The image shows a massive star (mass greater than eight solar masses) that has reached the end of its life. The core is already composed of iron and cannot be fused to heavier elements any more since the fusion of iron to heavier elements would require external energy instead of releasing it. Depending on the pressure and temperature there are various fusion shells and the end-product of each shell rains down onto the next shell. For example, Oxygen (O) fuses to Silicon (Si). Temperatures and pressures in the Oxygen fusion shell are not high enough to fuse Silicon into Iron. The heavier Silicon therefore falls towards the centre into the Silicon fusion shell where temperatures and pressure are finally high enough to allow the fusion of silicon into iron.
At a certain time enough iron has been fused and rained down onto the core of the star to increase the mass of the iron core to the Chandrasekhar limit. At this point in time nothing can stop the collapse of the star's core and the supernova event starts.
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