When a high-mass star, about 10 times the size of our Sun, burns up the elements within it in layers, a core-collapse supernova is imminent. When it fuses one element, the ash fuels the star, making it hotter, so that it can fuse heavier and heavier elements. The core continues to change in composition, and contract. Once the core becomes iron, the star can’t use it as fuel, and begins to cool down. When this happens, the star can’t support itself, and it collapses. Core temperatures rise to 10 billion Kelvin, at which photodisintegration occurs. The iron and other heavy elements undergo fission—the splitting of atoms. The core collapses at an even greater rate, until the neutrons come into contact with each other to form an neutron star. Once the collapse slows to a stop, it is so compacted that it immediately expands again as a supernova. The collapse and expansion only takes a few seconds, but unleashes an unimaginable about of energy.

All that remains is the neutron star. This super-dense perfect sphere of matter has the mass of a star, but size of a city, as a common comparison states. The matter that once belonged to the star, called the supernova remnant, rapidly expands away from the center for many centuries. Most of the time, the neutron star spins rapidly, and emits beams of X-ray and radiation of other wavelengths. These are called pulsars. The radiation causes the supernova remnant to glow.
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