The predicted merger of the Milky Way and Andromeda and the further fate of galaxies in the expanding Universe
All galaxies constantly change over cosmic time: they grow through mergers, gradually evolve under the influence of internal processes, and sometimes inevitably approach collisions with neighboring galaxies. The Milky Way, where we live, is no exception: it moves within the Local Group of Galaxies (LG), and observations show that it is heading toward a collision with its largest satellite – the Andromeda Galaxy (M31). This spectacular merger, also called “Milkomeda,” will fundamentally change our local cosmos in a few billion years. However, even after this event, the rapid expansion of the Universe will determine an even broader story of galaxy isolation and final fate. In this article, we will discuss why and how the Milky Way will collide with Andromeda, the possible consequences of the merger for both galaxies, and the broader long-term future of galaxies in the context of the expanding Universe.
1. Approaching merger: the Milky Way and Andromeda
1.1 Evidence for the collision trajectory
Accurate measurements of Andromeda's motion relative to the Milky Way show that it is in a blueshifted state – approaching us at about 110 km/s. Early radial velocity studies indicated a possible future collision, but Andromeda's transverse velocity remained unclear for a long time. Data from the Hubble Space Telescope and later refinements (including Gaia observations) allowed the determination of Andromeda's proper motion, confirming that in about 4–5 billion years it should collide with our Milky Way [1,2].
1.2 Context of the Local Group of Galaxies
Andromeda (M31) and the Milky Way are the two largest galaxies in the Local Group of Galaxies – a small galaxy cluster with a diameter of about 3 million light-years. The Triangulum Galaxy (M33), orbiting near Andromeda, may also be included in the future collision. Various dwarf galaxies (e.g., the Magellanic Clouds, other satellites) located on the edges of the LG may also experience tidal disturbances or become satellites of the merged system.
1.3 Timescales and collision dynamics
Simulations show that the first collision between Andromeda and the Milky Way will occur in about 4–5 billion years, possibly with several close flybys before the final coalescence ~6–7 billion years in the future. During these approaches:
- Tidal forces will stretch the disk structure, possibly creating tidal tails or ring features.
- Star formation will briefly intensify in regions where gas clouds overlap.
- Black hole “feeding” may intensify in the nuclear regions if gas flows to the center.
Ultimately, these galaxies are expected to merge into a massive elliptical or lenticular galaxy called “Milkomeda,” where the stars of both spirals will combine [3].
2. Possible outcome of the “Milkomeda” merger
2.1 Elliptical or massive spheroidal remnant
Major mergers, especially of two similar-mass spirals, usually destroy disk structures and form a pressure-supported spheroid characteristic of elliptical galaxies. The final appearance of “Milkomeda” will likely depend on:
- Orbital geometry – if the interaction is centrally symmetric, a typical elliptical structure may form.
- Remaining gas amount – if unused or unexpelled gas still exists, a lenticular (S0) galaxy with a faint disk or ring structure may form.
- Dark matter halo – the combined halo of the Milky Way and Andromeda will create the gravitational environment that determines how stars redistribute.
Models examining gas-rich spirals show strong starburst activity during mergers, but after 4–5 billion years, the Milky Way's gas reserves will be more modest, so star formation during the merger may be less intense than in the early Universe [4].
2.2 Central SMJS interaction
The Milky Way's black hole (Sgr A*) and the larger Andromeda black hole may eventually merge due to dynamic friction. In the final moments of the merger, strong gravitational waves could be emitted (though not as intense on a cosmological scale as in other more massive or distant systems). The merged black holes will remain at the center of the new elliptical galaxy, possibly radiating as an AGN for some time if there is enough gas.
2.3 Fate of the Solar System
At the time of the merger, The Sun will be about as old as it is now – to the Universe, approaching the end of the late hydrogen burning phase. The Sun's brightness will increase, making Earth inhospitable to life, despite the galactic collision. Dynamically, the Solar System will most likely remain orbiting the center of the new galaxy (or further out at the edge of the halo), but it is unlikely to be ejected or absorbed by the black hole [5].
3. Other Local Group galaxies and dwarf satellite evolution
3.1 Triangulum galaxy (M33)
M33, the third largest spiral galaxy in the LG, orbits Andromeda and could be involved in the Milkomeda process. Depending on its orbit, M33 may merge with the merged Andromeda–Milky Way system later or be disrupted by tidal forces. This galaxy has quite a lot of gas, so its final merger could trigger a later increase in star formation in the combined system.
3.2 Interactions of dwarf satellites
The LG has dozens of dwarf galaxies (e.g., Magellanic Clouds, Sagittarius dwarf, etc.). Some of them may be disrupted or merge into the Milkomeda system during upcoming mergers. Over billions of years, many small mergers can further grow the stellar halo, thickening the final system. Thus, hierarchical interaction continues even after the main spiral coalescence.
4. Further context of Universe expansion
4.1 Accelerating expansion and galactic isolation
After the formation of Milkomeda, the rapid expansion of the Universe, driven by dark energy, means that galaxies not gravitationally bound recede and eventually it becomes impossible to establish causal contact with them. After tens of billions of years, only the Local Group (or its remnant) will remain gravitationally bound, while all more distant cluster structures will recede faster than light can connect. Eventually, Milkomeda and its satellites will become an "island Universe", separated from other clusters [6].
