When two galaxies collide, they do not usually smash their stars together in a spectacular cosmic crash. Instead, their immense gravitational forces distort their shapes, pull out long streams of stars and gas, trigger bursts of star formation, and gradually reshape both galaxies. Over millions to billions of years, the two systems can merge into a single, larger galaxy.
Galaxy collisions are among the most dramatic events in the universe, but they are also important stages in galactic evolution. They influence how galaxies grow, how stars are born, how central black holes develop, and how matter moves through the universe. Understanding these encounters helps astronomers explain why galaxies have different shapes and how large structures evolve over cosmic time.
Why galaxies collide
Galaxies are enormous collections of stars, planets, gas, dust, and dark matter held together by gravity. Although they may appear isolated when viewed through telescopes, galaxies are not stationary objects. They move through space, interact gravitationally, and often belong to groups or clusters containing many other galaxies.
Gravity is the primary force behind galactic collisions. When two galaxies pass close to each other, each exerts a gravitational pull on the other. These forces can alter their paths, stretch their shapes, and gradually change their motions. If the galaxies lose enough orbital energy through gravitational interactions and the effects of their surrounding matter, they can become gravitationally bound and eventually merge.
Galaxies are particularly likely to interact in regions where many galaxies exist relatively close together. Galaxy groups, which contain several gravitationally associated galaxies, provide opportunities for repeated encounters. Larger galaxy clusters contain hundreds or thousands of galaxies, although their high relative speeds can make complete mergers between large galaxies less likely than in some smaller groups.
The expansion of the universe does not prevent galaxies from colliding. Although space is expanding on large scales, gravity can overcome this expansion within gravitationally bound systems. As a result, nearby galaxies can move toward one another even while the universe as a whole continues to expand.
Why stars rarely collide during a galactic merger
One of the most surprising aspects of galaxy collisions is that individual stars almost never collide.
A galaxy may contain hundreds of billions of stars, but the distances between them are enormous compared with their physical sizes. Even when two galaxies overlap, their stars generally pass through the other galaxy without making direct contact.
A useful analogy is two large swarms of insects passing through each other. Although the swarms overlap, most individual insects avoid touching one another. The analogy has limits because galaxies are governed by gravity rather than coordinated movement, but it helps illustrate how enormous distances separate individual stars.
The same principle applies to planetary systems. A collision between galaxies does not mean that planets routinely crash into other planets or that every solar system is destroyed. Most planetary systems remain physically intact during the encounter.
However, the gravitational environment can change substantially. Stars may be pulled into new orbits, displaced from their original locations, or thrown into extended streams around the merging galaxies. Some may eventually become part of the outer regions of the resulting galaxy, while others can be ejected into intergalactic space.
Although direct stellar collisions are extremely uncommon, gravitational interactions can still have significant consequences for individual stars and their planetary systems. A passing star or a changing gravitational environment can disturb distant planetary orbits, particularly in relatively crowded regions.
How gravity reshapes colliding galaxies
The first major effects of a galactic encounter usually involve changes in the galaxies’ overall shapes and motions.
As two galaxies approach, their gravitational fields interact. Each galaxy pulls on the stars, gas, dust, and dark matter associated with the other. These gravitational forces can produce tidal effects, which occur when gravity acts with different strengths across an extended object.
Tidal forces stretch and distort galaxies. Their outer regions are often affected first because the gravitational influence of the approaching galaxy can change the motions of stars and gas far from the center.
As the galaxies move around one another, material can be drawn into long, curved structures called tidal tails. These features may contain stars, gas, and other matter extending far beyond the galaxies’ original boundaries.
Some encounters also produce bridges of material connecting the galaxies. These bridges form when gravitational interactions pull matter from one system toward the other.
The resulting structures can be extraordinarily complex. A galaxy that once had a relatively symmetrical spiral shape may become stretched, bent, or distorted. Its spiral arms can be disrupted, its central regions can change, and its outer material can spread into enormous arcs.
