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 containing 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. These galaxies are gravitationally interacting; that is, they are exerting gravitational pull on each other.

Sometimes they actually collide, forming new galaxies. This interact and collision activity is, in fact, what helped build galaxies up throughout the history of the universe. 

Galaxy Interactions

Large galaxies, like the Milky Way and Andromeda galaxies, cab have smaller satellites orbiting close by. These are usually classified as dwarf galaxies, which have some of the characteristics of larger galaxies, but are on a much smaller scale and can be irregularly shaped. 

In the case of the Milky Way, its satellites, called the Large and Small Magellanic Clouds, are likely being pulled toward our galaxy due to its immense gravity. The shapes of Magellanic clouds have been distorted, causing them to appear irregular.

The Milky Way has other dwarf companions, many of which are being absorbed into the current system of stars, gas and dust that orbit the galactic center.

Galaxy Mergers

Occasionally, large galaxies can collide, creating new larger galaxies in the process.

Often what happens is that two large spiral galaxies will collide 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 kind of galaxy known as an elliptical. Occasionally, depending on the relative sizes of the merging galaxies, an irregular or peculiar galaxy is the result of the merger.

Interestingly, the merger of two galaxies often does not have a direct effect on most of the stars located throughout the individual galaxies. This is because most of what is contained in a galaxy is void of stars and planets, and is composed of primarily gas and dust (if any).

However, galaxies that contain a large amount of gas and enter a period of rapid star formation, greatly exceeding the average rate of star formation of either progenitor galaxy. Such a merged system is known as a starburst galaxy; aptly named for the large number of stars and are created in a short amount of time.

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.

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 as it, and the Milky Way, become warped due to the immense gravitational pull they will have on each 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.

What Happens 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, most of the stars should find new orbits around the combined galactic center.

In fact, 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; at which point it will engulf the Earth.

Life, it seems, will have died out long before the merger completes itself, as the increased radiation of the Sun will have irreparably damaged our atmosphere long as the Sun begins its own descent into old age in about 4 or so billion years.

Edited and updated by Carolyn Collins Petersen.