How are galaxies destroyed?

The clashing galaxies NGC 4568 (bottom) and NGC 4567 (top) as seen by the Gemini North telescope in Hawai‘i
The clashing galaxies NGC 4568 (bottom) and NGC 4567 (top) as seen by the Gemini North telescope in Hawai‘i (Image credit: International Gemini Observatory/NOIRLab/NSF/AURA Image processing: T.A. Rector (University of Alaska Anchorage/NSF's NOIRLab), J. Miller (Gemini Observatory/NSF's NOIRLab), M. Zamani (NSF’s NOIRLab) & D. de Martin (NSF’s NOIRLab))

Eventually, all galaxies, including our own Milky Way, will meet their end. 

But how do galaxies die? If you're in the mood to destroy an entire galaxy, you have several options, depending on your desired level of destructiveness.

Related: Monster black holes may have murdered their host galaxies in the early universe

Option 1: Wake up the monster black hole 

At the heart of almost every galaxy sits a supermassive black hole. In the case of the Milky Way, we have Sagittarius A*, a beast weighing more than 4.5 million suns. Usually, these giant black holes are quiet and dormant, just sipping on whatever bits of gas or stars that wander too close. But occasionally, they feast on a much larger meal. When they do, that gas swirls around them and compresses, reaching temperatures well over a billion degrees.

Those ridiculously high temperatures cause the gas to give off an enormous amount of radiation that then goes out to flood the entire galaxy, heating up any reserves of gas and preventing the formation of new stars. While things usually calm down after that, in the worst cases, the radiation from the vicinity of the black hole can outright eject massive amounts of gas from the galaxy altogether.

This doesn't quite destroy a galaxy, but it does effectively kill it by preventing new stars from forming for a very long time and, in some cases, forever. 

Option 2: Drop it into 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 hold vast 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 as shock waves compress gas clouds throughout the galaxy. But eventually, the pressure from the gas does its work, picking pieces of gas off the galaxy like debris flying off a meteorite.

This leads to a cute situation known as "jellyfish galaxies," so called because the stripped gas resembles the tentacles of a jellyfish. Although most galaxies survive their descent into a cluster's ICM, some smaller galaxies get completely evaporated.

The Hubble Space Telescope captured an edge-on view of JW100 — a jellyfish galaxy (located in the lower right of the image) — along with six small elliptical galaxies and a much larger elliptical galaxy at the top of the frame.  (Image credit: ESA/Hubble & NASA, M. Gullieuszik and the GASP team)

Option 3: Crash it into another galaxy 

Galaxy collisions represent one of the greatest releases of energy in the known universe, and that means it's not exactly a pretty sight. Our own Milky Way will collide with our neighboring galaxy Andromeda in about 5 billion years.

A slow and aching process lasting hundreds of millions of years, galaxy mergers can raise huge tidal tails that consist of streams of broken-off stars and gas that arch around the galaxies. During the collision and merger, countless stars are lost through random interactions. And once the respective supermassive black holes meet, a fresh round of radiation pummels the newly merged galaxy. The combined devastation depletes the galaxy of gas reservoirs, effectively shutting off star formation for good. 

Option 4: Feed it to a much bigger galaxy 

If a smaller galaxy and a much larger companion merge, it can spell the end of the smaller galaxy. Indeed, the European Space Agency's Gaia survey has discovered the bones and corpses of cannibalized galaxies littered throughout the Milky Way.

One such example is known as the Gaia sausage. This collection of stars scattered around the Milky Way's core share properties, like an abundance of heavy elements and orbital parameters, that are distinct from the rest of the population. Astronomers think the stars in the Gaia sausage are the tattered remnants of a small dwarf galaxy that was torn apart by its merger with the greater Milky Way.

Astronomers have identified dozens of other such collections, streams, clumps and remnants — a sign of the violent merger history of any decently sized galaxy like our own.

Option 5: Just wait 

Eventually, time will do its work. Galaxies are remarkably stable; many have existed for well over 10 billion years. But nothing lasts forever.

Far, far into 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 simply a matter of gravitational chance. Most stars spend most of their lives nowhere near each other, but occasionally, they wander too close. When they do, they perform a little gravitational dance, sending them in new directions. Very rarely, one can get enough energy to escape the galaxy altogether.

This is incredibly rare, but after trillions upon trillions of years, it's bound to happen. Eventually, everything in our galaxy will either make its way inside a giant black hole or be scattered into the wider universe. And that will truly be the end of our galaxy.

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Paul Sutter Contributor

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

  • MikeMc
    Okay, but I feel like it’s important to note that way before a trillion years gets here, there will be no such thing as a sun-like star. Life orbiting a white dwarf would be very extremely unlikely and life orbiting a red dwarf would be very highly unlikely. So when the sun-like stars burn out, that will very likely be the end of life as we know it.

    I would say, however, that there is not a zero percent chance that we could survive the merger of the Milky Way and Andromeda. If humans don’t go extinct in the next 100,000 years, I think it’s highly likely that we will figure out a way to accomplish interstellar travel. This is the century of Quantum Computing and Artificial Intelligence. If those technologies don’t kill us, they will transform our lives in unimaginable ways.
  • Catastrophe
    While the Andromeda Galaxy contains about 1 trillion (1012) stars and the Milky Way contains about 300 billion (3×1011), the chance of even two stars colliding is negligible because of the huge distances between the stars. For example, the nearest star to the Earth after the Sun is Proxima Centauri, about 4.2 light-years (4.0×1013 km; 2.5×1013 mi) or 30 million (3×107) solar diameters away.

    To visualize that scale, if the Sun were a ping-pong ball, Proxima Centauri would be a pea about 1,100 km (680 mi) away, and the Milky Way would be about 30 million km (19 million mi) wide. Wiki

    Cat :)

  • Questioner
    A galaxy's structure and form is highly dependent on the mass field currently by most consensus attributed to DM.

    Currently mass is the only assigned property of DM.

    If that's the case DM will, and probably already should have gravitationally aggregated at other mass centers,

    which in fact is not demonstrable in any way.

    If as an alternative the halo of mass is a secondary, additional mass field of black holes it creates a challenge to conventional establishment consensus,
    because that huge halo would entail a superluminal speed of gravity.

    So whether the mass halo is either sloppy fluid DM (which would be easily perturbed) or a coherent secondary black hole mass curve (vastly more resilient), or something else altogether should be there buried in the detailed movements of colliding galaxies.
  • MikeMc
    There is a lot more to the merger of the Milky Way and Andromeda other than the distance between stars. The merger could create the energy equivalent to 100 million supernova explosions which could kill all life on Earth. Another effect would be planets getting all mixed up and moving around. The galaxy would look nothing like it currently does and Earth would be in a completely different location in the galaxy. There is a good chance that Earth will be destroyed in this merger along with probably life everywhere in the merged galaxy. It doesn’t matter what the chances are of stars colliding. Earth could be destroyed in multiple different ways. Besides the fact that Earth will already be destroyed anyway by our own Sun. In about a billion years at the most, but probably much sooner than that, all life on Earth will cease to exist because the Sun is getting larger and hotter as it runs out of fuel.
  • Catastrophe

    Such an event would have no adverse effect on the system and the chances of any sort of disturbance to the Sun or planets themselves may be remote.
    Andromeda–Milky Way collision - Wikipedia
  • Classical Motion
    It would really be great to watch from Earth. One set of stars would be moving and the other not.

    Only the intruders would move for us. They would streak with exposure.

    ALL the acceleration expressed(both galaxies) would show up on those streaks.

    Quite a display I would imagine.

    Both accelerations should curve the streaks.