These distortions do not happen instantly. A galactic encounter unfolds over hundreds of millions of years or longer, depending on the galaxies’ masses, relative speeds, orbital paths, and surrounding environments.
The galaxies may pass close to each other several times before finally merging. Each encounter can transfer energy and angular momentum, meaning the amount of rotational motion associated with their orbits, into the motions of stars and other material. This helps alter their trajectories and can bring them closer together.
What happens to gas and dust during a collision
Although stars generally pass through one another, gas and dust can interact much more directly.
Galaxies contain large quantities of gas, especially hydrogen, which provides the raw material for forming new stars. When two galaxies interact, their gas clouds can be compressed, disturbed, and redirected by gravitational forces.
Unlike stars, gas clouds can collide, interact, and experience friction through their constituent particles. These processes can create regions of increased gas density. When gas becomes sufficiently dense and gravitationally unstable, it can collapse under its own gravity and form new stars.
This can produce a period of enhanced star formation known as a starburst. A starburst occurs when a galaxy forms stars at a substantially higher rate than it normally would.
The gravitational interaction does not automatically produce a starburst in every collision. The outcome depends on factors such as the galaxies’ gas content, orbital geometry, internal structure, and the efficiency with which gas can cool and collapse.
Some encounters generate intense star formation across large regions. Others produce more limited increases, while certain mergers can actually suppress future star formation by heating, expelling, or consuming much of the available gas.
Dust also plays an important role. Dust consists of tiny solid particles mixed with interstellar gas. During a merger, it can become concentrated in dense regions, where it absorbs and scatters visible light.
As a result, some active star-forming regions are difficult to observe using ordinary visible-light telescopes. Infrared observations can reveal much of the activity because infrared radiation can pass through dusty regions more effectively than visible light.
How galactic collisions create new stars
The increased compression of gas during some galaxy mergers can encourage the formation of massive groups of new stars.
Star formation begins when sufficiently dense regions of cold gas become gravitationally unstable. Gravity pulls the material inward, causing it to contract. As the cloud becomes denser, smaller regions can collapse and eventually form individual stars or groups of stars.
A galaxy merger can encourage this process by disturbing gas throughout the interacting systems. Gas may flow toward the central regions, collide with other gas, or collect in dense structures.
The resulting starburst can produce large numbers of young, hot stars. Massive stars are particularly bright and emit substantial amounts of ultraviolet radiation. They can make regions of intense star formation glow brightly.
However, massive stars have relatively short lifetimes compared with smaller stars. They consume their nuclear fuel rapidly and may end their lives in powerful explosions called supernovae.
Supernovae release energy into surrounding gas, producing shock waves that can compress or disperse nearby material. They also distribute heavier chemical elements into interstellar space. These elements, including oxygen, carbon, and iron, become part of the material from which later generations of stars and planets can form.
This creates a complicated feedback process. A merger can encourage star formation, but the resulting massive stars and supernovae can heat and disrupt the gas that would otherwise form additional stars.
Consequently, a starburst does not necessarily continue throughout the entire merger. Its intensity can rise and fall as gas moves, collapses, heats up, and becomes depleted.
What happens to the supermassive black holes
Most large galaxies are believed to contain supermassive black holes in their central regions. These objects have masses ranging from hundreds of thousands to billions of times the mass of the Sun.
When two galaxies merge, their central black holes can eventually become gravitationally bound to each other.
Initially, the black holes may remain far apart even after the surrounding galaxies begin to combine. Their motion through the dense stellar environments of the merged system gradually changes because of gravitational interactions with nearby stars and other matter.
This process can cause the black holes to lose orbital energy and move closer together. Eventually, they may form a binary black hole system, meaning two black holes orbiting a common center of mass.
As the pair becomes more tightly bound, it can transfer energy to surrounding stars and other matter. Under suitable conditions, the black holes can move close enough for gravitational radiation to become the dominant mechanism driving their final approach.
