--- title: "Energy Transport and Nuclear Burning in Stars" description: "Energy made by fusion in the core leaves by radiation or by convection depending on how steeply temperature falls, and the proton-proton chain and the CNO cycle dominate in different mass ranges. Phot" canonical: https://lightmysky.com/learn/science/energy-transport-and-nuclear-burning-in-stars-mt_2rc4UAMyl- source: https://lightmysky.com/learn/science/energy-transport-and-nuclear-burning-in-stars-mt_2rc4UAMyl-.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Energy Transport and Nuclear Burning in Stars Energy made by fusion in the core leaves by radiation or by convection depending on how steeply temperature falls, and the proton-proton chain and the CNO cycle dominate in different mass ranges. Photons take a very long time to escape. Subject: Science · Area: Astronomy & Astrophysics · Ages 20 to 22 Page: https://lightmysky.com/learn/science/energy-transport-and-nuclear-burning-in-stars-mt_2rc4UAMyl- ## Ready when they can - Compares the proton-proton chain and the CNO cycle and says which suits which stellar mass - Distinguishes radiative from convective transport and states what decides between them - Explains why energy made in the core takes so long to reach the surface ## Lesson: How starlight gets out Sun-like cores fuse hydrogen through the proton-proton chain. Hotter, heavier cores switch to the CNO cycle, where carbon, nitrogen, and oxygen act as catalysts that hurry hydrogen fusion along. A star near twice the Sun's mass therefore runs mostly on CNO. Energy then crawls outward two ways. Where hot gas is transparent, photons carry it as radiation; where cooler gas turns opaque, rising and falling currents carry it as convection. Conduction barely matters in a star. **Example.** The crawl is absurdly slow: each photon flies only about a centimeter before atoms swallow and re-emit it, so escaping takes tens of thousands of years of random walk. Neutrinos skip the queue entirely and reach Earth in minutes, which is how we know fusion powers the Sun right now. **Tip.** Four protons fusing into helium free about 26 MeV, yet the Sun must run this reaction staggering numbers of times per second to radiate. Contraction alone could fund only tens of millions of years, far short of billions, so gravity cannot be the main supply. **Recap.** Hydrogen fuses by chain or cycle, photons wander out over millennia, and neutrinos confirm the burning is current. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Hydrostatic Equilibrium and the Structure of a Star](https://lightmysky.com/learn/science/hydrostatic-equilibrium-and-the-structure-of-a-star-mt_RKeMW8iX56) ## Opens up - [The Sun as a Star: Photosphere, Corona and the Activity Cycle](https://lightmysky.com/learn/science/the-sun-as-a-star-photosphere-corona-and-the-activity-cycle-mt_AMIkiU7wRz) - [The Hertzsprung-Russell Diagram and Stellar Evolution](https://lightmysky.com/learn/science/the-hertzsprung-russell-diagram-and-stellar-evolution-mt_zMZDIB5TGc)