Milky Way May Have Formed From Thousands of Galaxies, New Simulations Suggest

The Milky Way may have begun as a far more fragmented system than its present spiral shape suggests. New simulations indicate that thousands of smaller galaxies and early galactic building blocks could have merged over billions of years to create the galaxy we see today.

This finding supports a more complex answer to the question of how the Milky Way formed. Rather than emerging as one coherent galaxy, it may have grown through countless interactions involving stars, gas and dark matter.

The phrase “thousands of galaxies” does not mean that the young Milky Way swallowed thousands of modern spiral galaxies. Most contributing systems were likely much smaller, including dwarf galaxies and early protogalactic structures. Because the estimate comes from simulations, it represents a possible formation history rather than a directly observed census.

The Conventional Picture of Milky Way Formation

The Milky Way Formed Within the Cosmic Web

The Milky Way developed within the cosmic web, the immense network of dark matter, gas, galaxies and galaxy clusters that spans the universe. Gravity gradually pulled matter toward denser regions, allowing small structures to form and grow.

Dark matter played a central role. Although it does not emit or reflect light, its gravity helped gather ordinary matter into halos. Gas collected inside those halos, cooled and formed stars. Over time, the halos interacted, merged and became the foundations of larger galaxies.

The Milky Way therefore did not appear fully formed. Its disk, central bulge and extended halo emerged through a process that lasted billions of years. Star formation continued as new gas arrived, while interactions with nearby systems reshaped the galaxy.

Earlier Models Emphasized Fewer Major Mergers

Simplified explanations often focused on a limited number of important mergers between relatively large protogalaxies. These events could strongly influence the shape, rotation and stellar populations of a developing galaxy.

Astronomers distinguish among several types of galactic interaction:

• Major mergers involve galaxies with relatively similar masses.
• Minor mergers occur when a much smaller galaxy interacts with a larger one.
• Dwarf-galaxy accretion involves the gradual capture and disruption of small satellite systems.

The new simulation-based picture gives greater importance to many minor mergers. Each event may have been modest, but thousands of interactions could have produced major cumulative effects.

Thousands of Smaller Galaxies May Have Helped Build the Milky Way

The Main Finding

The central claim is that the Milky Way may have assembled from thousands of smaller galaxies that merged, dissolved or contributed material during its long development. These systems may have supplied:

• Stars and stellar remnants.
• Cold gas for future star formation.
• Dark matter halos.
• Chemical elements produced by earlier generations of stars.
• Orbital energy and angular momentum.
• Material that helped build the stellar halo and central regions.

The estimate should not be interpreted as an exact historical count. It describes the many small systems that may have contributed to the Milky Way throughout its formation.

Some of these galaxies may have been extremely faint by modern standards. Others may have contained only a small number of stars while remaining dominated by dark matter. Many were likely disrupted long ago, leaving no obvious surviving galaxy.

Why Simulations Matter

Astronomers cannot watch the Milky Way form from beginning to end. Its earliest major growth occurred billions of years ago, long before humans developed telescopes. Simulations provide a way to reconstruct possible histories using physical laws and observations of the present galaxy.

Galaxy-formation simulations can model:

• The gravitational behavior of dark matter.
• The movement of stars and gas.
• The growth of galactic halos.
• Star formation and stellar feedback.
• The disruption of dwarf galaxies.
• The long-term effects of repeated mergers.

Researchers compare simulated galaxies with the real Milky Way. A strong model should reproduce several observed features at once, including the galaxy’s disk, halo, satellite population, stellar motions and chemical diversity.

Simulations do not prove one unique history. They test whether a proposed scenario can produce a galaxy resembling the Milky Way.

A More Fragmented Early Milky Way

The early Milky Way may have been a crowded and unstable environment. Numerous small systems moved through the same region of space, interacting with one another and with the growing central galaxy.

Repeated encounters could have gradually built or reshaped the Milky Way’s:

• Central bulge, where older and densely packed stars are concentrated.
• Stellar disk, where much of the galaxy’s later star formation occurred.
• Thick disk, containing older stars with larger vertical motions.
• Stellar halo, a broad population of ancient stars outside the main disk.
• Dark matter halo, extending far beyond the visible galaxy.

