Despite the vastness of space, things do bump into each other. Even on large cosmic scales, clusters of galaxies slam into each other and last tens of millions of years.
Galaxies are drawn to one another by forces that can be felt but aren’t visible. It is a mutual gravitational attraction and is inevitable: Inch by inch and light-year by a light-year; as the cosmic clock ticks, the galaxies grow closer. As galaxies near each other, their thin tendrils of gases begin to embrace, and the stars begin to reach out through the tenuous medium between them. Read on to find out what happens when galaxies collide with one another.
Stars will be thrown out of galaxies.
The galactic collision will cause stars to be thrown out of the galaxy, while others will be destroyed when they crash into the merging supermassive black holes. The delicate spiral structure of both the galaxies will collapse as they become a single, elliptical, giant galaxy. The process sounds cataclysmic, but in reality, it is a natural part of galactic evolution.
Galaxies are held together by an orbit and mutual gravity around a common centre. Intergalactic interactions are a common phenomenon, especially between satellite and giant galaxies. Galactic collisions may lead to mergers. If one of the colliding galaxies is relatively larger than the other, it will retain its shape and remain largely intact, while the smaller galaxy will be stripped apart and will eventually become part of the larger galaxy.
A galactic collision can trigger star formation.
Such galactic interactions can trigger the formation of a small number of stars. When the galaxies collide, it leads to the creation of vast hydrogen clouds that become compressed and trigger a series of gravitational collapses. A galactic collision also causes galaxies to age prematurely, as much of their gas gets converted into stars.
Formation of a quasar
Galaxies contain supermassive black holes in their centres, which will converge near the centre of the newly-formed galaxy. This merging of the black holes will result in the transfer of orbital energy to stars. When two black holes come within a light-year of one another, they will supposedly emit gravitational waves, causing massive radiation of orbital energy until they merge completely. Furthermore, gas consumed by the merged black hole might create a luminous quasar. It may also create an active nucleus to form the centre of the galaxy.
Note that inside clusters, galaxies routinely collide with one another and merge to become one.