What Happens When Galaxies Collide?

by Carson

The Universe is expanding at an accelerating rate, but some galaxies still interact with each other. What will happen when they do? We’ll explain that in this article.

Why Do Galaxies Still Collide?

Galaxies are pulled away from each other constantly as the cosmos expands. However, galaxies sometimes form close enough together that their mutual gravitational pull outpowers dark matter. Therefore, they can move toward each other until they start interacting. An example is our Milky Way Galaxy and the Andromeda Galaxy, which are getting closer by 110 kilometers per second, and they will eventually collide in a few billion years.

During The Collision

When the two galaxies get so close to each other that they start to interact, the gases in them will be compressed significantly by gravity. That means many new stars will form at this stage since there are more materials and conditions for clumps of gases to collapse further. Sometimes stars might even be ejected away from the interacting galaxies.

However, don’t expect to see stars actually collide. Star systems are usually very far away from each other (sometimes light-years away), and it’s extremely unlikely that a star will collide with another one, even when two galaxies interact.

If one galaxy is significantly more massive than the other, it will result in the smaller galaxy being swallowed and the more massive galaxy being almost unaffected. However, suppose both galaxies have similar mass (like in the Milky Way-Andromeda Galaxy collision). In that case, both will interact vigorously and become distorted, like in the case of Antennae Galaxies (NGC 4038 and NGC 4039) and Mice Galaxies (NGC 4676). If these galaxies have any spiral arms, those structures will be destroyed too. Eventually, the two will become a large elliptical galaxy with almost no star formation.

Although this process may look like it happens in a few hundred years, it is actually a multi-billion-year process because of the relatively slow speeds that galaxies are moving toward each other, even when they’re close together.

When the Supermassive Black Holes Merge

In the center of every massive galaxy, there is almost always a supermassive black hole. Therefore, the collision of the supermassive black holes is the climax of the event, although it may happen billions of years after the first contact between the two.

When the supermassive black holes get sufficiently close to each other, they will orbit each other. In the meantime, they emit gravitational waves that distort spacetime, which shrinks its orbit gradually until they collide, which simply turn them into a larger black hole.

Why Will Star Formation Cease When Two Spiral Galaxies Merge?

It’s natural to ask why two spiral galaxies with active star formation will eventually form an elliptical galaxy with no star formation. In galactic collisions, star formation is rapid, and large amounts of gases get ejected. These processes essentially eliminate the interstellar gases that create stars, and consequently, the delicate spiral arms will disappear.


If two galaxies collide, they will gradually form a larger galaxy by forming new star formation regions, swallowing or destroying galaxies, and making the black holes collide (if they ever get close enough). Keep mind that one type of peculiar galaxies, where it is neither spiral, elliptical, or irregular, are formed from two interacting galaxies. If you want to learn more about interacting galaxies, please visit the websites in the references below.

References and Credits

  1. (2004, October 14). When Galaxies Collide. Retrieved October 29, 2021, from https://www.nasa.gov/audience/foreducators/k-4/features/F_When_Galaxies_Collide.html
  2. David J. Eicher. (2019, July 1). What Happens When Galaxies Collide? Retrieved October 29, 2021, from https://astronomy.com/magazine/greatest-mysteries/2019/07/40-what-happens-when-galaxies-collide
  3. Jonathan O’Callaghan. (2018, August 14). What happens when two galaxies collide? | – Space Answers. Retrieved October 29, 2021, from https://www.spaceanswers.com/deep-space/what-happens-when-two-galaxies-collide1/
  4. Matt Williams. (2016, October 3). What happens when galaxies collide? Retrieved October 29, 2021, from https://phys.org/news/2016-10-galaxies-collide.html
  5. Annagrazia Puglisi. (2021, January 12). Galaxies eject gas when they merge, preventing new stars forming – new research. Retrieved October 29, 2021, from https://theconversation.com/galaxies-eject-gas-when-they-merge-preventing-new-stars-forming-new-research-153118
  6. (2020, August 20). Why do colliding spiral galaxies form elliptical galaxies? Retrieved October 29, 2021, from https://astronomy.com/magazine/ask-astro/2020/08/why-do-colliding-spiral-galaxies-form-elliptical-galaxies
  7. (2018, April 23). Colliding Galaxies (NGC 2207). Retrieved October 29, 2021, from https://svs.gsfc.nasa.gov/30941
  8. Rob Garner. (2017, October 19). Messier 51 (The Whirlpool Galaxy). Retrieved October 29, 2021, from https://www.nasa.gov/feature/goddard/2017/messier-51-the-whirlpool-galaxy
  9. (2013, November 14). NASA Hubble Sees Sparring Antennae Galaxies. Retrieved October 29, 2021, from https://www.nasa.gov/content/goddard/nasa-hubble-sees-sparring-antennae-galaxies

Related Posts

Leave a Comment

* By using this form you agree with the storage and handling of your data by this website.