This history contrasts with the image of a galaxy forming smoothly in isolation. The modern Milky Way appears organized, but its structure may preserve evidence of a much more chaotic beginning.

How Small Galaxies Merge Into One Large Galaxy

Gravity Drives the Process

Gravity pulls nearby galaxies toward one another. As a small galaxy enters the larger galaxy’s gravitational field, its orbit can change. The system may lose orbital energy through interactions with surrounding dark matter and stars, causing it to move closer to the galactic center.

A galaxy merger does not usually involve stars striking one another directly. The distances between stars are enormous, so direct stellar collisions are rare. The galaxies’ gravitational fields interact even when individual stars pass without contact.

Gas behaves differently. Gas clouds can collide, compress and cool. These processes may trigger bursts of star formation or send material toward the center of the growing galaxy.

Mergers Rearrange Stars and Gas

A merger can produce several recognizable effects:

• Stars may be pulled into long tidal streams.
• Some stars can be scattered into the outer halo.
• Gas may flow toward the galactic center.
• New star formation may follow gas compression.
• The orbit and rotation of the main galaxy can change.
• The central black hole may receive additional gas.

One small merger may leave only a subtle trace. Thousands of mergers, however, can reshape an entire galaxy. Their combined effects may explain why the Milky Way contains stellar populations with different ages, chemical compositions and orbital patterns.

The Building Blocks Were Not Modern Spiral Galaxies

The systems involved in the Milky Way’s early growth were probably unlike present-day spiral galaxies. Many may have been dwarf galaxies, dark-matter-dominated systems or loosely organized protogalaxies.

Some contained relatively few stars. Others may have experienced limited chemical enrichment because they formed stars for only a short period. Their structures and internal motions could also have differed from those of larger, mature galaxies.

The distinction matters: the Milky Way did not suddenly absorb thousands of fully developed galaxies comparable to itself. It likely grew through the gradual incorporation of much smaller building blocks.

Evidence That Preserves the Milky Way’s Merger History

The Stellar Halo Holds Ancient Clues

The Milky Way’s stellar halo contains old stars outside the bright disk. Some formed within the Milky Way, while others may have arrived inside smaller galaxies that were later disrupted.

Astronomers study halo stars by measuring their ages, chemical compositions, positions, velocities and orbital directions. Groups of stars that share unusual motions and similar chemical patterns may have originated in the same former galaxy. These groups act as archaeological evidence for systems that no longer exist.

Tidal Streams Reveal Past Encounters

Tidal streams are elongated trails of stars left behind when a dwarf galaxy or star cluster is pulled apart. The Milky Way’s gravity stretches these systems along their orbits.

A stream’s shape, direction and speed can help researchers estimate:

• When the disruption occurred.
• The satellite’s original mass.
• The orbit of the former galaxy.
• The distribution of dark matter in the Milky Way.
• How strongly the Milky Way influenced the satellite.

The chemical composition of stream stars provides another clue. Shared chemical patterns can help astronomers distinguish stream stars from those formed inside the Milky Way’s disk.

Satellite Galaxies May Be Remaining Relics

The Milky Way is surrounded by satellite galaxies, including numerous dwarf galaxies. Some may be surviving examples of the small systems that once interacted with the growing galaxy.

Others may eventually be disrupted and absorbed. Their stars could spread across the halo, while their dark matter becomes part of the Milky Way’s larger halo.

Not every satellite galaxy necessarily contributed to the Milky Way’s formation. Some remain separate systems, and their histories must be established through observation and modeling. Their existence nevertheless offers a nearby view of the hierarchical growth process that may have shaped the Milky Way in the past.

The Disk and Bulge Provide Additional Evidence

The Milky Way’s disk is not a single uniform population. It includes a thin disk of relatively young stars and a thicker component containing older stars with more dispersed orbits.

Earlier mergers may have heated the original disk, pushing stars into larger vertical orbits. They may also have contributed stars directly to the thick disk or helped drive gas toward the central bulge.

