SALLY BENSUSEN (1988) / SCIENCE PHOTO LIBRARY SALLY BENSUSEN (1988) / SCIENCE PHOTO LIBRARY
Illustration showing the evolution of a high-mass star, ending in a supernova explosion. Stage one (left) shows a normal blue star, powered by hydrogen fusion. As the hydrogen fuel in the core becomes depleted, the helium 'ash' contracts, heats up and ignites. The higher temperatures of helium-burning cause the star to expand & become a red giant (stage 2). In a similar way, successive heavier elements begin to burn in shells around the core. Eventually the core is converted to iron & the fusion ceases. Both the core & atmosphere then collapse (3). When nuclear density is reached, the core rebounds (4), sending out a shock wave which tears the star apart.
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