What is an 'N-body simulation'?

An N-body simulation is a powerful computer model used in astrophysics to simulate the motion of particles under the influence of gravity. In this case, 'N' represents millions of particles that stand in for stars, gas, and dark matter, allowing us to model how entire galaxies interact.

Will the Earth and Sun be destroyed in this collision?

It is extremely unlikely. While the galaxies will collide, the space between individual stars is vast. The simulation shows that our solar system will almost certainly not collide with another star. Instead, it will be gravitationally flung into a new, much larger orbit within the newly formed 'Milkomeda' galaxy.

What is a 'starburst' event?

A starburst is an intense period of rapid star formation. When the gas clouds of the Milky Way and Andromeda collide and are compressed during the merger, it will trigger a spectacular burst of new star birth, briefly making the galactic center shine incredibly brightly.

Why will the final galaxy be an 'elliptical' galaxy and not a spiral?

The chaotic gravitational interactions during the merger will disrupt the orderly, rotating disk structures of both the Milky Way and Andromeda. The stars will be scattered into more random orbits, forming a large, spheroidal, or 'elliptical,' galaxy, which is the common outcome of major galaxy mergers.

Simulating the Milky Way-Andromeda Galaxy Collision

The Local Group of galaxies is dominated by two massive spirals: our own Milky Way and the Andromeda Galaxy (M31). Spectroscopic measurements have confirmed that these two giants are on a collision course, destined to merge in several billion years. Using the computational power of the Galactic Collision Simulation (GCS) project, we have constructed the most detailed N-body simulation to date of this future event. This publication outlines the simulation's parameters and presents a detailed timeline of the merger, the formation of the resulting "Milkomeda" galaxy, and the ultimate fate of our Solar System.

Introduction: The Inevitable Collision

Decades of observational data have precisely measured the radial velocity of M31, revealing a blueshift that indicates it is moving towards our Milky Way at approximately 110 kilometers per second. While its tangential velocity is still subject to some uncertainty, all credible models show that a collision and subsequent merger are unavoidable. This future event provides a unique, local test case for the hierarchical model of galaxy formation. Modeling it is crucial for understanding the evolution of our cosmic neighborhood.

The Simulation Framework: Building a Virtual Local Group

Our simulation was run on a state-of-the-art supercomputer, employing a modified GADGET-2 code. We modeled the Milky Way and Andromeda as multi-component systems, each containing a central bulge, a stellar disk, a gas disk, and a massive dark matter halo. The simulation utilized several million particles to represent these components, with mass resolutions allowing for the detailed tracking of large-scale structural changes. The initial positions, velocities, and mass distributions were constrained by the latest observational data for both galaxies. The simulation was run forward for 10 billion years (Gyr) to capture the entire merger process.

Simulation Results: A Timeline of the Merger

The simulation reveals a dynamic and visually spectacular multi-stage merger process.

The First Pass: A Gravitational Dance (4 Billion Years)

At approximately 4 Gyr from the present, the two galaxies will make their first close pass. They will not collide head-on but will interpenetrate. The immense tidal forces will warp both galactic disks, pulling out long, spectacular tidal tails of stars and gas into intergalactic space. The night sky from a future Earth would be a breathtaking sight, with the distorted disk of Andromeda dominating the view.
The Main Merger and Starburst Event (6 Billion Years)

After the first pass, the galaxies will separate before gravity pulls them back together for a final, decisive collision. At approximately 6 Gyr, the cores of the Milky Way and Andromeda will merge. This event will cause a massive compression of the interstellar gas clouds from both galaxies, triggering an immense starburst. The rate of star formation will increase dramatically, and the newly-formed galactic center will shine with the light of billions of new, hot, blue stars.
The Birth of 'Milkomeda': A Giant Elliptical Galaxy (10 Billion Years)

Over the next several billion years, the chaotic motions will subside. The tidal tails will fall back and be absorbed, and the stars from both galaxies will settle into new, randomized orbits. The beautiful spiral structures of the parent galaxies will be completely erased. By 10 Gyr from now, the merger will be complete, resulting in the formation of a single, massive, and relatively featureless elliptical galaxy, which has been nicknamed 'Milkomeda'.

The Fate of the Solar System

A key question is what happens to our own Sun and Earth. The simulation shows that despite the galactic chaos, the vast distances between individual stars make a direct stellar collision highly improbable. Instead, our Solar System will experience a gravitational relocation. Our analysis indicates there is a high probability that the Sun will be flung from its current orbit into the extended halo of the new Milkomeda galaxy. While life on Earth will have long since ended due to the Sun's natural evolution into a red giant, any future observers would see a night sky filled with a dense, uniform starfield, very different from the familiar band of the Milky Way we see today.

Conclusion: A Glimpse into Our Cosmic Destiny

This high-resolution simulation provides our clearest glimpse yet into the long-term future of the Local Group. It demonstrates the power of the hierarchical formation model and provides a detailed, physically-grounded prediction for the evolution of our own galactic neighborhood. The merger of the Milky Way and Andromeda represents the most significant future event in our galaxy's history, a dramatic and creative act of cosmic evolution that will ultimately give birth to a new, grander stellar system.