galaxy merger

When two galaxies drift too close, gravity grabs hold and they begin a slow, spectacular crash that ends with them fused into one. A galaxy merger is exactly that: the gravitational coming-together of two galaxies, drawn out over hundreds of millions of years, that distorts both and eventually blends them into a single larger galaxy.

Despite the word 'collision', the stars almost never hit each other — galaxies are mostly empty space, so the stars sail past like two swarms of gnats passing through one another. What really collides and clumps is the gas; and the real action is gravity. Tidal forces fling out long curving tails and bridges of stars and gas, scramble orderly disks, and funnel gas toward the centers, often igniting a furious starburst and feeding the central black holes. A merger of two big, comparable galaxies is called a major merger; swallowing a much smaller galaxy is a minor merger. The whole event can take a billion years or more from first touch to final settling.

Mergers matter because they are a main engine of galaxy evolution. The leading idea is that when two gas-rich spirals merge, their disks are destroyed and the remnant settles into a smooth elliptical — turning a living, star-forming spiral into a red, quiescent ball. Mergers drive the brightest starbursts and quasars, build up the most massive galaxies, and, in the hierarchical picture, are simply how galaxies grow. The Milky Way and Andromeda are on a collision course and will merge in a few billion years.