Galaxies are immense cosmic structures, vast collections of billions of stars, gas, dust, and dark matter, all held together by gravity. These sprawling systems are not static objects in the universe; instead, they are constantly in motion. Understanding how fast galaxies move offers insights into the dynamic nature of the universe itself.
Different Kinds of Galactic Movement
Galaxies exhibit different types of motion, including local movements within their immediate cosmic neighborhoods and participation in the overall expansion of the universe.
One type of movement is local motion, often referred to as peculiar velocity. This describes how galaxies move relative to each other due to gravitational attraction. For instance, our own Milky Way galaxy and its closest large neighbor, the Andromeda galaxy, are currently approaching each other. This gravitational interaction is common in galactic clusters, where galaxies orbit or fall toward common centers of mass.
Beyond these local interactions, galaxies are also carried along by the expansion of the universe itself, known as the Hubble flow. This is not galaxies moving through space, but rather the space between galaxies stretching, carrying them farther apart. The farther two galaxies are from each other, the faster the space between them expands. This cosmic expansion dominates galactic movement on very large scales, where gravitational pulls between individual galaxies become less significant.
How We Measure Galactic Speeds
Astronomers rely on the properties of light to determine how fast galaxies are moving. A fundamental tool for this is the Doppler effect, which describes how the wavelength of light changes when its source is moving relative to an observer. Just as the pitch of a siren changes as an ambulance passes, the color of light from a galaxy shifts depending on its motion.
When a galaxy is moving away from us, its light waves are stretched, shifting them towards the red end of the electromagnetic spectrum; this is known as redshift. Conversely, if a galaxy is moving towards us, its light waves are compressed, resulting in a shift towards the blue end, called blueshift. By analyzing the unique spectral lines present in a galaxy’s light, which act like a barcode for elements, astronomers can precisely measure these shifts.
The amount of redshift or blueshift directly correlates to the galaxy’s radial velocity (its speed directly toward or away from Earth). For galaxies that are far away, this technique also allows astronomers to apply Hubble’s Law. This principle states that a galaxy’s recessional velocity (its speed away from us due to cosmic expansion) is proportional to its distance from us.
What Are the Speeds
Galaxies exhibit a wide range of speeds. Local peculiar velocities, driven by gravitational interactions, typically range from a few tens to hundreds of kilometers per second. Galaxies within small groups, for instance, often have peculiar velocities around 100 kilometers per second.
A prominent example of local motion is the Andromeda galaxy, which is approaching the Milky Way at approximately 110 kilometers per second. This closing speed suggests a future collision between our two galaxies, though the stars within them are so widely spaced that direct stellar impacts are highly improbable. In richer galaxy clusters, where gravitational forces are stronger, individual galaxies can move at speeds up to 1000 kilometers per second, and some can even surpass 2000 kilometers per second as they plunge through the cluster’s center.
Recessional velocities, caused by the expansion of space, increase with distance. For very distant galaxies, these speeds can appear to exceed the speed of light. This seemingly faster-than-light motion does not violate the laws of physics because it is space itself that is expanding, not the galaxies moving through space at such speeds. Observable galaxies at extreme distances can have recession velocities exceeding twice the speed of light, such as GN-z11, which recedes at approximately 2.2 times the speed of light.
The Forces Behind Galactic Motion
The movements of galaxies are governed by two fundamental forces that operate on different cosmic scales. Gravity is the primary driver of local galactic motion, shaping the structure of the universe on smaller scales. It causes galaxies to attract each other, forming groups, clusters, and even larger superclusters.
This gravitational pull is responsible for galaxies orbiting each other and, in some cases, merging over billions of years. The impending encounter between the Milky Way and Andromeda is a direct consequence of their mutual gravitational attraction. Gravity also dictates the internal rotation of galaxies, keeping billions of stars in their orbits around a galactic center.
On the largest cosmic scales, a mysterious force known as dark energy dominates, driving the accelerated expansion of the universe. While gravity pulls matter together, dark energy acts as a repulsive force, pushing galaxies farther apart at an ever-increasing rate. This force explains why distant galaxies recede from us faster over time. The interplay between gravity, which organizes matter into structures, and dark energy, which spreads these structures apart, shapes galactic motion across the vastness of the cosmos.