When it comes to the Milky Way and Andromeda galaxies, there’s a whole “will they or won’t they” thing going on. In 2012 scientists published their results of Hubble Space Telescope observations examining the motion of Andromeda, the closest large spiral galaxy to our own. They found that, within the observational uncertainties at the time, Andromeda was essentially heading straight for us and would collide with our galaxy in approximately four billion years.
Subsequent studies have cast doubt on this supposedly inevitable smashup. Some showed a clean miss, and others showed a collision after much more time. The latest research, which includes the trajectory-tweaking gravitational effects of several satellite galaxies, indicates the odds of a collision are 50–50—a coin toss.
In a purely pragmatic sense, you shouldn’t lose sleep over an impending collision with Andromeda because this new study suggests it won’t happen (if at all) for another eight billion years or so. But assuming it does occur all those eons in the future, would residents of either galaxy have anything to worry about? This depends, of course, on how exactly such a galactic train wreck unfolds.
On supporting science journalism
If you’re enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.
A Milky Way–Andromeda collision would see these two titans crashing together at roughly a million kilometers per hour. That’s incredibly fast on human scales but much less so on a cosmic one. The Milky Way and Andromeda each boast a flattened stellar disk well more than 100,000 light-years, or a quintillion kilometers, across, so a million-kilometer-per-hour collision can take hundreds of millions of years to unfold. And even then, the aftermath will still resonate within the newly merged galaxy for billions of years to come.
Andromeda has a mass that is 1.5 trillion times that of the sun, and our Milky Way’s mass is about 800 billion times that of our home star. That is a lot of mass, which means the gravitational attraction between the two galaxies is huge. But importantly, both galaxies are quite large, which means the gravitational effects are more than just a simple attraction.
Let’s imagine a point at which the approaching two galaxies, edge to edge, are separated by 120,000 light-years—the approximate diameter of the Milky Way’s disk. That would be the distance between the two closest edges of each disk. But the far side of the Milky Way’s disk would be twice that distance from Andromeda, well more than 200,000 light-years. Gravity weakens with the square of the distance between the two, so a star that was on the far side of the Milky Way from the edge-on collision point would feel a much less powerful gravitational attraction than a star on the near side would. This change in gravity with distance is called the tidal force.
The star on the near side would be pulled toward Andromeda much harder than one at the center of the Milky Way, which would itself be pulled harder than the star on the far side. This would have the effect of stretching the Milky Way. As the two galaxies converged, each would pull the other apart like taffy, creating long tendrils of stars, gas and dust called tidal tails.
All this, you might think, would be a prelude to the main event of the two galaxies’ respective disks slamming together, which surely would involve a terrifying amount of damage, like a head-on collision between two 16-wheeler freight trucks. But galaxies aren’t like trucks; they aren’t solid bodies at all! Because of this, they can pass right through each other like ghosts in the night, their mutual gravity gradually drawing them back together in a series of collisions that can result in the two galaxies merging into one. And even in scenarios where two such galaxies avoid a direct collision, they can still swing around each other in a complex gravitational dance, slinging out curving tidal tails that, despite their cosmic violence, are astonishingly lovely. These gentler, more glancing interactions often result in a merger, too.
But just because galaxies can pass through each other doesn’t mean a collision will have no negative effects. Planetary systems caught up in a tidal tail could be ejected from their host galaxy entirely, for example, although this change of location would be relatively slow and mostly harmless. A bigger concern could be collisions between stars, which aren’t as ghostly as galaxies. But the chances for this are astronomically low—at least for our neighborhood in the Milky Way. A typical star is very roughly a million kilometers across. The distance between stars around the sun’s location averages about four light-years, or approximately 40 trillion kilometers. So an average star in our vicinity is a forty-millionth the size of its separation from its nearest neighbor. That’s a mighty small target and one very unlikely to hit.
The likelihood of stellar collisions ramps up toward the centers of galaxies, where millions of stars can be packed in the same volume of space we enjoy out here in the galactic suburbs. And when stars collide, the typical result are messy celestial fireworks you wouldn’t want erupting in your backyard. The eye-catching stellar system of V838 Monocerotis is an example of such an event.
And while stars are small, the sprawling clouds of gas and dust from which they’re born are not. These can be hundreds of light-years across, making collisions not only inevitable but common during a galactic merger, potentially sparking bursts of star formation. The intense radiance from dozens or even hundreds of massive newborn stars may be beautiful from afar but can cause all sorts of trouble for “local” observers.
But the most worrisome aspect of any collision between the Milky Way and Andromeda is the supermassive black hole that lurks at the center of each. Located at the core of the Milky Way, the black hole Sagittarius A* is about four million times the mass of the sun. And in the heart of Andromeda, also called Messier 31 (M31), the black hole M31* is more like 140 million times our home star’s mass. During a collision, gas clouds could be thrown down toward each galaxy’s center and ultimately fall into decaying orbits around each awaiting black hole, forming huge disks there that would become extremely hot and potentially extremely bright. If so, both galaxies could become “ active,” blasting out tremendously hazardous high-energy radiation that would make the emission from mere bursts of star formation seem like a mild sunburn.
Even worse, a few billion years after the collision, the two monster black holes could themselves merge. If the black holes were to do so, they’d send out a blast of gravitational waves so energetic that they could be about as powerful as all the stars in the visible universe combined. Because this energy wouldn’t manifest as electromagnetic radiation but rather the wobbling of spacetime itself, it’s hard to gauge what, if any, effects the unification of two gargantuan black holes would have on surrounding stars and planets. Out of caution, though, I’d advise against watching from the front row.
The “good” news is that the sun and Earth will be long gone by then: by eight billion years from now, our star will have swollen into a red giant, cooked our planet and then shrunk to a brilliant but tiny white dwarf. We’ll miss all the fireworks.
In a sense, that’s too bad because seeing a galactic collision from the inside would be an astronomer’s dream! But eight billion years or so is a long time to wait. For now, we’ll have to be happy with watching other, more distant galaxies and using them to better understand what will happen to our own in the coming eons, when it will get to know its nearest spiral neighbor a whole lot better.