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The energy produced and emitted by stars (such as our Sun) is a result of the process of nuclear fusion taking place at the star’s core. At the beginning of a star’s life, it is mostly composed of hydrogen, and, depending on the star’s mass, the star will spend millions or billions of years producing energy through the fusion of hydrogen into helium. Once the hydrogen runs out, the star starts fusing the helium into heavier elements, such as carbon and oxygen. When the helium runs out, the star switches to fusion of carbon and oxygen into still heavier elements, such as magnesium, silicon and sulfur. When all that’s left in the star is silicon and sulfur, the star switches to the production of iron. With each change in the fusion process, the amount of time before the new fuel is used up decreases, and the amount of energy produced drops.
As the series of successive fusion processes continues, the structure of the star evolves to reflect its history. After the switch to fusion of heavier elements, and leftover lighter fuel elements float out of the star’s core to a higher layer. So a star in its later stages has an outer layer that predominantly consists of any leftover hydrogen. Further in, is a layer of helium, and still further in, carbon and oxygen, and so on until you get to the iron core.
When the silicon and sulfur run out, the fusion process in the star ceases; this is because fusion of iron and elements heavier than iron do not produce energy when they undergo fusion. Low mass stars (such as our Sun) that get to this stage simply stop emitting radiation and remain essentially as burned out cinders. Heavy stars, on the other hand, undergo a supernova. In any star, the energy produced by the fusion process counters the force of gravity pulling all the mass of the star to its center. When the process of fusion ceases, gravity no longer has a counterforce, and all mass starts to collapse. In a heavy star, the collapsing occurs at a dramatic velocity. The core the collapses into either a neutron star or a black hole, while the elements in the outer layers are ejected outward, where they can seed other gas clouds and perhaps become incorporated into planets orbiting new stars.
Because the process of fusion occurs at the core of the star, it is shrouded in a certain amount of mystery. However, a supernova occurred in 2021 (SN2021yfj) that pierced that shroud. Spectroscopic analysis of the light from the star before the explosion occurred showed that all layers of the star had already been expelled from the star, down to the silicon and sulfur layer. The researchers called this an “extremely stripped supernova.” How this happened is not clear. Usually, the stripping of the star’s layers can be attributed to the stellar wind–essentially the force of all the light and other material expelled by the star in the course of its usual operation–but a normal stellar wind would never have the force to expel the deeper layers. It is a challenge for theorists to explain what kind of process could have stripped the star down to its deepest layers.
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