Interacting Galaxies Have Interesting Results

Galaxy Mergers and Collisions

Eyes in the Sky
Two galaxes are merging together in this view from the Spitzer Space Telescope. The colors indicate where clouds of gas and dust and starbirth regions exist in the galaxies. NASA/JPL-Caltech/STScI/Vassar

Galaxies are the largest single objects in the universe. Each one contains upwards of trillions of stars in a single gravitationally bound system. While the universe is extremely large, and many galaxies are very far apart, it is actually quite common for galaxies to group together in clusters. It's also common for them to collide with each other. The result is the creation of new galaxies. Astronomers can trace the construction of galaxies as they collided throughout history, and now know that this is the main way galaxies are built.  

There's a whole area of astronomy devoted to the study of colliding galaxies. The process not only affects the galaxies themselves, but astronomers also observe that starbirth is often triggered when galaxies merge together. 

Galaxy Interactions

Large galaxies, like the Milky Way and Andromeda Galaxy, came together as smaller objects collided and merged. Today, astronomers see smaller satellites orbiting nearby both the Milky Way and Andromeda. These "dwarf galaxies" have some of the characteristics of larger galaxies, but are on a much smaller scale and can be irregularly shaped.  Some of the companions are being cannibalized by our galaxy. 

The Milky Way's largest satellites are called the Large and Small Magellanic Clouds. They seem to be orbiting our galaxy in a billions-of-years-long orbit, and may not actually ever merge with the Milky Way. However, they are affected by its gravitational pull, and may only be approaching the galaxy for the first time. If so, there still could be a merger in the distant figure. The shapes of Magellanic clouds have been distorted by that, causing them to appear irregular. There's also evidence of large streams of gas being pulled from them into our own galaxy. 

Galaxy Mergers

Large-galaxy collisions do occur, which creates huge new galaxies in the process. Often what happens is that two large spiral galaxies will merge, and due to the gravitational warping that precedes the collision, the galaxies will lose their spiral structure. Once the galaxies are merged, astronomers suspect that they form a new structure known as an elliptical galaxy. Occasionally, depending on the relative sizes of the merging galaxies, an irregular or peculiar galaxy is a result of the merger.

Interestingly, while galaxies themselves may merge, the process doesn't always hurt the stars they contain. This is because while galaxies do have stars and planets, there's a LOT of empty space, as well as giant clouds of gas and dust. However, colliding galaxies that do contain a large amount of gas enter a period of rapid star formation. It's usually much greater than the average rate of star formation in a non-colliding galaxy. Such a merged system is known as a starburst galaxy; aptly named for a large number of stars that are created in a short amount of time as a result of the collision.

Merger of the Milky Way with the Andromeda Galaxy

A "close to home" example of a large galaxy merger is the one that will occur between the Andromeda galaxy with our very own Milky Way. The result, which will take millions of years to unfold, will be a new galaxy. 

Currently, Andromeda is about 2.5 million light-years away from the Milky Way. That's about 25 times as far away as the Milky Way is wide. This is, obviously quite a distance, but is quite small considering the scale of the universe. Hubble Space Telescope data suggests that the Andromeda galaxy is on a collision course with the Milky Way, and the two will begin to merge in about 4 billion years.

Here's how it will play out. In about 3.75 billion years, the Andromeda galaxy will virtually fill the night sky. At the same time, it and the Milky Way will start warping due to the immense gravitational pull each will have on the other. Ultimately the two will combine to form a single, large elliptical galaxy. It is also possible that another galaxy, called the Triangulum galaxy, which currently orbits Andromeda, will also participate in the merger.  The resulting galaxy might be named "Milkdromeda", if anybody is still around to be naming objects in the sky. 

What Will Happen to Earth?

Chances are that the merger will have little effect on our solar system. Since most of Andromeda is empty space, gas, and dust, much like the Milky Way, many of the stars should find new orbits around the combined galactic center. That center may have as many as three supermassive black holes until they, too, merge. 

The greater danger to our solar system is the increasing brightness of our Sun, which will eventually exhaust its hydrogen fuel and evolve into a red giant. That will start to happen in about four billion years.  At that point, it will engulf Earth as it expands. Life, it seems, will have died out long before any kind of galaxy merger takes place. Or, if we're lucky, our descendants will have figured out a way to escape the solar system and find a world with a younger star. 

Edited and updated by Carolyn Collins Petersen.