If you remember, in an article two weeks earlier (12 Esfand 1392) I showed an image of what I called the most distant cosmic exclamation mark and discussed it. Scientists' analysis suggested that what we were seeing really was, figuratively speaking, the astonishment and surprise of two galaxies that had suddenly found themselves in each other's path.
The animation discussed in the original article makes the story clearer. What appears to us as an exclamation mark is actually a moment* before the two galaxies collide.**
With major advances in computer simulation, the future development of galaxy collisions like this can be modelled. In the animation, the collision of two large spiral galaxies is simulated at a rate of several million years per second of animation. Real Hubble Space Telescope observations of other galaxy interactions are then shown alongside the simulation, providing an idea of what these two galaxies may experience over the coming millions of years.
Galaxy collisions are very common in the universe, and an individual interaction may continue for more than a billion years. Because most of the volume inside galaxies is empty, the probability that individual stars from the two galaxies will directly collide is very small. Instead, the gravity of the galaxies drives the large-scale changes, potentially severely altering one or both galaxies and even causing them to merge into a larger galaxy—perhaps similar to processes involved in forming the Milky Way.
The material most strongly disturbed in such an encounter is the gas and dust of the interacting galaxies. The source describes this disturbance as producing waves through the galaxies and triggering the formation of large numbers of new stars, which themselves continue to evolve during the long interaction.
Our own Milky Way is thought to have absorbed several smaller galaxies during its history, and the article notes the expectation that in several billion years it will collide with neighbouring Andromeda, ultimately producing a larger galaxy.
* On cosmic scales, “a moment” can mean several million years.
** The author humorously compares the impending head-on encounter to Zinedine Zidane's headbutt on Marco Materazzi in the 2006 World Cup.
Animation by Frank Summers of the Space Telescope Science Institute. Computer simulation by Chris Mihos of Case Western Reserve University and Lars Hernquist of Harvard University.