What happens if the Milky Way collides with Andromeda? Will the Earth survive?

Imagine you somehow wake up after 4.5 billion years. When your alarm goes off at 5 am, you step outside because you have made a promise to yourself to get some fresh air first thing in the morning. The air is still cool, so you wrap your jacket tighter. The world has not yet shaken off the weight of sleep. Suddenly, you notice a strange light and look up. The sky looks otherworldly, as if it were a painting. It is no longer the quiet, empty expanse you remember, but a vast, luminous canvas. There are streaks like flowing rivers, soft and filled with stars. It feels almost hypnotic. You also notice faint glowing bands and arcs stretching into the horizon. It is certainly not a sky you can take in all at once, but something you would slowly soak in.

This is only true if you are still alive in a world that exists about 4.5 billion years from now. In approximately 4.5 billion years, the night sky above Earth will undergo a transformation unlike anything in human experience. Our Milky Way’s neighbour, the massive Andromeda Galaxy, is approaching at ~250,000 mph. Over the course of the next 2 billion years, the galaxies will collide in a gravitational sumo match that will ultimately bind them forever. But the bigger question is: will the Earth survive this? Let’s find out.

The Milky Way–Andromeda collision

The Milky Way galaxy is a large spiral galaxy containing an estimated 100 to 400 billion stars. Our solar system lies about 26,000 light-years from the galactic centre, in a smaller spiral region called the Orion Arm. But our galaxy is not alone. The neighbouring massive galaxy, the Andromeda Galaxy (M31), in the Local Group of galaxies, is moving towards us. It contains about 1 trillion stars and is more or less the same mass as the Milky Way — about 800 billion times the mass of the Sun. It spans approximately 220,000 light-years across, making it the largest galaxy by diameter in our local universe. Just like the Milky Way, it is a spiral galaxy with a supermassive black hole (about 100 million solar masses). Andromeda lies approximately 2.54 million light-years from the Milky Way, perhaps the most distant thing you can see with your naked eye in the night sky. This proximity is crucial because most galaxies are far more distant.

As early as 1912, astronomers knew that Andromeda was headed our way. Andromeda is moving towards us at approximately 110 kilometres per second (relative to the Local Group’s centre of mass). The two galaxies will approach each other, eventually becoming ‘Milkomeda’, as it is now nicknamed.

A century later, with the help of NASA’s Hubble Space Telescope, scientists were able to measure how Andromeda was moving. They discovered that the sideways motion was negligible and that it is mostly coming straight at us. So a head-on collision seemed bound to happen. However, it would not be a violent smash as one might imagine, but a slow merger that would result in a spheroidal collection of stars known as an elliptical galaxy. Because of this, new stars, explosions (supernovae), and possibly even a shift in the Sun’s path were expected. Scientists thought the collision was essentially inevitable, as certain as ‘death and taxes’.

Not inevitable, though, new study

What was thought to be inevitable may not actually be. Scientists from Helsinki, Durham and Toulouse universities used data from Hubble and the European Space Agency’s (ESA) Gaia space telescope to simulate how the Milky Way and Andromeda will evolve over the next 10 billion years. They ran 100,000 simulations of both galaxies based on the latest observational data and re-examined the long-held prediction of a collision. In findings shared in the Nature Astronomy journal, scientists found that there is approximately a 50–50 chance of the two galaxies colliding within the next 10 billion years, contrary to previous belief. Although the new research challenges the previously accepted fate of our galaxy, the study authors say it is very difficult to make a precise prediction.

“The Universe is a dynamic place, constantly evolving. We see external galaxies often colliding and merging with other galaxies, sometimes producing the equivalent of cosmic fireworks when gas, driven to the centre of the merger remnant, feeds a central black hole, emitting an enormous amount of radiation before irrevocably falling into the hole. Until now, we thought this was the fate that awaited our Milky Way galaxy,” leading cosmologist Professor Carlos Frenk of Durham University said in a statement.

“Although its mass is only around 15% of the Milky Way’s, its gravitational pull, directed perpendicular to the orbit with Andromeda, perturbs the Milky Way’s motion enough to significantly reduce the chance of a merger with the Andromeda galaxy. While earlier studies only considered the most likely value for each variable, we ran many thousands of simulations, which allowed us to account for all the observational uncertainties,” lead author Dr Till Sawala of the University of Helsinki said in a statement.

The study’s co-author, Professor Alis Deason of Durham University’s Institute for Computational Cosmology, thinks these results are significant for the fate of our galaxy. “It used to appear destined to merge with Andromeda, forming a colossal ‘Milkomeda’. Now, there is a chance that we could avoid this fate entirely,” she said.

Will Earth survive?

The new study suggests there is only a small probability, about 2%, of a head-on collision between the galaxies in the next five billion years. Even if the Milky Way and Andromeda do eventually collide and merge, the solar system will be pushed much farther from the centre of the galaxy, to around three times its current position. Right now, our solar system sits in a small branch called the Orion Spur. So even if we were flung further out, we would end up at the very outer edge of this arm.

However, by the time the two galaxies collide, the surface of Earth will have become too hot for liquid water to exist. Earth’s real threat is the Sun. In about a billion years, the Sun’s luminosity will gradually increase, ending all terrestrial life. By 5 billion years, the Sun will expand into a red giant and burn out, likely engulfing Earth. Humankind will have disappeared billions of years before that. “We now know that there is a very good chance that we may avoid that scary destiny,” Professor Frenk added. Even if the idea of galaxies colliding sounds apocalyptic, the irony is that it is not going to be our biggest concern.