Researchers compare the ages, chemical compositions and movements of stars in these regions with predictions from simulations. Unusual stellar orbits can reveal gravitational disturbances that occurred long ago.

What the Model Could Explain

Diverse Stellar Populations

Stars formed in different environments do not have identical chemical fingerprints. Their compositions depend on the gas available in their birth systems and the types of stars that existed there previously.

A merger-rich history naturally produces a mixture of stars formed inside the early Milky Way, stars born in dwarf galaxies, ancient halo stars and younger stars formed from gas brought in during later interactions.

This diversity helps explain why the Milky Way contains stellar populations with different ages, elemental abundances and orbital behaviors.

An Extended Galactic Halo

The halo surrounds the visible disk and extends across a vast region of space. Disrupted dwarf galaxies could have supplied many of its stars.

As small galaxies were pulled apart, their stars spread along long orbits. Over time, streams became more diffuse, but their debris remained part of the halo. Dark matter from the same systems also joined the Milky Way’s larger dark matter structure.

Ancient stars and faint streams far from the galactic center support the idea that the halo contains debris from earlier accretion events.

The Milky Way’s Dark Matter

Each small galaxy likely occupied its own dark matter halo. When galaxies merged, their dark matter became part of the larger gravitational structure surrounding the growing Milky Way.

Dark matter cannot be observed directly, but astronomers infer its presence from gravitational effects, including the speeds of stars, the motions of satellite galaxies and the behavior of gas across the galaxy.

Simulations use these measurements to estimate how the Milky Way’s dark matter halo developed. Contributions from many smaller halos could help explain its present mass and structure.

A Galaxy That Is Not Perfectly Simple

The Milky Way’s spiral appearance can make it seem like a simple, stable object. In reality, its components formed at different times and under different conditions.

Its disk, bulge, halo, satellite galaxies and dark matter halo all preserve parts of its history. Earlier mergers may have left streams, unusual orbits, chemically distinct stars and structural irregularities.

The galaxy’s present form is therefore the result of gradual assembly, continued gas accretion and repeated gravitational disruption.

How This Fits Into Broader Galaxy Research

Galaxy formation is generally considered hierarchical: smaller structures emerge first and later combine into larger ones. Gravity brings dark matter halos together, while gas collects inside them and forms stars.

The Milky Way is one example of this broader process. Each galaxy develops a different history depending on its environment, mass and merger activity.

Other research suggests that the early universe contained complex and rapidly changing galaxies. Studies have examined whether collisions between early galaxies helped shut down star formation in some massive systems. This research concerns the evolution of giant galaxies, not the specific assembly history of the Milky Way, but both lines of work point to a broader conclusion: galaxy evolution can be more complicated than older models suggested.

Researchers are also investigating early black holes, unusual objects known as “little red dots” and the possibility that rocky planets began forming before the first galaxies. These studies do not directly explain the Milky Way’s formation, but they show that astronomers are revising many assumptions about the early universe.

What the Simulations Do Not Prove

No Single Definitive Formation Timeline

Different simulations can produce different assembly histories. Results depend on initial conditions, dark matter assumptions, gas physics, star-formation models and computational resolution.

The estimate of thousands of contributing galaxies should therefore be treated as a simulation-based possibility, not an exact historical census.

Not Every Small Galaxy Merged Completely

A contributing system may have been:

• Fully disrupted.
• Partially absorbed.
• Stripped of stars.
• Reduced to a dark matter remnant.
• Left intact as a satellite galaxy.

The phrase “contributed to the Milky Way” can describe several outcomes. It does not necessarily mean that every system was completely destroyed.

Simulations Need Observational Tests

Researchers must compare models with data from stellar surveys, Gaia measurements, spectroscopy, tidal-stream maps and dwarf-galaxy observations.

The strongest formation models reproduce several independent features of the Milky Way at the same time. A model that explains the halo but fails to reproduce the disk or satellite population remains incomplete.

How Astronomers Can Test the Scenario

Map Stellar Motions

Precise measurements of stellar positions and velocities can reveal groups of stars that share a common origin. Coherent motions in the halo may identify debris from former dwarf galaxies.

