Neon burning occurs in massive stars, converting neon into oxygen and magnesium and releasing light and heat. It takes only a few years and occurs after carbon burning but before oxygen burning. If a star has more than 8 solar masses, it will start burning neon, and if it continues to burn heavier nuclei, it will eventually reach iron, leading to core collapse and a supernova.
Neon burning is a nuclear reaction that occurs in the core of massive stars (8 solar masses or more) near the end of their lives. It converts neon into oxygen and magnesium atoms, releasing light and heat in the process. The burning of neon is so rapid that it takes place over the course of only a few years, a blink of an eye in astrophysics, where times are usually measured in millions or billions of years. The combustion process of neon occurs after the combustion of carbon and before the combustion of oxygen.
For most of a star’s lifespan, it will slowly burn the hydrogen in its core, fusing hydrogen nuclei into helium nuclei, slowly increasing the percentage of helium in its core. If the star is massive enough, it will begin to fuse helium via the triple alpha process, leaving the main sequence and becoming a giant star. If the star has even more mass, it will start fusing helium into carbon, a process that takes only about 1,000 years.
What happens next separates the really massive stars from the smaller ones. If a star has less than about 8 solar masses, it expels most of its envelope via the solar wind and leaves behind an oxygen/neon/magnesium white dwarf. If it has more, the core condenses in size, heats up, and starts burning neon. The combustion of neon requires temperatures in the range of 1.2×109 K and pressures around 4×109 kg/m3. This is about four million tons per square meter.
Above the neon-burning core, the burning of the carbon, helium, and hydrogen continues in shells located at progressively greater distances from the core. The burning of neon is fundamentally based on photodisintegration, the process by which gamma rays of extreme energy are created and strike atomic nuclei so strongly that they knock off protons and neutrons or even break the nucleus in half. In the core of a dying star, photodisintegration strips alpha particles (helium nuclei) from neon nuclei, producing oxygen and alpha particles as byproducts. The energetic alpha particles then fuse with the neon nuclei to create magnesium.
Over time, the star consumes its neon and the core condenses again, at which point the burning of oxygen begins. If the star continues to burn heavier and heavier nuclei, it eventually reaches iron, which cannot be sustainably ignited, and core collapse occurs, followed by a supernova.
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