Gravitational waves are disturbances in spacetime produced by accelerating masses. They carry energy away from the orbiting black holes, causing their separation to decrease.
If the black holes finally merge, they form a single, more massive black hole. A portion of the original mass is converted into energy carried away by gravitational waves, in accordance with the relationship between mass and energy.
The merger can release an enormous amount of energy through gravitational waves, although this energy does not generally emerge as a bright explosion of ordinary light.
The process is not guaranteed to happen quickly. The final stages of bringing two supermassive black holes together remain an important area of research. In particular, astronomers are investigating how efficiently black hole pairs can lose enough energy to reach the distances at which gravitational waves drive their final merger.
Can galaxy collisions destroy entire galaxies?
A galaxy collision can dramatically alter the structure of both galaxies, but it does not usually destroy their matter in the sense of making it disappear.
Stars, gas, dust, and dark matter are redistributed. Some material becomes part of the central merged galaxy, while other material spreads into extended outer regions or becomes gravitationally unbound.
One possible outcome is the transformation of two spiral galaxies into a much larger elliptical galaxy.
Spiral galaxies have flattened disks containing stars, gas, and often recognizable spiral arms. Elliptical galaxies have smoother, more rounded structures and generally lack the prominent spiral patterns found in disk galaxies.
During a major merger, the original ordered stellar orbits can become more randomized. This process, known as violent relaxation, occurs when a rapidly changing gravitational field redistributes the energies and motions of stars.
The resulting galaxy may have a more rounded structure, although its exact shape depends on the masses, initial structures, orbital geometry, gas content, and other properties of the merging galaxies.
Not every merger produces an elliptical galaxy. Some interactions preserve or eventually rebuild a rotating disk, particularly when sufficient gas remains available to settle into an organized structure.
Smaller galaxies can also merge with larger ones without completely transforming their overall appearance. Such events are called minor mergers. They can add stars to a galaxy’s outer regions, disturb its disk, and contribute to its gradual growth.
The distinction between major and minor mergers is based primarily on the relative masses of the galaxies involved. Major mergers involve galaxies with broadly comparable masses, while minor mergers involve a substantially smaller companion.
How scientists observe galaxy collisions
Astronomers cannot watch most galaxy mergers from beginning to end because these events unfold over timescales far longer than a human lifetime.
Instead, they study different galaxies at different stages of interaction. By comparing these observations, scientists can reconstruct the processes that occur during a merger.
Telescopes operating at different wavelengths reveal different components of the encounter.
Visible-light observations show the distribution of stars and can reveal tidal tails, distorted spiral arms, and other structural changes. Ultraviolet observations help identify regions containing young, hot stars, making them useful for studying recent star formation.
Infrared observations are especially valuable for examining dusty galaxies. They can reveal star-forming regions that are obscured in visible light and help astronomers study how gas and dust behave during mergers.
Radio observations can detect neutral hydrogen gas, providing information about the extended gaseous structures surrounding interacting galaxies. Other radio and millimeter observations can reveal molecular gas, which is particularly important because cold molecular clouds are major sites of star formation.
Astronomers also use computer simulations to investigate how mergers develop. These simulations model the gravitational behavior of stars and dark matter, as well as the more complicated physical processes affecting gas, star formation, and black hole activity.
By changing the masses, orbital paths, gas content, and other initial conditions, researchers can compare simulated mergers with real galaxies. This helps them identify which physical processes are most likely responsible for observed structures.
However, simulations are not perfect representations of reality. Some processes, especially those involving gas turbulence, star formation, and energy released by black holes, occur on scales that are difficult to model in detail. Scientists therefore compare simulations with multiple kinds of observations rather than treating any single model as definitive.
Famous examples of interacting galaxies
Several observed systems provide particularly useful examples of what happens when galaxies interact.
The Antennae Galaxies are among the best-known examples. Located roughly 60 million light-years from Earth, this interacting pair consists of two galaxies whose gravitational encounter has produced long, curved tidal tails. Their overlapping regions contain extensive star-forming activity, illustrating how gravitational interactions can compress gas and encourage the formation of new stars.
