The Milky Way and Andromeda Galaxies Are on A Collision Course

Andromeda and the Milky Way colliding, as seen from the surface of a planet inside our galaxy.
Andromeda and the Milky Way colliding, as seen from the surface of a planet inside our galaxy. Credit: NASA; ESA; Z. Levay and R. van der Marel, STScI; T. Hallas; and A. Mellinger

It sounds almost like something out of a science-fiction movie: two giant barred spiral galaxies on a collision course with each other. In a movie, there would be aliens and planets crashing together in mighty cataclysm. In reality, however, these cosmic collisions provide a different kind of excitement as they mingle stars for an orbital dance that could take millions of years. 

As it turns out, our own galaxy, the Milky Way Galaxy, is involved in collisions right now. It's currently cannibalizing tiny dwarf galaxies that happen to stray into its path. However, there's a much bigger orbital event in the far future: the meeting and mingling of the Milky Way and the  Andromeda galaxies. It's a future fate that none of us will live to see, but thousands of generations from now, our umpty-ump great-great-great-great grandchildren will live through the titanic experience. And, they will experience a process that has occurred for billions of years as other galaxies have merged to form ever-larger ones! .

The Galactic Collision Course

Scientists have long suspected that this merger will happen. In recent years, astronomers have used Hubble Space Telescope to confirm that the two are on a collision course. And, as part of galaxy studies, they have observed many other such collisions across the universe. That's in addition to some very detailed studies of the Andromeda Galaxy itself (by Hubble), that show us a lot of detail in its spiral arms and core. This might not be the first (or the last) merger Andromeda will experience. 

When Will Our Galaxies Merge?

Given their current velocity and direction through space, the two galaxies will meet in about 4 billion years. In about 3.75 billion years, they will have gotten close enough together that the Andromeda galaxy will virtually fill the night sky. The Milky Way will be visibly warped by the gravitational pull of the approaching galaxy.

The result of the collision and cannibalization will create a giant elliptical galaxy containing billions of stars. In fact, researchers hypothesize that all giant elliptical galaxies are the result of mergers of spiral galaxies (or in this case, barred spiral galaxies). So, such a galactic dance may be part of the cosmic scheme of things, just as is the practice of smaller galaxies getting eaten up by bigger ones.

Not Just Andromeda Will be in the Dance

As it turns out, another galaxy or two might get into the act. The nearby Triangulum Galaxy is the third largest galaxy (behind the Milky Way and Andromeda) in our Local Group. That's a group of at least 54 galaxies that gravitationally interact in this region of the universe. Triangulum Galaxy is actually a satellite of Andromeda. Since it's bound to its neighbor by mutual gravity there's a pretty good chance that it will also get dragged into the Milky Way first. It is more likely, however, that the Triangulum will be absorbed by the Andromeda/Milky Way merged galaxy at some later point.

The More the Merrier

In addition to the Triangulum galaxies, it also turns out that two more galaxies, the Magellanic clouds, could become part of our home galaxy as well. The difference, really, is only the scale of the galaxy we are merging with, and Andromeda is quite large and massive. The Magellanics and the other dwarf galaxies are relatively small in comparison. Still, the combination of several galaxies merging in a billion-year-spree is tantalizing. Even more fascinating is what this new galaxy will be called. Some researchers have suggested "Milkdromeda." 

Effects on Human (or Alien) Life Forms

The effects of a giant galaxy merger on our little bitty solar system are not entirely clear. Much of what happens to our far-flung galactic neighborhood depends on how the Milky Way and Andromeda collide. It's possible there will be little effect on us and our home world. Space is big and there's a LOT of distance between objects. It's also possible that things could get very interesting for our descendants in the far future as the galaxies spiral through their lengthy gravitational dance. 

Simply because the Milky Way is merging with another galaxy does not mean that the planetary systems within it are in much danger. In fact, the Milky Way is currently absorbing three other, much smaller galaxies and so far, there's been no evidence of planets being affected. However, the jury's still out, since planets are tough to detect from a distance. Most of the galaxies being "eaten up" likely have few (if any planets), since they are metal-poor (and planets need heavier elements to form). 

The most likely scenario is that our solar system, or perhaps just our planets, will be flung into some new part of the new galaxy by the gravitational interactions. However, because of the relatively large distance between stars in the galaxies (and the fact that we are nowhere near the galactic center), it's unlikely that there would be some catastrophic collision between our Sun (or Earth) and some other object. The Sun, however, will find a new orbit around the core of the newly formed galaxy. Some scenarios suggest that the Sun and Earth could get flung out of the galaxy altogether, to wander the depths of intergalactic space. It's not a very comforting thought, although there's not much anybody can do to change things.

Living in a New Galaxy

As for life? Well, we (meaning the Sun and Earth) certainly will not be here anymore. As the Sun's luminosity continues to increase over time, just a part of the stellar evolution process, eventually any life on Earth will be snuffed out as the Sun dies. It may well be on its way to its final death throes before the galaxy collision gets well underway. So, humans may not have much of a planet to worry about, and may well have decamped for another planet somewhere in a safer part of the galaxy.

In theory, however, any life forms in the two merging galaxies should be able to survive just as long as their solar systems remain relatively intact, which is a very reasonable possibility.

Key Takeaways

  • The Milky Way Galaxy and the Andromeda Galaxy are on a collision course. Their projected merger could start occurring in just a few billion years.
  • Other galaxies, such as the Triangulum Galaxy, plus the Magellanic Cloud irregular galaxies, could also be involved.
  • When galaxies mingle, the gravitational actions may affect the Sun and planets. 
  • The Milky Way is currently swallowing up the stars from smaller, dwarf galaxies. 

Edited and updated by Carolyn Collins Petersen.