GALaxy

A galaxy is a gravitationally bound system consisting of stars, stellar remnants, interstellar gas and dust, and dark matter. Galaxies vary widely in size and structure, from dwarf galaxies with only a few million stars to giant galaxies containing trillions. They range in diameter from tens of thousands to over one hundred thousand light-years and exist in a variety of environments, from small groups to rich clusters.

Most galaxies contain a mix of stars in different populations, gas in various phases, dust lanes, and dark matter halos. Their visible components may take the form of a rotating disk with a central bulge, a spheroidal halo, or irregular shapes. Star formation occurs in dense molecular clouds and is fueled by gas accretion and mergers. Dark matter dominates their total mass and shapes their gravitational potential.

Galaxies are commonly classified by morphology into spiral, barred spiral, elliptical, lenticular, and irregular types, with a separate class for dwarf galaxies. The Hubble sequence notes correlations between morphology, color, and star formation, while more quantitative methods use kinematics and stellar populations.

Formation and evolution occur within a cosmological framework in which galaxies assemble from small structures within dark matter halos. Interactions and mergers transform morphologies, trigger bursts of star formation, and grow central black holes. Feedback from stars and active nuclei regulates gas content and future star formation.

Observationally, galaxies are studied across wavelengths, from radio to X-ray, revealing their stellar content, gas dynamics, dust, and central engines. The Milky Way and Andromeda are the best-studied nearby galaxies; the Local Group contains about 50 known members. In the broader cosmos, billions of galaxies populate the observable universe.