4.2 Exhaustion of star formation
As cosmic time progresses, gas resources will diminish. Mergers and feedback can heat or remove remaining gas, and the inflow of new gas from cosmic filaments decreases in the late epoch. After hundreds of billions of years, star formation will nearly cease, leaving mostly aging red stars. The final elliptical galaxy will fade, dominated only by dim red stars, white dwarfs, neutron stars, and black holes.
4.3 Dominance of black holes and remnants
After trillions of years, many stars, influenced by gravitational interactions, may be ejected from the Milkomeda halo. Meanwhile, SMBH will remain in the galaxy's core. Eventually, black holes may be the only significant mass concentrations in this bleak cosmic background. Hawking radiation over incredibly long periods could evaporate even black holes, but this lies far beyond usual astrophysical epochs [9, 10].
5. Insights from observations and theoretical analysis
5.1 Monitoring Andromeda's motion
Hubble Space Telescope measured Andromeda's velocities in detail, confirming the collision trajectory with a small lateral component. Additional data from Gaia further refines the orbits of Andromeda and M33, allowing better determination of the approach geometry [7]. Future space astrometric missions may determine the first collision time even more precisely.
5.2 N-body Simulations of the Local Group
Models developed at the NASA Goddard Space Flight Center and elsewhere show that the first collision will begin in about 4–5 billion years, after which M31 and the Milky Way may pass close to each other several times. Eventually, they will merge over a few hundred million years, forming a huge elliptical-like galaxy. Simulations also examine M33's involvement, the tidal tails left behind, and nuclear starburst events [8].
5.3 The Fate of Distant Clusters Beyond the Local Group
Due to cosmic acceleration, distant clusters separate from us – over time, they will exceed our visibility limits. Observations of high-redshift supernovae show that dark energy dominates the Universe's expansion, so on a larger scale the galaxy network will split into isolated "islands." Thus, even if galaxies merge locally, the broader cosmic structure recedes and fades from our view.
6. The Distant Cosmic Future
6.1 The "Degenerate" Era of the Universe
After star formation exhausts, galaxies (or merged systems) gradually enter the "degenerate era", where the main source of population mass is stellar remnants (white dwarfs, neutron stars, black holes). Occasionally, random collisions of brown dwarfs or stellar remnants may briefly revive star formation, but on average the Universe is much dimmer.
6.2 The Final Reign of Black Holes
After hundreds of trillions of years, gravitational interactions may eject many stars from the galaxy halo, while the largest black holes will remain at the centers. Eventually, they may be the only significant mass reservoirs in lonely space. Hawking radiation over unimaginably long timescales could even evaporate these black holes, although this far exceeds normal astrophysical epochs [9, 10].
6.3 Legacy of the Local Group
"In the 'Dark Age,' Milkomeda will likely be the only massive elliptical structure containing the stellar remnants of the Milky Way, Andromeda, M33, and dwarf galaxies. If more distant galaxies/clusters move beyond our cosmological horizon, locally this merged island will remain, gradually sinking into cosmic darkness."
7. Conclusions
The Milky Way and Andromeda are inevitably approaching a galaxy merger – an event that will cause a huge change at the center of the Local Group. In about 4–5 billion years, these two spiral galaxies will begin interacting with tidal distortions, bursts of star formation, and waves of black hole "feeding," until they finally merge into one massive elliptical – "Milkomeda." Smaller galaxies, such as M33, may be drawn into this union, while dwarf satellites will be tidally disrupted or integrated.
Looking further ahead, the expansion of the Universe will separate this new structure from the remaining ones, enclosing it in loneliness, where star formation will eventually cease. Over tens or hundreds of billions of years, only aging stars will remain, until finally only black holes and stellar remnants dominate. However, for the next few billion years, our cosmic corner will remain quite vibrant, and the approaching collision with Andromeda will become the last grand galaxy assembly event in the Local Group.
References and further reading
- van der Marel, R. P., et al. (2012). “The M31 Velocity Vector. III. Future Milky Way–M31–M33 Orbital Evolution, Merging, and Fate of the Sun.” The Astrophysical Journal, 753, 9.
- van der Marel, R. P., & Guhathakurta, P. (2008). “M31 Transverse Velocity and Local Group Mass from Satellite Kinematics.” The Astrophysical Journal, 678, 187–199.
- Cox, T. J., & Loeb, A. (2008). “The Collision Between the Milky Way and Andromeda.” Monthly Notices of the Royal Astronomical Society, 386, 461–474.
- Hopkins, P. F., et al. (2008). “A unified, merger-driven model of the origin of starbursts, quasars, and spheroids.” The Astrophysical Journal Supplement Series, 175, 356–389.
- Sackmann, I.-J., & Boothroyd, A. I. (2003). “Our Sun. III. Present and Future.” The Astrophysical Journal, 583, 1024–1039.
- Riess, A. G., et al. (1998). “Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant.” The Astronomical Journal, 116, 1009–1038.
- Gaia Collaboration (2018). “Gaia Data Release 2. Observational Hertzsprung–Russell diagrams.” Astronomy & Astrophysics, 616, A1.
- Kallivayalil, N., et al. (2013). “Third-epoch Magellanic Cloud proper motions. III. Kinematic history of the Magellanic Clouds and the fate of the Magellanic Stream.” The Astrophysical Journal, 764, 161.
- Adams, F. C., & Laughlin, G. (1997). “A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects.” Reviews of Modern Physics, 69, 337–372.
- Hawking, S. W. (1975). “Particle Creation by Black Holes.” Communications in Mathematical Physics, 43, 199–220.