The Mice Galaxies provide another striking example. Their long, thin tidal tails give the interacting system an appearance resembling two mice with extended tails. These structures demonstrate how gravity can pull stars and gas far beyond the original boundaries of galaxies.
The Whirlpool Galaxy, also known as Messier 51, is interacting with a smaller companion galaxy. Their gravitational relationship contributes to the prominent structure of the larger galaxy’s spiral arms. This system illustrates how interactions can influence galactic structure even when the galaxies have not completed a major merger.
These examples show that galaxy interactions do not all produce identical results. Some generate spectacular tidal structures and intense star formation, while others primarily disturb a galaxy’s existing structure.
What will happen when the Milky Way and Andromeda approach each other?
One of the most familiar examples of a future galactic encounter involves our own galaxy, the Milky Way, and the nearby Andromeda Galaxy.
Andromeda is the nearest large spiral galaxy to the Milky Way, located approximately 2.5 million light-years away. Both galaxies belong to the Local Group, a collection of galaxies held together by gravity.
For many years, astronomers expected the Milky Way and Andromeda to undergo a major merger in several billion years. Their gravitational attraction and measured motion suggested that they were approaching one another.
However, predicting the exact outcome requires more than knowing their distance and motion toward each other. Their sideways motion, the gravitational influence of other Local Group galaxies, and uncertainties in the masses and distribution of dark matter all affect their future trajectories.
Consequently, the eventual encounter is not as certain as earlier simplified predictions sometimes suggested.
If the galaxies do merge, the process will unfold gradually over billions of years. Their disks will become distorted, their stars will be redistributed, and their central regions will eventually combine. The resulting galaxy would likely be a much larger system with a structure different from either original spiral galaxy.
The Sun and its planetary system would not necessarily be destroyed in the process. Direct collisions between stars would remain extremely unlikely. However, the Sun’s position and orbit within the resulting galaxy could change substantially.
There is also an important distinction between the future of the Milky Way as a galaxy and the future of Earth as a planet. The merger itself is not expected to produce a direct collision between the Sun and another star. Nevertheless, Earth has a separate long-term challenge: the Sun will gradually become brighter as it ages, and its later evolution will eventually make Earth uninhabitable, regardless of whether the galactic merger occurs.
Why galaxy collisions matter for the evolution of the universe
Galaxy collisions are not simply destructive encounters. They are part of the processes through which galaxies grow and change over cosmic time.
In the early universe, matter was distributed less uniformly than it is today. Small concentrations of matter grew under gravity, attracting surrounding material and gradually developing into larger structures. Over billions of years, smaller galaxies combined and interacted, contributing to the formation of the diverse galaxy populations astronomers observe today.
Mergers can increase a galaxy’s mass, alter its shape, redistribute its stars, and influence its central black hole. They can also trigger periods of rapid star formation or help bring that activity to an end by changing the conditions within the galaxy.
These events are especially important for understanding why galaxies differ so much. Some retain orderly rotating disks, while others develop rounded structures with more randomized stellar motions. Their histories of gravitational interactions, gas accretion, and mergers help explain these differences.
Galaxy collisions also contribute to the distribution of chemical elements throughout the universe. Stars created during merger-driven starbursts eventually return some of their material to surrounding space through stellar winds and supernova explosions. This enriched material can later become part of new stars, planets, and other astronomical objects.
The broader lesson is that galaxies are dynamic systems rather than permanent, isolated collections of stars. Their shapes, contents, and internal structures change over immense periods through gravity, gas dynamics, star formation, and interactions with their surroundings.
When two galaxies collide, the stars generally pass one another, but the galaxies themselves can undergo profound transformation. Gas clouds interact, gravity reshapes stellar orbits, new stars may form, and central black holes may eventually merge. What begins as an encounter between two separate galaxies can ultimately produce a single, larger system with a very different structure and history.