Neutron stars are the remains of massive stars after they go supernova; while the outer layers of the star explode outward creating fireworks literally on a cosmic scale, the core of the star collapses, becoming incredibly compressed. If the core has enough mass it'll become a black hole, but if it's shy of that limit it’ll become an ultra-dense ball made up mostly of neutrons.
The stats for neutron stars are sobering. They have a mass of up to over twice the Sun, but the density of an atomic nucleus: Over 100 trillion grams per cubic centimeter. That's hard to grasp, but think of it this way: If you compressed every single car in the United States into neutron-star-stuff, you’d get a cube 1 centimeter on a side. The size of a sugar cube, or a six-sided die. All of humanity compressed into such a state would be less than twice that width.
Neutron stars have a surface gravity hundreds of billions times Earth's, and magnetic fields even stronger. A neutron star half the galaxy away from us had a seismic event on it that physically affected us here on Earth, 50,000 light years distant.
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