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A $15M_{\odot }$ star burns core hydrogen chiefly by the CNO cycle for about $10^7$ years. It commonly evolves towards a red supergiant, though winds and mixing alter its track. Core helium burning lasts of order $10^6$ years. Continued contraction then permits carbon, neon, oxygen and silicon burning, producing successively heavier nuclei up to an iron-group core. Neutrino cooling makes the advanced phases much shorter: carbon burning can last thousands of years, while the final burning stages fall to years or days, with substantial model dependence.

An iron-group core cannot gain energy by ordinary fusion into heavier nuclei. Electron captures, photodisintegration and loss of pressure support lead to stellar core collapse. A core-collapse supernova can eject the envelope and leave a neutron star; strong fallback or a failed explosion can instead produce a black hole. The massive-star endpoint involves core collapse, rather than the quiet carbon-oxygen white-dwarf cooling of the two lower-mass examples.