The phrase "nuclear furnace" refers to a very hot place where nuclear reactions occur. It is usually used to describe the sun or stars, which release incredible amounts of heat and light through nuclear fusion reactions. Full definition
All heavier elements like carbon, nitrogen, and oxygen were created much later in the hot nuclear furnaces of stars. (keckobservatory.org)
Creating so much oxygen takes a fiercer nuclear furnace than is needed for a carbon - rich mixture, so the stars that became these white dwarfs must have been hot and massive. (newscientist.com)
These elements are forged in the stars» fiery nuclear furnaces. (discovermagazine.com)