Eventually, all galaxies, including our own Milky Way, will come to an end.
But how do galaxies die? If you’re in the mood to destroy an entire galaxy, you have several options depending on the level of destructiveness you want.
Related: Monstrous black holes may have killed their host galaxies in the early universe
Option 1: Awaken the monstrous black hole
At the heart of almost every galaxy is a supermassive black hole. In the case of the Milky Way, we did Sagittarius A*, a beast that weighs more than 4.5 million suns. Most of the time, these giant black holes are silent and dormant, just sipping on the bits of gas or stars that get too close. But occasionally they enjoy a much larger meal. When they do, that gas swirls around them and compresses, reaching temperatures well over a billion degrees.
These ridiculously high temperatures cause the gas to emit an enormous amount of radiation, which then escapes and floods the entire area universe, which heats up any gas reserves and prevents the formation of new stars. While things usually settle down afterwards, in the worst case the radiation from the environment can black hole can eject downright enormous amounts of gas from the Milky Way.
This doesn’t completely destroy a galaxy, but it does effectively kill it by preventing the formation of new stars for a very long time and, in some cases, forever.
Option 2: Place it in a cluster
Galaxy clusters are the dense urban centers of the cosmos, usually home to a thousand or more galaxies. But these clusters contain more than just galaxies; they also contain enormous reservoirs of a hot, thin gas known as the intracluster medium (ICM).
The ICM is so thin that it would register as a vacuum in laboratories on Earth. But when galaxies fall into a cluster, they still have to swim through it. Initially, this leads to a brief round of star formation, with shock waves compressing gas clouds throughout the galaxy. But eventually the pressure of the gas does its work, plucking bits of gas from the Milky Way, like debris flying off a meteorite.
This leads to a fun situation known as “jellyfish systems”, so called because the stripped gas resembles the tentacles of a jellyfish. Although most galaxies survive their descent into the ICM of a cluster, some smaller galaxies become completely vaporized.
Option 3: Crash it into another galaxy
Collisions between galaxies represent one of the largest energy releases in the known universe, which means it’s not exactly a pretty sight. Our own Milky Way will collide with our neighbor galaxy Andromeda in approximately 5 billion years.
A slow and painful process that takes hundreds of millions of years. Merging galaxies can create enormous tidal tails made up of streams of broken-up stars and gas that arc around the galaxies. During the collision and merger, countless stars are lost due to random interactions. And once the respective supermassive black holes meet, the newly merged galaxy is ravaged by another round of radiation. The combined destruction depletes the galaxy’s gas reservoirs, halting star formation forever.
Option 4: Take it to a much larger galaxy
If a smaller galaxy and a much larger companion merge, it could mean the end of the smaller galaxy. Indeed, that of the European Space Agency Gaia Research has discovered the bones and corpses of cannibalized galaxies scattered throughout the Milky Way.
An example of this is known as the Gaia sausage. This collection of stars spread across the core of the Milky Way has properties in common, such as an abundance of heavy elements and orbital parameters, that distinguish it from the rest of the population. Astronomers think the stars in the Gaia sausage are the ruptured remains of a small dwarf galaxy that was torn apart by its merger with the larger Milky Way.
Astronomers have identified dozens of other such collections, streams, clumps and remnants – a sign of the violent merger history of any decent-sized galaxy like ours.
Option 5: Just wait
Ultimately, time will do its work. Galaxies are remarkably stable; many have been around for more than 10 billion years. But nothing lasts forever.
Far, far in the future, when the universe is many times older than it is today, the merged Milky Way-Andromeda galaxy will begin to dissolve. It’s just a matter of gravity. Most stars spend most of their lives away from each other, but occasionally they wander too close. When they do, they perform a little gravity dance, sending them in a new direction. Very rarely can someone gain enough energy to escape the galaxy completely.
This is incredibly rare, but after trillions and trillions of years it will definitely happen. Ultimately, everything in our galaxy will either find its way into a giant black hole or become scattered throughout the wider universe. And that will truly be the end of our Galaxy.