A galaxy is a large gravitationally bound system containing stars, stellar remnants, gas, dust, dark matter and other components. Galaxies vary enormously in size, mass, structure and activity. They can contain millions, billions or even trillions of stars, although the exact number depends on the galaxy and on how faint a population can be detected.
Most of the total mass of a typical galaxy is believed to reside in its dark matter halo rather than in its visible stars and gas. Galaxies are not simply collections of stars randomly scattered through space. Gravity organizes their matter into coherent structures, while gas dynamics, star formation, stellar evolution, black hole activity, mergers and interactions shape their appearance and evolution. Galaxies range from small dwarf systems to giant ellipticals and massive spiral galaxies. They are fundamental building blocks of the large scale universe and provide the environments in which stars, planetary systems and many of the chemical elements necessary for complex matter are produced.
Galaxies are commonly classified according to their visible morphology. Spiral galaxies have flattened rotating discs containing stars, gas and dust, often with spiral arms and a central bulge. Elliptical galaxies generally have smooth, rounded or elongated shapes and contain predominantly older stellar populations with relatively little cold gas available for ongoing star formation. Lenticular galaxies possess disc like structures but have less prominent spiral arms and generally lower current star formation than typical spirals.
Irregular galaxies lack a simple regular morphology and can be strongly affected by interactions, tidal forces or internal processes. Dwarf galaxies form an especially diverse category and can be dwarf irregulars, dwarf spheroidals or other types. Modern classification also considers quantitative properties such as stellar mass, colour, star formation rate and structural parameters because visual morphology alone does not capture the full physical diversity of galaxies.
A galaxy has a complex internal structure. A spiral galaxy can contain a central bulge, a thin stellar disc, a thicker stellar component, spiral arms, a gas rich interstellar medium and an extended dark matter halo. Many massive galaxies contain a supermassive black hole in their central region. The Milky Way, for example, contains a central supermassive black hole called Sagittarius A star. Spiral arms are not simply permanent solid structures containing the same stars forever.
They can be understood through density waves, transient structures, gravitational instabilities and other dynamical processes, and individual stars can move through them. Galaxies can also contain globular clusters, open clusters, nebulae, molecular clouds and stellar streams. Their visible structure is therefore the result of many interacting components operating over enormous ranges of scale.
Star formation occurs when sufficiently dense regions of cold interstellar gas collapse under gravity. Molecular clouds provide important environments for this process because low temperatures allow gas to become dense enough for gravitational collapse. As gas collapses, it fragments into smaller dense regions that can form individual stars or groups of stars.
Newly formed massive stars emit ultraviolet radiation and strong stellar winds that can heat and disperse nearby gas, while supernova explosions can enrich the surrounding interstellar medium with heavy elements and drive large scale gas flows. Galaxies therefore operate as ecosystems in which gas cycles between stars and the interstellar medium. Material expelled by stars can later become part of new molecular clouds and subsequent generations of stars. This recycling contributes to the progressive chemical enrichment of galaxies over cosmic time.
Galaxies grow through both internal processes and interactions with other galaxies. Smaller galaxies can merge with larger systems, contributing stars, gas and dark matter. Major mergers can substantially transform morphology, potentially converting disc galaxies into more spheroidal systems and triggering intense bursts of star formation. Minor mergers and tidal interactions can create stellar streams, distortions and asymmetries. Galaxies also acquire material through the gradual accretion of diffuse gas and smaller systems.
Cosmological simulations indicate that galaxies form within a hierarchy of dark matter structures, with dark matter halos merging and growing over time. The history of a galaxy is therefore strongly connected to the history of its surrounding cosmic environment. A galaxy observed today represents the accumulated effects of billions of years of gravitational assembly, gas accretion, star formation, stellar feedback and interactions.
Galaxies are also major sites of chemical evolution. The early universe consisted primarily of hydrogen and helium with small amounts of lithium and other light nuclei produced during primordial nucleosynthesis. Stars then produced heavier elements through nuclear fusion and other processes. Massive stars manufacture elements up to various stages of the nuclear burning chain, while supernovae and other explosive events distribute newly produced material into surrounding space. Neutron star mergers and other processes contribute additional heavy elements.
Gas enriched with these elements can later form new stars and planets. Consequently, the chemical composition of successive stellar generations can differ from that of earlier generations. Astronomers describe this enrichment using concepts such as metallicity, where metals means elements heavier than helium. Studying metallicity provides information about the formation history and evolution of galaxies.
Galaxies interact with their environments on scales much larger than their own dimensions. They can reside in groups, clusters and larger cosmic structures connected by filaments of the cosmic web. Within galaxy clusters, hot intracluster gas emits strongly in X rays and can exert ram pressure on galaxies moving through it. This pressure can strip gas from a galaxy and reduce its ability to form stars. Gravitational interactions between neighbouring galaxies can also redistribute stars and gas.
At even larger scales, galaxies trace the distribution of dark matter and form part of the cosmic web of filaments, walls and voids. Their locations, motions, colours, masses and star formation rates therefore provide information about the evolution of the entire universe rather than merely the properties of individual systems.
A galaxy is consequently much more than a visible collection of stars. It is a dynamically evolving gravitational system whose structure and history are controlled by dark matter, ordinary matter, gravity, gas physics, stellar evolution, black hole activity and interactions with other systems. Galaxies are observed across enormous distances and therefore provide a record of cosmic history because light from more distant galaxies shows them at earlier stages of their evolution.
Astronomers study galaxies using optical, infrared, radio, ultraviolet, X ray and gamma ray observations, because different wavelengths reveal different components and physical processes. Understanding galaxies is central to astronomy because galaxies contain stars and planetary systems, manufacture and distribute chemical elements, trace dark matter and large scale structure, and preserve information about how the universe developed from its early nearly uniform state into the complex cosmic structures observed today.
