While planets orbit stars in predictable paths, the movement of galaxies across the vastness of the cosmos presents a different and far more intricate picture. Galactic dynamics are influenced by multiple forces and operate on scales far grander than individual stars and planets.
How Galaxies Move Through Space
Galaxies’ movements are complex, primarily driven by the gravitational pull of other galaxies and the larger structures they inhabit. Galaxies are often found grouped together in clusters, such as our own Local Group, which contains over 50 galaxies. Within these clusters, galaxies exert gravitational forces on each other, causing them to move towards and past one another in streaming motions.
The Milky Way and the Andromeda galaxy, the two largest members of the Local Group, are currently approaching each other at approximately 110 kilometers per second. This means they are on a collision course, expected to merge in about 4.5 billion years. The Local Group itself is not stationary; it is moving collectively towards more massive structures, such as the Virgo Supercluster and regions known as the Great Attractor.
The Unseen Hand of Gravity and Dark Matter
Gravity dictates galaxy motion, but much of this influence comes from dark matter, an invisible component constituting about 85% of the universe’s total mass. Without it, observed galaxy behavior wouldn’t align with gravitational laws.
Dark matter forms vast, invisible “halos” that envelop galaxies and galaxy clusters, providing the additional gravitational pull needed to hold these structures together. For instance, stars at the outer edges of galaxies move much faster than expected based solely on visible matter, a phenomenon explained by dark matter’s gravitational presence. The bending of light from distant galaxies, known as gravitational lensing, also evidences dark matter’s pervasive influence on cosmic scales.
When Galaxies Meet: Collisions and Mergers
Galactic movement often leads to encounters, such as collisions and subsequent mergers. Unlike collisions between stars, which are rare due to immense distances, galaxies frequently pass through one another. During a galactic collision, individual stars typically do not strike each other, but the vast clouds of gas and dust within the galaxies interact significantly.
These interactions can compress the gas, triggering intense bursts of new star formation. Over extended periods, these encounters can alter galactic structures, often transforming spiral galaxies into larger, more rounded elliptical galaxies. The impending collision between the Milky Way and Andromeda is projected to eventually form a new, larger elliptical galaxy often dubbed “Milkomeda.”
The Universe’s Expansion and Galactic Dynamics
While gravity governs galaxy movements and interactions on local scales, such as within clusters, the universe as a whole is expanding. This expansion causes unbound galaxies to move further apart over time, preventing a universal “orbit” around a single point.
The expansion is observed to be accelerating, meaning distant galaxies are receding from us at ever-increasing speeds. This cosmic dynamic differs from gravitational interactions within clusters. While galaxies within a cluster can draw closer or even merge due to their mutual gravitational attraction, the overall fabric of the universe continues to expand, carrying unbound galaxies away from each other.