Galaxies are enormous gravitationally bound systems containing stars, stellar remnants, gas, dust, dark matter and other components distributed across a range of spatial scales. They can contain millions to trillions of stars and vary enormously in mass, size, luminosity, colour, gas content and structure. Galaxies are not isolated collections of stars in a simple sense. Their visible components are embedded within much larger dark matter structures, and their evolution is affected by internal processes and interactions with other galaxies and the surrounding cosmic environment. The observable universe contains vast numbers of galaxies, and their diversity provides a record of the history of cosmic structure formation.
Galaxies are commonly classified according to visible morphology. Major broad categories include spiral galaxies, elliptical galaxies and irregular galaxies, although real systems can display combinations of features and specialised structures. Spiral galaxies generally possess rotating stellar and gaseous discs, central concentrations and spiral structure. Elliptical galaxies are typically dominated by older stellar populations and have smooth, approximately ellipsoidal light distributions with relatively little cold gas. Irregular galaxies lack a simple regular morphology and often contain substantial gas and active star formation. Classification is useful but does not by itself explain how every galaxy formed.
The Milky Way is a barred spiral galaxy containing the Solar System and is an important reference system for understanding galaxies because astronomers can study many of its components in detail. Galaxies also contain stellar populations of different ages and chemical compositions. Young stars tend to trace regions of active star formation, while old stars can populate extended halos and spheroidal components. Gas exists in several phases, including cold molecular gas, neutral atomic gas and hot ionised plasma. Dust absorbs and scatters electromagnetic radiation and can obscure portions of a galaxy in visible wavelengths while producing detectable infrared emission.
Galaxies contain dark matter that cannot be observed directly through ordinary electromagnetic emission but is inferred from gravitational effects. Evidence includes galaxy rotation curves, gravitational lensing, satellite galaxy motions and the dynamics of galaxy clusters. The dark matter halo generally extends far beyond the bright stellar component. The relationship between visible matter and dark matter is fundamental to galaxy formation because dark matter provides gravitational potential wells in which ordinary matter can accumulate. The precise microscopic nature of dark matter remains unknown, making galaxy observations an important source of constraints on cosmological models.
Star formation within galaxies depends on the availability and physical state of gas. Dense molecular clouds can collapse under gravity when conditions permit, producing protostars and eventually stellar populations. Massive young stars emit ultraviolet radiation and drive winds that can heat and disperse surrounding gas. Supernova explosions return chemically enriched material and energy to the interstellar medium. These processes can regulate subsequent star formation through feedback. The balance between gas accretion, star formation, feedback and gas removal therefore helps determine whether a galaxy remains actively star forming or becomes relatively quiescent.
Galaxies interact gravitationally with one another and with smaller satellite systems. Close encounters can distort spiral arms, trigger bursts of star formation, strip gas or rearrange stellar orbits. Mergers can transform morphology and produce strong gravitational and hydrodynamic disturbances. Galaxy clusters contain many galaxies embedded within a common gravitational environment and can alter member galaxies through tidal interactions, ram pressure and other processes. Interactions are therefore an important part of galaxy evolution rather than rare exceptions to an otherwise isolated existence.
Many galaxies contain supermassive black holes at their centres. When substantial quantities of gas accrete onto such an object, the resulting active galactic nucleus can become extraordinarily luminous and can influence the surrounding galaxy through radiation, winds and jets. Quasars are particularly luminous active galactic nuclei observed at great distances. Not every galaxy has an actively accreting central black hole at a given time, and the relationship between black hole growth and galaxy evolution is an active research area.
Galaxies are fundamental units of large scale astronomical structure and are strongly connected to cosmology. Their positions trace the cosmic web of filaments, groups, clusters and voids. Their observed properties change with cosmic time, allowing astronomers to study how galaxies assembled and transformed over billions of years. Modern surveys combine optical, infrared, radio, ultraviolet and X ray observations to construct multiwavelength pictures because different wavelengths reveal different components and physical processes. Understanding galaxies therefore requires simultaneous consideration of gravity, dark matter, gas dynamics, star formation, stellar evolution, black holes, radiation and cosmological structure formation.
