The galaxy is not a static "island island," but it is dynamic and always changes its shape over time as it continues to move through the darkness of the universe. Sometimes, galaxies can even collide with other galaxies.
Located 350 million light-years away in the constellation Cetus, here's a pair of spiral galaxies that have just begun an incredible merging process. Known collectively as Arp 256, this image features two galaxies that have been torn apart, throwing gas, dust, and stars in a mess due to the gravitational forces that pull the two galaxies together.
Although the nuclei of these two galaxies are still separated by great distances, but the structure of the two galaxies in Arp 256 already looks very distorted. Galaxies at the top of the image even already have a kind of tail features that are very prominent and long due to the interaction of gravity.
Due to collision, these galaxies are also "endowed" with extreme levels of star formation. Can be seen along the path of the arm of the two galaxies above, you can see the blue color caused by hot young stars just formed.
Arp 256 itself was first catalyzed by Halton Arp astronomer in 1966, as one of 338 galaxies that he found in the Atlas of Peculiar Galaxies catalog. The purpose of the catalog is to illustrate examples of strange and beautiful structures found in the universe, providing an overview of the various stages of galactic evolution.
These strange galaxies are like natural experiments that are played on a cosmic scale. By cataloging, astronomers can better understand the physical processes that will occur after the two galaxies collide.
Many galaxies in this catalog are dwarf galaxies with unclear structures, or active galaxies that produce powerful jets. A large number of galaxies in this catalog are also being collided, such as Messier 51 galaxies, galaxies Antennae, and Arp 256.
Then why did the galaxy collide when he said the universe expands? Should not each galaxy stay away from each other? What happens is actually the battle between the forces of gravity between two galaxies (which attract each other) and the expansion of the universe (which seeks to separate the two galaxies).
The gravitational forces between these two galaxies can make them both close enough to each other. And when that happens, the force of gravity between the two galaxies is the winner compared to the expansion of the universe, so that eventually the two galaxies will collide.