Galàxies

Galàxies are gravitationally bound systems that contain stars, stellar remnants, gas, dust, and dark matter. They range from dwarfs with a few million stars to giants with hundreds of billions. Galàxies are the fundamental building blocks of the universe’s large-scale structure and host most of the visible matter.

They are commonly categorized by morphology into elliptical, spiral, and irregular types. Elliptical galaxies have smooth, ellipsoidal shapes and little gas; spiral galaxies feature a flattened disk with spiral arms and ongoing star formation; irregulars lack a coherent structure. A bar-shaped feature is observed in many spirals, creating barred spirals. Classification correlates with color, star formation rate, and kinematics.

Formation and evolution: Galàxies form within dark matter halos in the early universe as gas cools and fragments. They grow through accretion and mergers, a process known as hierarchical assembly. Interactions can trigger starbursts, transform morphology, strip gas, and feed central supermassive black holes that power active galactic nuclei.

Observations and significance: Distances to galaxies are measured using standard candles and redshift; Hubble’s law relates redshift to cosmic expansion. Studies of galaxies reveal the distribution of matter in the universe, galaxy evolution over cosmic time, and the physics of star formation, feedback, and dark matter. Galàxies cluster in groups and clusters as part of the cosmic web and can interact and merge.

Notable examples include the Milky Way, the Andromeda Galaxy (M31), and the Triangulum Galaxy (M33).