What Happens When Galaxies Collide

The universe is filled with motion. Moons orbit planets, which orbit stars, which revolve or move in regard to other stars within a galaxy. Even entire galaxies are moving at phenomenal speeds. The sizes and distances involved are so tremendous that it isn’t often noticeable to the human eye, but they still do move and gravity has a lot to do with the movement. All of this also means that galaxies sometimes collide. It is interesting to look at what happens when this occurs.

A galaxy is a vast collection of stars held together by gravity. They vary by type, shape, and size. 

There are four types of galaxies; spiral, barred-spiral, elliptical, and irregular. The Milky Way, our galaxy, is a spiral galaxy, though some astronomers class it as a barred-spiral. Elliptical galaxies vary from being shaped rather like a football to being almost spherical. The motion of the stars in an elliptical galaxy is quite complex, rather than rotating around a common center, as seen in spiral galaxies.

Our galaxy is about 100,000 light-years in diameter and it contains an estimated 100 billion to 400 billion stars.

One of the best known ‘nearby’ galaxies, Andromeda, is only 2 million light-years away. Andromeda is a spiral galaxy that is about half again larger than the Milky Way. There are two other galaxies that are closer; the Large and Small Megalenic clouds; 165,000 and 195,000 light-years away, respectively. These are irregular galaxies that are substantially smaller than the Milky Way. 

The distance to the Megelenic clouds is great, but it is close enough that we can see the effects the gravity of the galaxies has on each other and how the Milky Way affects them both. This is a matter of more than just passing curiosity because Andromeda and the Milky Way are racing toward each other and will almost certainly collide in about 4.5 billion years.

So what happens when large spiral galaxies collide? This is where it gets interesting. We might be tempted to think of galaxies as more or less solid objects. They are far from it. Despite the number of stars, the star density (number of stars per light-year) is quite low. This means that most of a spiral galaxy is space.

If a spiral galaxy smashes into another spiral galaxy, the two galaxies can pass right through one another without a single star colliding with another, if the only thing that is considered is the space that is involved. However, that isn’t the only thing that is involved. There is also gravity, which not only holds a galaxy together around the center but also attracts the two galaxies to each other. 

The amount of gravity exerted by several hundred billion stars in one galaxy on several hundred billion stars in another galaxy is enormous. It is easily enough to pull stars this way and that. This does more than increasing the chance that stars will smash into one another. It also disrupts the motion of the stars in each galaxy. The pull of the Milky Way is already doing this to the Megalinic Clouds, spawning the birth of new stars in the process.

So what does this mean? It means that in roughly the amount of time that the sun has been around, Andromeda and the Milky Way are due to collide. When they do, both galaxies will cease being spiral galaxies and will merge to become an elliptical galaxy. When that happens, there will be an enormous period of new stars being born.

Of course, it isn’t anything to worry about because none of us will be alive in 4.5 billion years. However, we can see this happening in distant galaxies. Some astronomers believe that this is how all elliptical galaxies formed.