A new photograph from the Spitzer Space Telescope shows how supernova remnant Cassiopeia A evolved over time. The original star contained 15 to 20 times the mass of our Sun, and was made up of concentric shells of elements. The lightest elements, like hydrogen, were in the outermost shell, while the heaviest elements sunk to the centre. The shells of the exploded material match up quite well with the original layers in the star before it detonated as a supernova.

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