Animation of the formation of a black hole, following the death of a massive star in a supernova explosion. At the end of its life the star contains a core of several incredibly hot and dense layers, with an huge overlying envelope of hydrogen (red). This has expanded to an enormous size, and the star will appear as a red supergiant, such as Betelgeuse. Zooming in to the core of the opened star, there is a thin layer of hydrogen fusion (red), which supports the envelope. Successive layers of fusing nuclei are then revealed: a layer of helium fusion, carbon fusion, neon, oxygen and silicon, surrounding an inert core of nickel and iron (white). Only stars with more than eight times the mass of the Sun can produce pressures and temperatures high enough to ignite the fusion of carbon and beyond. Once this has begun, the life of the star is very short. Carbon burning lasts several thousand years, neon just a few years, oxygen less than a year and silicon burning lasts just a few days. Although the pressure on the inert core is immense, fusion of iron and nickel nuclei takes more energy than it releases, and without energy production the core cannot support itself against gravity, and it rapidly collapses. The core collapses in a fraction of a second to the density of an atomic nucleus, and the infalling outer layers rebound as a type ii supernova explosion, briefly outshining an entire galaxy and leaving an expanding remnant of gas and heavy elements. For stars of more than 20 times the Sun's mass, the core is so dense that it forms a black hole at the centre of the supernova remnant. |