A star is a huge nuclear reactor, fusing light elements into heavier elements. Each element builds up a shell, from a light hydrogen skin to a dense core. For stars big enough to build a massive iron core, the star eventually explodes in a supernova, like Cassiopeia A.
If you played around with the supernova simulator, you're familiar with the reaction pathways. If haven't, go play it — we'll wait. So, now you've got the order of fusion: hydrogen into helium, helium into neon and oxygen, oxygen into sulfur and silicon, and finally into iron. Each one leaves a shell behind, never truly burning through all available fuel.
So, what happens to those careful shells when the supernova blows? Cassiopeia A only went supernova a brief three centuries ago. The Chandra X-ray Observatory image of the supernova is colour-matched to the artist interpretation shells —blue for iron, green for sulfur and silicon, and red for magnesium, neon, and oxygen.
While oxygen should theoretically be the most abundant, the X-ray emissions identified by the element is strongly absorbed by the gas between us and the supernova remnant. What's more surprising is that iron, which starts in the interior, is blown farthest out into an outer shell. It looks like not only a supernova blows gas everywhere, but it also turns the star inside-out during the explosion!