The Gaia mission has transformed this work by providing detailed measurements for large numbers of Milky Way stars. Future data releases should improve the reconstruction of stellar orbits.

Compare Chemical Fingerprints

Stars preserve chemical information from their birth environments. Elements such as iron and magnesium can distinguish populations formed in different systems.

Chemical tagging may help separate stars born inside the Milky Way from stars introduced through mergers. Combining chemistry with stellar motion provides stronger evidence than using either type of data alone.

Search for More Tidal Debris

Faint streams and disrupted galaxies can be difficult to detect because their stars are spread across large areas of the sky. Deeper surveys may reveal structures that earlier observations missed.

Each new stream provides another test of the Milky Way’s merger history. Its orbit and chemistry can be compared with simulated predictions.

Improve Galaxy-Formation Simulations

Future simulations can include:

• Larger numbers of low-mass galaxies.
• Higher-resolution stellar data.
• More realistic gas physics.
• Improved star-formation models.
• Black-hole feedback.
• Better measurements of the Milky Way’s dark matter halo.

Combining improved simulations with observations will show whether the thousands-of-galaxies scenario explains the Milky Way better than simpler alternatives.

What This Means for Our Understanding of the Milky Way

The Milky Way is not a static object. It has grown through internal star formation, gas accretion, mergers and gravitational rearrangement.

Its current structure reflects many different periods. Some stars formed in the original galaxy, while others arrived from systems that no longer exist.

The Sun formed billions of years after the earliest stages of Milky Way assembly. The solar system therefore developed inside a galaxy already shaped by earlier mergers and accretion events. This does not mean that one specific merger directly caused the Sun to form; it means that the Sun inherited a galactic environment created by a long history of cosmic construction.

Galaxy formation has not stopped. The Milky Way continues to interact with nearby satellite galaxies, some of which are gradually losing stars and gas. These interactions provide a local example of hierarchical growth and offer astronomers a way to study processes that may have been common during the Milky Way’s early history.

Conclusion: A Galaxy Built From Many Smaller Histories

New simulations suggest that the Milky Way may have formed through the gradual contribution of thousands of smaller galaxies and early galactic systems. These building blocks supplied stars, gas, dark matter and chemical elements, while gravitational interactions shaped the galaxy’s disk, bulge and halo.

The result is not a proven, complete reconstruction of the Milky Way’s past. It is a model that must be tested against stellar motions, chemical compositions, tidal streams, satellite galaxies and the structure of the dark matter halo.

The broader significance is clear: the Milky Way may be less a single object than a collection of many histories. Its spiral form is the visible outcome of billions of years of hierarchical growth, repeated mergers and ongoing evolution.

Frequently Asked Questions

Did the Milky Way really form from thousands of galaxies?

New simulations suggest that thousands of smaller galaxies may have contributed to the Milky Way’s growth over time. The number is model-based rather than a directly observed count. Many systems may have been dwarf galaxies or early galactic building blocks rather than large, modern-style galaxies.

How did the Milky Way form?

The Milky Way formed gradually as gas, stars and dark matter collected under gravity. Smaller galaxies and protogalactic systems merged with one another and with the growing galaxy. Continued gas accretion and star formation helped create its disk, bulge and halo.

What evidence shows that smaller galaxies merged with the Milky Way?

Evidence includes ancient stars in the stellar halo, tidal streams, surviving satellite galaxies and groups of stars with shared motions or chemical compositions. Together, these clues can reveal material acquired during earlier mergers.

Are the simulations proof of the Milky Way’s exact history?

No. Simulations test possible formation histories by comparing their predictions with observations. The estimate of thousands of contributing galaxies represents a scientific scenario that requires further testing.

What happens when small galaxies merge with a larger galaxy?

Gravity can disrupt the smaller galaxy. Its stars may spread through the larger galaxy’s halo or form tidal streams. Its gas can trigger star formation, while its dark matter becomes part of the larger galaxy’s dark matter halo.

Could the Milky Way still be merging with other galaxies?

Yes. The Milky Way continues to interact with nearby satellite galaxies. Some are gradually being disrupted or absorbed, providing a nearby example of the hierarchical growth process that may have shaped the galaxy during its early history.