Question

Which of the following is unchanged at the end of the CNO cycle?

A. carbon-12

B. oxygen-15

C. nitrogen-13

D. the four protons

Answer 1

The correct option is: A. carbon-12

Step-by-step explanation:

The CNO cycle, the abbreviation for the carbon-nitrogen-oxygen cycle, is a catalytic cycle by which the stars produce helium from elemental hydrogen, via a series of nuclear fusion reactions.

This cycle involves the fusion of four protons with carbon (
`_(6)^(12)\textrm{C}`), nitrogen isotope (
`_(7)^(13)\textrm{N}`), and oxygen isotope (
`_(8)^(15)\textrm{O}`), to give an alpha particle and two electron neutrinos and positrons.

The reaction involves the regeneration of carbon (
`_(6)^(12)\textrm{C}`) nucleus in the last step.

`_(6)^(12)\textrm{C}` →
`_(7)^(13)\textrm{N}` →
`_(6)^(13)\textrm{C}` →
`_(7)^(14)\textrm{N}` →
`_(8)^(15)\textrm{O}` →
`_(7)^(15)\textrm{N}` →
`_(6)^(12)\textrm{C}`