What Happens When Stars Collide?

by Carson
stellar collision

Although the Universe is mostly empty in terms of matter, it may be natural to ask what happens when stars ever run into one another. Therefore, we’ll explain what happens when stars collide in this article.

Why Do Stars Collide?

The density of stars in the Universe is pretty low, demonstrated by the fact that the nearest star from the Sun, Proxima Centauri, is over 4 light-years away. However, stars do collide more often than you might think. That’s because many stars are part of star systems, in which multiple stars orbit each other. The stars are pretty far away most of the time, but as their orbits potentially shrink, they can get closer together. In fact, some stars are already in the process of merging with another star, as in the case of contact binaries.

What Happens When Stars Collide?

The consequences that will happen when stars collide highly depend on the type of the star. Do you know that there are luminous, blue stars in globular clusters that should have exploded as supernovae billions of years ago? These stars are called blue stragglers, and are the result of a collision between two stars. Stars in globular clusters are more tightly packed together than in any other place in the Universe. Thus, collisions are more common than usual, and they often gently merge into one massive star called a blue straggler.

On the other hand, when the stars are merging more quickly and violently, they may not form a blue straggler. Instead, the gases of the stars might be ripped off its core, eventually turning the two stars into a cloud of gases that mainly consists of hydrogen and helium. However, these collisions are probably rarer than the gentler collisions since it may require two stars to simply bump into each other instead of orbiting each other before the collision.

What Happens When Stellar Remnants Collide?

Stellar remnants are objects that form from the death of a star, namely white dwarfs, neutron stars, and stellar-mass black holes. What makes collisions of them so intriguing is that stellar remnants are so dense that peculiar results often arise from collisions of them.

First and foremost, dramatic collisions of stellar remnants will usually result in the emission of strong gravitational waves, especially those involving collisions between neutron stars or black holes. Those gravitational waves travel through and distort the fabric of spacetime and are sometimes picked up by our sensitive detectors on Earth.

Secondly, these collisions may also emit gamma-ray bursts. As seen from its name, gamma-ray bursts are bursts in the emission of gamma rays. Gamma-ray bursts can be classified into long and short gamma-ray bursts (GRBs). Specifically, mergers between neutron stars will result in short GRBs that only last for less than 2 seconds but release an unimaginable amount of energy.

How about the remaining objects that are left after the collision? Firstly, when black holes collide, they simply merge together, and the singularity gets more massive, and the radius of the event horizon expands. When neutron stars collide, they form what is known as a kilonova. They are bright bursts of energy that are about 1000 times brighter than a nova, but are definitely not as bright as a supernova. Last but not least, when two white dwarfs whose combined mass exceeds the minimum mass of a neutron star known as the Chandrasekhar limit, it will result in a type Ia supernova, which is the only type of supernova that doesn’t involve high-mass stars.


In this article, we mentioned what happens when stars and stellar remnants collide. If you want to learn more, please visit the webpages in the references below.

References and Credits

  1. Alastair Gunn. (n.d.). What happens when stars collide? Retrieved February 5, 2022, from https://www.sciencefocus.com/space/what-happens-when-stars-collide/
  2. (n.d.). Blue Stragglers. Retrieved February 5, 2022, from https://astronomy.swin.edu.au/cosmos/b/blue+stragglers
  3. (2020, May 21). What would happen if two stars collided? Retrieved February 5, 2022, from https://astronomy.com/magazine/ask-astro/2020/05/what-would-happen-if-two-stars-collided
  4. Adam Mann. (2020, January 15). What Is a Gamma-Ray Burst? Retrieved February 5, 2022, from https://www.space.com/gamma-ray-burst.html
  5. (n.d.). Gamma-ray Bursts. Retrieved February 5, 2022, from https://imagine.gsfc.nasa.gov/science/objects/bursts1.html
  6. Claudia Crowley. (2020, November 17). Hubble sees the brightest kilonova. Retrieved February 5, 2022, from https://earthsky.org/space/unusual-kilonova-infrared-light-neutron-stars-grb/
  7. Elizabeth Howell. (2013, August 5). What’s A Kilonova? You’re Looking At It! Retrieved February 5, 2022, from https://www.universetoday.com/103940/whats-a-kilonova-youre-looking-at-it/

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