A collision is not an explosion
The distances between stars are so large that individual stellar collisions are extremely unlikely when two galaxies pass through one another. Gravity still affects every orbit. Disks can bend, bars can form, and stars can be thrown into long tidal tails that remain visible for hundreds of millions of years.
The word collision describes the interaction between two gravitational systems rather than a single moment of impact. The galaxies may make several close passes before merging, or they may remain a bound pair for a very long time.
Gas clouds feel the encounter
Stars pass through one another, but gas clouds collide, compress and lose energy. This can trigger a burst of star formation, producing clusters of massive blue stars and bright emission nebulae.
The same encounter can also remove gas from the disk or heat it so much that it cannot collapse. Whether star formation rises or falls depends on the orbit, gas fraction, metallicity and feedback from young stars and black holes.
Tidal tails and streams
The outer parts of a galaxy are weakly bound, so they respond strongly to a companion. Stars and gas can be pulled into curved tidal tails, shells and bridges. These features are often faint and are best seen in long exposures.
Over time, the material may fall back into the remnant, form new dwarf systems or disperse into the surrounding halo. Tails therefore preserve information about the direction, speed and geometry of the encounter.
Black holes can meet
When two large galaxies merge, their central supermassive black holes may sink toward the new centre through interactions with stars and gas. Eventually they can form a binary system and, after losing orbital energy, coalesce.
The process can produce gravitational waves at frequencies different from those detected by ground-based interferometers. Future space-based observatories are designed to study these massive black-hole mergers.
The future of the Milky Way
The Milky Way and Andromeda are expected to interact in several billion years. The stellar disks will be distorted, gas clouds may form new stars and the final remnant may be a larger elliptical or lenticular galaxy.
Galaxy collisions are therefore part of normal growth, not rare cosmic accidents. They build mass, mix chemical elements, feed central black holes and help shape the population of galaxies seen today.
What remains after a merger
After two galaxies merge, the final remnant can look smooth even though its stars remember the encounter. Some stars occupy shells, streams or unusual orbits; gas may settle into a new disk; and globular clusters can trace material from both progenitors. Astronomers use these fossils to reconstruct events that happened hundreds of millions of years earlier. A quiet-looking elliptical can therefore be the end product of a very violent and complicated history.