.
Their common name honors the Portuguese explorer Ferdinand Magellan, whose expedition crew recorded them during the first circumnavigation of the Earth in the early 16th century. These objects were known to indigenous peoples of the Southern Hemisphere long before European explorers.