How Massive Stars Collapse Into Neutron Stars Only About 10 Kilometers Wide
Some massive stars swell into red giants, then explode and leave behind a neutron-star core only about 10 kilometers across.
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A neutron star is what can remain after a massive star reaches the end of its life and its core collapses during a supernova. Massive stars can pass through a red-giant stage before their cores run out of usable fuel and collapse under gravity. During that collapse, matter is squeezed to extraordinary densities as electrons and protons combine to form neutrons. The outer layers are blasted into space while the collapsed core can settle into a neutron star. The result is an astonishing size change: a stellar core can end up only around 10 to 12 kilometers in radius while still containing roughly a Sun-like amount of mass. Neutron stars are therefore among the densest objects known. Their gravity is enormous, their surface escape speed is more than half the speed of light, and some rotate hundreds of times per second. The fastest known neutron star rotates more than 700 times every second. Rapid rotation happens because the collapsing star becomes vastly smaller while retaining much of its angular momentum. Not every massive-star collapse produces a neutron star. If the remaining core is too massive to be supported, it can continue collapsing into a black hole instead. Neutron stars also remain an active area of research because scientists still do not fully know what matter is like under their extreme pressure and density.
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