carbon cycle 
noun
1The series of processes by which carbon compounds are interconverted in the environment, involving the incorporation of carbon dioxide into living tissue by photosynthesis and its return to the atmosphere through respiration, the decay of dead organisms, and the burning of fossil fuels.The total number of carbon atoms on Earth is fixed - they are exchanged between the ocean, atmosphere, land and biosphere, through a set of processes known as the carbon cycle....
- The series of chemical, physical, geological, and biological changes by which carbon moves through the Earth's air, land, water, and living organisms is called the carbon cycle.
- We have reversed the carbon cycle by about 400,000 years.
2 Astronomy The cycle of thermonuclear reactions believed to occur in stars, in which carbon nuclei are repeatedly formed and broken down in the conversion of hydrogen into helium.For his role in working out the energy source for stars more massive than the sun, the carbon cycle, Hans Bethe received the Nobel Prize in 1967....
- Carbon speeds the fusing of hydrogen nuclei into helium in the carbon cycle, so these stars burn faster and more brightly than do other hydrogen-burning stars.
- The carbon cycle is the dominant energy source for ordinary stars of mass greater than about 1.5 times the mass of the Sun.