What Happens When Galaxies Get Too Close?

Located in the constellation Lepus (the Hare), galaxy IRAS 06076-2139 is actually two galaxies speeding past each other in space. ESA/Hubble & NASA

About 500 million light-years from us, two galaxies are zooming by one another at a speed of 1,243,000 miles per hour.

Counted as a single celestial object, galaxy IRAS 06076-2139 — located in the constellation Lepus — is a stunning but not uncommon sight for the Hubble Space Telescope. Per the European Space Agency or ESA, the telescope is often seeking out galactic interactions as astronomers are always keen to see the influence that galaxies have on one another.

The two galaxies that make up IRAS 06076-2139, for instance, are about 20,000 light-years apart, but that's close enough that each galaxy "will distort one another through the force of gravity while passing each other, changing their structures on a grand scale."

The ESA doesn't go into what those "grand scale" changes are, but since astronomers have observed galaxies interacting for a while now, they've developed a classification system for them. The answer may lie in there.

Galaxy harassment

According to Swinburne University, galaxy harassment — what a vaguely outer space, human resources-esque word — is the sort of galactic interaction that involves "fly-bys" among galaxies. Harassment is more likely to occur in galaxy clusters where galaxies are constantly in motion near one another.

This sort of near-hit-and-run can still have sizable impacts on the galaxies involved, even if there's not an actual collision between them. When the term was first coined in 1996, scientists explained that gravitational forces can cause the galaxies to change their shape, or even lose it entirely, becoming a " featureless ellipse." Other events, like stars and gases being pulled out from the edges of a galaxy to form a sort of galactic tail.

While we can't know the possible effects of the fly-by happening between the galaxies of IRAS 06076-2139 just yet, it will probably qualify as galaxy harassment since the two bodies aren't crashing into each other.

Galaxy cannibalism

Galaxies start out small, collecting interstellar gas and dust with which to make new stars. But sometimes these young, small galaxies don't grow to a ripe old age. Instead, the gravitational forces of larger galaxies tug at passing smaller galaxies, pulling all those precious star-making materials into it. The end result is that the tiny galaxy gets stretched out, like taffy.

Our own Milky Way is guilty of munching on satellite galaxies. According to Aaron Robotham of the University of Western Australia, "The Milky Way hasn't merged with another large galaxy for a long time but you can still see remnants of all the old galaxies we've cannibalized," he explained to Motherboard. He suggests that our home galaxy's appetite has yet to be satisfied as well. It may end up eating "two nearby dwarf galaxies, the Large and Small Magellanic Clouds, in about 4 billion years."

Galaxy collision and merger

The Mice Galaxies in the process of merging
The Mice Galaxies are two spiral galaxies that have been in the process of merging 290 million years ago. NASA, H. Ford (JHU

When it's more than a fly-by or just a celestial snack, you end up with galaxies crashing into one another. Rather, it's more accurate to say that the galaxies' respective gravitational fields crash into each other. Galaxies, as the website for the Herschel Space Observatory explains, have stars, gas, dust and dark matter swirling about, creating and influencing its gravitational fields. So when those fields get too close to one another, what you end up with is more a tug of war between different forces of gravity than a physical collision between stars and planets. Like with galaxy harassment, galaxy collision can result in stars and gases getting pushed around and out of their original places. Shock waves created by the collision can drive interstellar dust and gas to compress and the result is a starburst, or the creation of many new stars.

Collisions don't always result in two galaxies becoming a single one. Sometimes the galaxies have enough speed that they collide and then just continue traveling through the cosmos, a little dented and different, but they keep on trucking as it were. But if they don't have enough speed, the galaxies begin the very slow — we're talking potentially billions and billions of years — process of merging into a single galaxy.

Galactic collisions and mergers are very common within the universe. Even our own Milky Way is suspected of having merged with another galaxy in the past.

The Milky Way's fate

The Andromeda galaxy
The Andromeda galaxy is coming for us. NASA/JPL-Caltech/Wikimedia Commons

Lest you think we're all safe from a galactic collision, the Andromeda galaxy, our closest major galactic neighbor, is on a direct path to collide with us.

Of course, it's going to take about 4 or 5 billion years for this to collision to occur.

And it's projected to be a slow process. Scientists told Smithsonian magazine that the Milky Way and Andromeda will likely just orbit each other at first. That orbiting will slow down both galaxies' movements and create the conditions for the two to merge to form a "three-dimensional ball of stars" instead of the spiral discs we're so used to.

This merger likely won't have a huge impact on our solar system. Stars and planets from Andromeda aren't going to start bouncing around in our solar system; indeed, bodies from Andromeda aren't even likely to get close to Earth. What will happen, as has been outlined above, is that the solar system will likely be pushed to a new place within this ball of a galaxy. The result is that our descendants will have a whole new patch of the universe to chart.