Galaxy Mergers & Interactions: Structures & Vocabulary

Galaxies frequently influence one another through tides, close passages, minor accretion events, and major mergers. Because stars occupy tiny volumes compared with galactic size, direct stellar collisions are rare, but gravitational forces can strongly rearrange stellar orbits and compress gas.

Differential gravity pulls stars and gas into elongated features. Gas clouds collide and lose orbital energy more readily than stars, producing compression and inflow that can trigger starbursts or feed central black holes. Dynamical friction causes massive satellites to lose orbital energy and spiral inward.

Nearby systems such as the Antennae show tidal tails and intense star formation, while deep imaging reveals shells and streams around apparently normal galaxies. Simulations reproduce many observed structures from gravitational interactions.

Interactions produce bridges, tails, shells, warped disks, double nuclei, and disturbed gas. Major and minor mergers differ in mass ratio and therefore in how strongly they reorganize the primary galaxy.