Galaxies, like the Milky Way, seem to float freely in space, but is that really the case? Are they orbiting something even larger?
To understand this, we must first grasp how orbits work. When two objects orbit each other, they exert mutual gravitational force. This phenomenon keeps them together, each orbiting around their common center of mass. In systems like the Solar System, this center is often located inside the most massive object, the central star, making its motion almost imperceptible.
The Andromeda galaxy.
Image Wikimedia
On larger scales, the situation becomes more complex. The Milky Way is part of the Local Group, a collection of galaxies that also includes Andromeda, Triangulum, and several dwarf galaxies. The Milky Way and Andromeda, being the most massive in this group, orbit around a center of mass located between them. Nothing larger is in the immediate vicinity, so these two galactic giants find themselves orbiting each other.
However, this orbit is neither circular nor elliptical like those of planets. The two galaxies follow primarily radial orbits, heading towards each other under the effect of their gravitational attraction. Their motion is not purely radial due to a slight lateral component, which further complicates the orbital pattern.
This radial motion implies a future collision between the Milky Way and Andromeda, expected in about 4.5 billion years. Despite this collision, the stars of the two galaxies will likely not collide due to the enormous distances separating them. The galaxies will pass through each other before separating and then crossing again, eventually merging into a single large elliptical galaxy.
The merger will cause significant gravitational disturbances. These will alter the arrangement of stars and potentially trigger the formation of new stars from the heated gas of the two galaxies' spiral arms. This process will transform the two spirals into a single elliptical structure.
Beyond the Local Group, orbits are less defined. We know that our galaxy group is heading towards the Virgo Cluster, which contains several hundred galaxies located about 65 million light-years away. However, the expansion of the Universe is moving the Local Group away faster than the gravitational pull of the Virgo Cluster can draw it in.
Each discovery brings us a little closer to understanding our place in the Universe.