A fireball in the supernova remnant W49B. with infrared data (red, green) with X-ray data (blue). [Caltech/J. Rho, T. Jarrett & NASA/CXC/SSC/J. Keohane et al.]


"I heard a star goes supernova after it has produced the element iron. Is this true? If so, how?"

Asked by: Taylor Sullivan


Supernovae have produced nearly every element occurring in nature. When a star is born, it is because it has enough mass to create enough heat, gravity and pressure to sustain nuclear fusion. Fusing hydrogen atoms to helium gives off enormous amounts of energy, and the star spends its life quietly fusing away. This process takes four hydrogen atoms to fuse into one helium atom. Hydrogen has only one proton, while helium has two protons along with two neutrons. This means that two protons are missing. Matter cannot be created or destroyed—it can only be turned into something else.

In this case, the two missing protons have turned into two neutrons. This energy is what makes the star shine and give off heat. Well.. after a while, the star has built up quite a bit of helium. This helium has found its way to the star’s center to create a helium core. Since hydrogen only has one proton, and helium has two protons and two neutrons, it’s heavier. That means the star has a little more mass in its core, which generates more heat. This heat builds up more and more, until it’s hot enough and has enough pressure to start fusing helium to carbon. This process generates a little less energy than fusing hydrogen to helium, but it still produces energy.


Star fusion (Credit: Nicolle Rager Fuller/NSF) (Source)

As a guideline, a star that has about one half the mass of the sun is too small and cool to fuse helium to carbon. So it will end up as a white dwarf made of helium. Stars between one half to four times the mass of the sun are massive and hot enough to fuse carbon to oxygen. Carbon and oxygen are fused more or less at the same time, and you’ll end up with a white dwarf made out of carbon and oxygen. I want to jump off topic here for a moment and ask a basic chemistry/physics question to all you readers. What happens when you introduce large amounts of heat and pressure to carbon? A girl’s best friend, diamonds!

The star has died and it’s a white dwarf made out of carbon: a giant diamond in the sky. Stars with masses greater than four times the mass of the sun are massive and hot enough to fuse oxygen to silicon. No, not the stuff they make implants out of, that’s "silicone" (sometimes, one letter can make a big difference).

Stars that have earned the title of "supergiant" are so massive and so hot that they begin fusing silicon to a solid core of iron. Once the star starts fusing iron, that’s it-- it’s doomed. Fusing silicon to iron takes more energy than it gives off. This means that the star is going to die soon; it is causing its own death by using more of its own energy than it is getting back from nuclear fusion.

Order of Nuclear Fusion in Dying Stars (Source)


When a star is fusing iron in its core, it's still giving off insane amounts of energy. The helium, hydrogen, carbon, oxygen, and silicon are still there in the star in different shells. Hydrogen is at the surface, still fusing to helium; a little further down, helium fusing to carbon and oxygen; further down we have silicon until the core, where silicon fuses to iron. This is why the star still exists and doesn't spontaneously explode the moment the first iron atom pops into existence.

At this point, the energy process is just no longer exothermic but endothermic. Iron cannot be fused into anything heavier because of the insane amounts of energy and force required to fuse iron atoms. The atomic structure of iron is very stable, more so than most other elements. I'm not saying all other elements are radioactive or unstable, just that iron is slightly more stable than the previous elements.

Stars this massive can turn into several things; it depends on how heavy it is. They can explode into supernova, collapse into various types of neutron stars, or even form a black hole. The iron in the star’s core isn’t the reason why the star went supernova, its overall mass made it explode. But, the iron in its core caused it to die.


Image Credit: NASA/CXC/SAO/JPL-Caltech

The rendering above illustrates the progression of a supernova blast. A star spends its life fusing hydrogen into helium. It then starts to fuse elements that are a bit heavier, leading up iron. Once iron comes into the equation, things get very bad very quickly. Suddenly, it's no longer able to sustain equilibrium, so its core collapses in on itself and it casts off its gaseous envelope in one fell swoop, sparking a supernova. Later on, the remainder of its gas gets energized by the core it leaves behind, either a pulsar or a neutron star (sometimes, if the star is massive enough, it leaves a stellarmass black hole behind instead), and it glows brilliantly for a time. We call these amazing things supernova remnants.



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