A galaxy is a vast system of stars, interstellar gas, dust, and usually dark matter, held together by gravity. Galaxies range from small systems containing a few thousand stars to giants containing trillions. Their shapes, stellar populations, and rates of star formation vary considerably. The Milky Way, which contains the Solar System, is one example. Galaxies are studied to understand both the behavior of individual stellar systems and the development of structure throughout the universe. (science.nasa.gov)
Components and scale
A galaxy’s visible appearance reflects only part of its contents. Stars provide much of its optical light, while gas and dust supply material for new stars. In spiral systems, these components occupy a flattened disk, often accompanied by a central bulge. A surrounding halo contains older stars, star clusters, and dark matter. Young stars are especially prominent in gas-rich spiral arms, whereas older populations occur throughout the disk, bulge, and halo. (science.nasa.gov)
Galactic dimensions span several orders of magnitude. The smallest known systems can extend only a few hundred light-years, while the largest can exceed a million light-years. A light-year expresses distance rather than time. The Milky Way’s stellar disk spans more than 100,000 light-years; its surrounding components extend beyond the bright disk. Physical size, stellar population, and total mass are distinct properties, so apparent brightness alone does not describe a galaxy’s scale. (science.nasa.gov)
Morphological classification
Galaxies are commonly classified by their visible structure:
- Spiral galaxies possess a disk and spiral arms, usually surrounding a central concentration of stars. Barred spirals also have an elongated stellar structure crossing their centers.
- Elliptical galaxies have smooth, rounded or elongated appearances. They generally contain older stellar populations, relatively little cold gas, and limited ongoing star formation.
- Lenticular galaxies have disks and central bulges but lack conspicuous spiral arms. Their stellar populations and low star-formation activity often resemble those of ellipticals.
- Irregular galaxies lack a regular spiral or elliptical outline. Some have been distorted by gravitational encounters and contain substantial gas and dust. (science.nasa.gov)
These categories describe appearance, not a mandatory sequence through which every galaxy passes. Interacting systems can show structures intermediate between familiar categories, and mergers may substantially alter a galaxy’s morphology. Classification by central activity is a separate approach: an active galaxy can also be spiral, elliptical, or irregular. (science.nasa.gov)
Dynamics and dark matter
The motions of stars and gas reveal how mass is distributed. A galaxy rotation curve plots orbital speed against distance from the center. In many disk galaxies, outer orbital speeds remain approximately constant rather than declining as would be expected if most mass were concentrated centrally. These measurements are important evidence for an extended, unseen mass component, conventionally interpreted as a dark-matter halo. (cosmos.esa.int)
Dark matter is inferred through gravitational effects rather than ordinary emitted light. Measurements of galactic rotation and other motions can require substantially more mass than visible stars and gas provide. Its physical identity remains unresolved, although it supplies the gravitational scaffolding for galaxy assembly in prevailing models. (science.nasa.gov)
Formation and evolution
Galaxy formation began as matter accumulated in denser regions of the early universe following the Big Bang. In dark-matter-based models, smaller structures combine into larger ones through hierarchical growth. Gas gathers within these structures, and successive episodes of star formation build their stellar populations. Observations of distant galaxies allow researchers to investigate stages of this process rather than relying solely on nearby, mature systems. (science.nasa.gov)
Evolution continues through gas acquisition, star formation, and galaxy mergers. During an encounter, mutual gravitational forces deform galaxies, while compressed gas can produce intense episodes of star birth. Individual stars generally do not collide because the spaces between them are enormous. If the systems merge, the resulting galaxy may differ markedly from either predecessor. (science.nasa.gov)
Stars also change galactic chemistry. Elements heavier than hydrogen and helium are produced through stellar processes, including nucleosynthesis, and are dispersed by stellar mass loss and supernova explosions. Consequently, the gas incorporated into later generations of stars need not have the same composition as the original material. Enriched matter can also escape into the space between galaxies. (science.nasa.gov)
Central black holes and activity
Most large galaxies contain a central supermassive black hole. When surrounding material accumulates in an accretion disk, it heats up and radiates strongly, producing an active galactic nucleus. Some systems also launch high-speed jets. Particularly luminous active nuclei include quasars. Their observed properties depend partly on luminosity and viewing direction. The relationship between the growth of central black holes and their host galaxies remains an important research question. (science.nasa.gov)
Environment and observation
Galaxies frequently belong to gravitationally bound groups or larger galaxy clusters. The Milky Way and Andromeda Galaxy belong to the Local Group. Clusters contain hundreds or thousands of galaxies, along with hot gas and dark matter. Groups, clusters, filaments, and comparatively empty voids collectively form the cosmic web; larger supercluster associations are generally not gravitationally bound as complete systems. (science.nasa.gov)
Modern understanding of galaxies emerged partly from distance measurements. In 1923, Edwin Hubble identified a Cepheid variable in Andromeda; subsequent observations established that Andromeda lay beyond the Milky Way. Today, deep imaging samples galaxies across cosmic history, while spectroscopy investigates their chemistry and motions. Because distant light takes time to arrive, observations show remote galaxies at earlier stages of their development. (science.nasa.gov)