When two neutron stars collide, they viciously shred each other before merging into a black hole punctuated by a gamma ray burst. A new supercomputer simulation sets a mismatched pair of neutron stars in a death-spiral to investigate the process. The merger completes in seconds, ringed by a halo of diffuse gas.
When a massive star, one of 8 to 30 times the mass of our sun, explodes in a supernova, it leaves behind a neutron star. These extremely dense, compressed cores can pack the mass of one and a half suns into a tiny sphere just 20 kilometers in diameter. That's so mind-bogglingly dense, every comparison and metaphor ends up sounding utterly ridiculous. A neutron star is like packing our sun and its sister down to something smaller than most asteroids, with a density so high, a single cubic centimeter of a neutron star outweighs the entirety of Mount Everest.
It is possible for a neutron star to form in a binary system without destroying its companion (immediately), so what would happen in a binary system of two neutron stars?
The NASA Astrophysics group released a simulation by the Albert Einstein Institute of a mismatched pair of neutron stars: one weighing in at 1.4 solar masses, and the other at 1.7 solar masses. They started the stars off at a separation distance of just under their diameters, a mere 18 kilometers apart.
In the simulation, red is the least dense, scaling up through orange, yellow, and finally to white for extremely high-density.
From the animation description:
By 7 milliseconds, tidal forces overwhelm and shatter the lesser star. Its superdense contents erupt into the system and curl a spiral arm of incredibly hot material. At 13 milliseconds, the more massive star has accumulated too much mass to support it against gravity and collapses, and a new black hole is born. The black hole's event horizon — its point of no return — is shown by the gray sphere. While most of the matter from both neutron stars will fall into the black hole, some of the less dense, faster moving matter manages to orbit around it, quickly forming a large and rapidly rotating torus. This torus extends for about 124 miles (200 km) and contains the equivalent of 1/5th the mass of our sun.
A merger between a pair of neutron stars might produce short gamma-ray bursts. Gamma ray bursts are quick, massive flashes of energy: a single burst lasting just two seconds outshines an entire year of light from all the stars in the galaxy. Since the bursts are over quickly and fade fast, it's challenging for astronomers to observe what happens. Part of the NASA Swift mission is to quickly detect and locate the source of gamma ray bursts, sharing that data with ground-based telescopes to capture the afterglow.
Image credits: NASA Goddard. Related paper on ArXiv [open access]. Gamma ray bursts might also originate in our own atmosphere! We've also found two black holes in a single galaxy.