CNO cycle

\ ˌsē-ˌen-ˈō- How to pronounce CNO cycle (audio) \

Definition of CNO cycle

: the process by which large stars convert elemental hydrogen into helium through a series of nuclear fusion reactions To become a supernova, the progenitor star had to be at least eight times our Sun's mass, so it must have fused hydrogen into helium via the CNO cycle, a reaction that uses carbon, nitrogen, and oxygen as catalysts.— Wayne J. Parker, Sky & Telescope, January 1995

Note: Stars that are roughly twice the size of the Sun or larger convert hydrogen by means of the CNO cycle, in which hydrogen nuclei combine with and then separate from carbon, nitrogen and oxygen in a specific repeating loop that generates both helium and extra energy. Smaller stars also fuse hydrogen into helium, but the process is different and does not involve other elements.

Love words?

You must — there are over 200,000 words in our free online dictionary, but you are looking for one that’s only in the Merriam-Webster Unabridged Dictionary.

Start your free trial today and get unlimited access to America's largest dictionary, with:

  • More than 250,000 words that aren't in our free dictionary
  • Expanded definitions, etymologies, and usage notes
  • Advanced search features
  • Ad free!

First Known Use of CNO cycle

1958, in the meaning defined above

Keep scrolling for more

Learn More about CNO cycle

Time Traveler for CNO cycle

Time Traveler

The first known use of CNO cycle was in 1958

See more words from the same year

Statistics for CNO cycle

Cite this Entry

“CNO cycle.” Dictionary, Merriam-Webster, Accessed 18 Jan. 2021.

Comments on CNO cycle

What made you want to look up CNO cycle? Please tell us where you read or heard it (including the quote, if possible).


Test Your Vocabulary

Words of Snow and Ice Quiz

  • image1037863653
  • Which of the following refers to thin, bending ice, or to the act of running over such ice?
True or False

Test your knowledge - and maybe learn something along the way.


Anagram puzzles meet word search.

Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!