Stars that are more massive than the Sun fuse hydrogen into helium in their cores through the CNO cycle. Explain briefly what the CNO cycle is, and why it occurs in the cores of massive stars.
The CNO cycle, or the carbon-nitrogen-oxygen cycle, is a nuclear reaction that occurs in high-mass stars. It is one of the two main processes through which stars convert hydrogen into helium, the other being the proton-proton chain.
In the CNO cycle, nuclei of carbon, nitrogen, and oxygen act as catalysts to speed up the fusion of hydrogen nuclei (protons) into helium. The main reaction in this cycle involves the fusion of four hydrogen nuclei to form a helium nucleus. This reaction occurs in several steps, with carbon, nitrogen, and oxygen isotopes acting as intermediaries.
The CNO cycle occurs primarily in the cores of massive stars because the high temperatures and densities in these regions are necessary for the fusion reactions to take place. The core of a massive star provides the ideal conditions for the CNO cycle to occur as it has sufficient pressure and temperature for the fusion reactions to be sustained. As a result, massive stars are able to burn their hydrogen fuel more efficiently and at a faster rate compared to lower-mass stars, leading to their shorter lifespans.