Carbon burning is a nuclear reaction that occurs in the core of massive stars. It only begins towards the end of a star’s life and requires at least four solar masses. The process produces elements heavier than carbon and can lead to the formation of a red supergiant or a planetary nebula.
The carbon burning process is a nuclear reaction that takes place in the core of massive stars under conditions of tremendous temperature and pressure. Carbon burning only begins towards the end of a star’s life. For a star to eventually build up enough pressure in its core to start burning carbon, it must contain at least four solar masses at its birth. Carbon burning begins only after large portions of the star’s hydrogen and helium have been burned off.
The most abundant element in the universe is hydrogen. Hence, most stars begin their lives consisting mostly of hydrogen. When nuclear fusion ignites in the core of a young star, hydrogen slowly begins to burn, its atomic nuclei fuse into helium via the pp chain – in stars the mass of the Sun or less – or the CNO cycle – in stars more massive. This is the nuclear reaction that generates the heat and light from the Sun we see when we go outside every day.
Depending on the size of the star, it burns its nuclear fuel at different rates. More massive stars have denser, hotter centers and burn their fuel faster. Some of the largest stars burn out most of their hydrogen fuel within a few million million years, while the Sun is predicted to continue fusing hydrogen for 4.5 billion years and the lightest stars will fuse hydrogen for a trillion years. As the helium “ash” builds up, it eventually reaches the critical density to cause the helium to ignite. The byproducts of helium combustion are carbon and oxygen.
As carbon and oxygen build up in the star’s core over millions of years of helium burning, eventually a large percentage of the helium is depleted and the star’s core cools, unable to generate any more nuclear energy. This cooling causes the core to contract, further increasing the density and pressure. In stars above about four solar masses, the temperature and density necessary for carbon combustion are achieved. This heats the core of the star and it expands to become a red supergiant.
Carbon burning is one of the main reasons why elements heavier than carbon exist in the universe. The main reaction consists of several components. In one, two carbon nuclei fuse to form a neon atom and a helium atom. Eventually, these break down into sodium and hydrogen, then magnesium and a free neutron. Due to all the nuclear processes going on simultaneously in the core of the star, large quantities of neon, oxygen and magnesium are produced. The entire carbon burning process takes only about 1000 years.
If the star has between four and eight solar masses of material, it will expel its outer layer as the carbon burn burns out, creating a planetary nebula and leaving behind a white dwarf core. If it has more than eight solar masses, it will eventually begin burning neon, the next stage in the evolution of massive stars.
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