Neutron Stars Collide in a Bright Millisecond Flash

two neutron stars colliding
Two neutron stars colliding. In just a few milliseconds they crash into each other and create huge flash seen as a gamma-ray burster and a flare of visible light. NASA/AEI/ZIB/M. Koppitz and L. Rezzolla

There are some really weird denizens of the cosmic zoo out there in space. You've probably heard about colliding galaxies and magnetars and white dwarfs. Have you ever read about neutron stars? They're some of the weirdest of the weird — balls of neutrons packed together very tightly.  They have incredible gravitational field strength, plus strong magnetic field. Anything getting close to one would be changed forever.

When Neutron Stars Meet!

Anything that gets near the neutron star is subject to its strong pull of gravity. So, a planet (for example) could be torn apart as it nears such an object. A nearby star loses mass to its neutron star neighbor.

Given that ability to rip things apart with its gravity, imagine what it would be like if two neutron stars met! Would they blow each other part? Well, maybe.  Gravity would obviously play a huge role as they get closer together and eventually merge. Beyond that, astronomers are still trying to figure out exactly what would happen in such a case (and what would cause one).  

What occurs during such a collision depends on the mass of each of the neutron stars. If they're smaller than about 2.5 times the mass of the Sun, they'll merge and create a black hole in a very short amount of time. How short? Try 100 milliseconds! That's a tiny fraction of a second.  And, because you have a tremendous amount of energy released during the merger, a gamma-ray burst would be produced. (And, if you think that's a huge explosion, imagine what might happen when black holes themselves collide!)

Gamma-Ray Bursts (GRBs): Bright Beacons in the Cosmos

Gamma-ray bursts are just what the name sounds like: bursts of high-energy gamma rays from an intensely energetic event (such as a neutron star merger). They have been recorded all over the universe, and astronomers are still finding likely explanations for them, including in neutron star mergers. 

If the neutron stars are larger than 2.5 times the mass of the Sun, you get a different scenario: there will be what's called a neutron star remnant.  No GRB is likely to take place. So, for right now, the conclusion is that you will either get a neutron star remnant or a black hole. If a black hole emerges from the collision, then it will be signaled by a gamma-ray burst. 

One other thing: when neutron stars merge, gravity waves are formed, and those can be detected with such instruments as the LIGO facility (short for Laser Interferometer Gravitational-Wave Observatory), built for to look for just such events in the cosmos.  

Forming Neutron Stars

How do they form? When very massive stars many times more massive than the Sun explode as supernovae, they blast a LOT of their mass to space. There's always a remnant of the original star left behind. If the star is massive enough, the leftovers are still very massive and they can shrink down to become a stellar black hole. 

Sometimes there's not quite enough mass left, and the remains of the star crush down to form that ball of neutrons — a compact stellar object called a neutron star. It can be quite small — perhaps the size of a  small town a few miles across.  Its neutrons are crushed together very tightly, and there's no way of knowing what's happening inside. 

Gravity Rules

A neutron star is so massive that if you tried to lift a spoonful of its material, it would weigh a billion tons. As with any other massive object in the universe, a neutron star has an intense gravitational pull. It's not quite as strong as a black hole's, but it can definitely have an effect on nearby stars and planets (if there's anything left after the supernova explosion). They also have very strong magnetic fields, and often also give off bursts of radiation that we can detect from Earth. Such noisy neutron stars are also called "pulsars". Given all that, neutron stars definitely rate as one of the top types of weird objects in the universe! Their collisions are among the most powerful events we can imagine.