Stars with initial masses less than about 8 M ☉ never develop a core large enough to collapse and they eventually lose their atmospheres to become white dwarfs. Stars with at least 9 M ☉ (possibly as much as 12 M ☉) evolve in a complex fashion, progressively burning heavier elements at hotter temperatures in their cores. The star becomes layered like an onion, with the burning of more easily fused elements occurring in larger shells. Although popularly described as an onion with an iron core, the least massive supernova progenitors only have oxygen- neon (- magnesium) cores. These super-AGB stars may form the majority of core collapse supernovae, although less luminous and so less commonly observed than those from more massive progenitors.