Galaxia

Galaxia, or galaxy, is a gravitationally bound system consisting of stars, stellar remnants, gas, dust, and dark matter. They vary widely in size and composition, from dwarf galaxies with millions of stars to giant galaxies containing trillions. Typical galaxies span thousands of light-years across and are the fundamental building blocks of the visible universe, often found in groups and clusters.

Galaxies are commonly categorized by appearance as spiral, elliptical, or irregular. Spiral galaxies have rotating disks with spiral arms and a central bulge; elliptical galaxies are more rounded and show little ongoing star formation; irregulars lack a regular shape, often due to gravitational interactions. Lenticular galaxies are an intermediate class with a disk but without prominent spiral structure.

Inside a galaxy, the main components include a stellar disk or bulge, a halo containing old stars and globular clusters, and usually a central supermassive black hole. Most of a galaxy’s mass is inferred to reside in a dark matter halo, which dominates the gravitational potential and shapes rotation curves.

Galaxy formation and evolution involve the collapse of gas in the early universe and subsequent growth through accretion and mergers. Interactions can distort shapes, trigger bursts of star formation, and transform spirals into lenticulars or ellipticals. Observations across cosmic time show galaxies changing in mass, color, and structure as the universe evolves.

Distances are estimated from redshift and standard candles; large surveys map the distribution of galaxies, revealing a cosmic web of clusters, filaments, and voids. The Milky Way is a barred spiral galaxy that contains our Solar System.