In this series we are exploring the weird and wonderful world of astronomy jargon! You’ll feel a little dense after reading about today’s topic: neutron stars!
Neutron stars are the densest objects in the entire universe. They’re so dense, in fact, that if they were even a tiny bit denser, they would collapse to become black holes.
Neutrons stars are what you get after a giant star (at least 10 times the mass of the Sun) dies. Towards the ends of their lives, those giant stars build heavier and heavier elements in their cores, all the way up to iron. But star can’t gain energy from the fusion of iron, and so the whole star begins collapsing.
If you have a few Suns’ worth of gas falling onto you at a good fraction of the speed of light, you would probably feel the pressure too. In less than a second, the incredible pressures shove electrons into atomic nuclei, converting protons into neutrons. This leaves behind a solid mass of neutrons which can (temporarily) halt the collapse the star. This triggers an ensuing supernova explosion, which leaves behind the core. That exposed core is now called a neutron star.
Neutron stars are some of the most exotic objects in the cosmos. They have up to twice the mass of the Sun crammed into a volume no bigger than a typical city. Their gravity is so intense that it bends light in the path of a circle, meaning that light can orbit a neutron star. Their densities are so extreme (they are, after all, essentially gigantic atomic nuclei) that a handful of neutron-star-stuff would outweigh a mountain on Earth.
The extreme gravity means that the tallest “hills” on a neutron star are only a centimeter or so high. If you were to fall off of one of those “mountains”, by the time you hit the ground you would be traveling at a good fraction of the speed of light.
While there are probably about a billion neutron stars in the Milky Way, astronomers can’t see most of them because they are relatively small and dim. When they first form they have a temperature of around half a million Kelvin, but they quickly cool off.
Sometimes neutron stars collide, and when they do they are responsible for an explosion known as a kilonova.