A cosmic train wreck may be looming in our future.
The Andromeda galaxy, a near twin of our Milky Way, is barreling toward us at a staggering 110 kilometers per second. You can already see it coming, if you know where to look—Andromeda’s great spiral appears as a fuzzy smudge, faint but larger than the full moon in Earth’s late summer and autumn skies. Over the eons, as our neighboring galaxy approaches, it will grow larger in the heavens until it engulfs our entire view—at which point it will physically collide with our galaxy, spawning chaos and flinging stars asunder in the resulting merger and perhaps even ejecting our own solar system into intergalactic space.
Or this might not happen. It’s hard to say.
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.
Although a lot of research indicates a potential collision between these two colossal structures—and certainly there’s been no shortage of reporting on the possibility over the years—the galactic smashup is by no means inevitable. In fact, an international team of European scientists contests the idea. In a (not yet peer-reviewed) research paper posted on the preprint server arXiv.org, the researchers show that when the effects of other nearby galaxies are taken into consideration, the odds of the Milky Way colliding with Andromeda are only about 50 percent. A decent chance, but nowhere near a sure bet.
The Milky Way is flattish disk galaxy that is about 120,000 light-years wide. It contains hundreds of billions of stars, as well as clouds of gas and dust, assorted dead stars and one very large black hole at its center. It’s also surrounded by a huge halo of old stars and invisible dark matter. The total mass of the Milky Way is approximately 1.5 trillion times the mass of the sun. Our galaxy is immense.
Andromeda is much the same, though it’s perhaps 30 percent more massive. The galaxy is located across a vast reach of intergalactic space, about 2.5 million light-years away from us. Andromeda and the Milky Way are the two biggest members of the Local Group, a small clutch of about 100 galaxies, most of which are much smaller and fainter. The next two largest are the Triangulum galaxy (M33), which is about 2.7 million light-years from the Milky Way (and only 700,000 light-years from Andromeda; they lie close together in the night sky), and the Large Magellanic Cloud (LMC), which is a satellite of the Milky Way that is about 160,000 light-years away. These four dominate the Local Group.
In galaxy clusters—large collections of hundreds or thousands of galaxies—we see members colliding fairly often. The gravitational interactions between galaxies can draw them together in a cosmic dance that can take the better part of a billion years to complete. A pair of interacting galaxies usually first make a close pass and then circle back around and slam into each other some time later. The two can merge, forming a single larger object that eventually settles down into a cotton-ball-shaped elliptical galaxy.
That simple summary belies the mind-crushing chaos of such an event, though. A close pass can stretch the pair of galaxies like taffy as each gravitationally grasps at the other, drawing out streams of gas and stars called tidal tails, which can be hundreds of thousands of light-years long. The tidal tails gracefully curve as a result of angular momentum as the two galaxies swing by each other. This process also steals orbital energy from the galaxies, allowing them to slow, drop back toward each other and merge. The pandemonium involved in the collision can drastically change the orbits of stars, hurling some toward the galactic center or flinging them far out into the galactic suburbs.
Worse, the myriad gas clouds orbiting in a galaxy are huge—some are hundreds of light-years wide. They very much can collide, and when they do, they can collapse and form lots of stars, creating huge outbursts of energy. Not only that, but enormous amounts of dust and gas can be dislodged to plummet toward each galaxy’s central supermassive black hole, piling up just outside to form an infernally hot accretion disk. As all that accumulated material approaches its final plunge into the black hole’s maw, it can blast out more energy than all the stars in both galaxies combined.
So despite taking eons to unfold, galactic mergers can still be quite ostentatious occurrences. They’re not usually great places to have a habitable planet.
This naturally raises the question of whether Andromeda and the Milky Way are doomed to merge. Early studies of Andromeda showed that (unlike nearly every other galaxy in the universe) it’s blueshifted, meaning it’s moving through space toward the Milky Way. This motion isn’t subtle either; it’s approaching at a speed of about 110 kilometers per second.
That makes it seem like a collision is inevitable! How could something approaching so fast not score a hit? The answer is that while Andromeda is headed mostly toward us, it may also be moving to the side. If this lateral shift—astronomers call it transverse velocity—is large enough, the galaxy could miss us.
The problem is measuring that transverse motion. Andromeda is a long way away, and its apparent motion across the sky is incredibly small. It was only very recently that astronomers were able to measure this tiny movement at all. The uncertainties are still large, but the overall transverse velocity indicates that Andromeda and the Milky Way will, at the very least, experience a close pass. But how close, we can’t as yet say.
The new research takes that into account. The scientists created computer simulations of the motions of the two galaxies and ran them forward in time to see if a collision occurred. The team took a different approach from others in the past, running the simulations over and over again, changing the input parameters a little bit each time—for example, increasing or decreasing the velocities a tad—to cover the uncertainties in the measured numbers. This approach builds up such statistical models, giving a likelihood of collision. When the researchers did this for just the Milky Way and Andromeda, they found the galaxies collided slightly less than half the time. It’s almost a coin flip: heads means a collision, and tails gives us a close pass but avoids an actual merger—although the interaction will cause a somewhat less disruptive version of the chaos described above.
What’s also new in this study is the inclusion of both M33 and the LMC, both of which are massive enough to gravitationally affect the trajectories of their much larger siblings. Including just the LMC reduces the odds of a crash to only about 30 percent—it orbits the Milky Way, and in most cases, it essentially pulls us to the side just enough to dodge a collision. Adding M33 in as well, however, pulls us back toward Andromeda, once again giving us even odds of an impact.
That’s better than inevitability but perhaps not terribly reassuring. If you prefer to breathe easier, note this tryst won’t occur—if it ever does—for roughly another eight billion years. By then the sun will have evolved past its red giant stage, cooked Earth and shrunken into a white dwarf. That’s cold comfort, I know.
On the other hand, collisions and mergers such as this are how galaxies grow. The Milky Way is a bruiser among galaxies, and we got here by what is essentially galactic cannibalism. And despite the chaos, the future merged object—which some astronomers call Milkomeda—may for a time be rejuvenated, with millions of new stars born in the aftermath. (To be honest, I’m not a fan of the name Milkomeda, but I can’t think of anything better. Andromeway is way worse.)
There are worse fates. And again, this may not happen at all. As time goes on and astronomers make more observations of Andromeda, we’ll get better data, and we’ll know for certain what literally lies ahead.
