Cosmic Dance: 100 years ago, the known Universe was restricted to the limits of the Milky Way. Although other galaxies have been observed since at least the second century, at that time there were still no precise ways to measure the distances to these large and diffuse objects to determine whether or not they belonged to our galaxy.
In 1923, American astronomer Edwin Hubble calculated the distance to the Andromeda Galaxy and solved this problem, finding a value much greater than the known diameter of the Milky Way and confirming that these objects were, in fact, stellar island universes distinct and separate from the our.
Since then, knowledge about these structures has grown exponentially, enabling astronomers to increasingly understand the properties of galaxies, how they form and how they evolve over billions of years.
Today, it is well known that galaxies are, on average, moving away from each other due to cosmic expansion. How is it possible, then, for two galaxies to collide? This is because some, especially those in environments such as groups and clusters, are not very far apart compared to their individual sizes.
One of the implications of this is that the effect of gravity between them is greater than the effect caused by the expansion of the Universe, which consequently makes two galaxies interact gravitationally with each other. These interactions provoke close encounters between them and, at times, make them collide with each other and merge completely, in a true “cosmic dance“.
Although it is not possible to observe all the steps of this “dance” in a single pair of galaxies, as the whole process can take millions of years, there are numerous dancing pairs in the Universe, each one of them at a distinct moment and stage, which allows astronomers to combine theory with imagination and creativity, putting together the complete picture with the steps and the general idea of how these dances work.
Furthermore, several computer simulations and modeling (using point masses and test particles) have been able to successfully reproduce the structures that are observed in interacting galaxies, further strengthening theories of how these processes occur.