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Q: Why does iron kill stars?

Physicist: Every now and again a physicist finds themselves in front of a camera and, either through over-enthusiasm or poor editing, is heard to say something that is “less nuanced” than they may have intended. “Iron kills stars” is one of the classics.

Just to be clear, if you chuck a bunch of iron into a star, you’ll end up with a lot of vaporized iron that you’ll never get back. The star itself will do just fine. The Earth is about 1/3 iron (effectively all of that is in the core), but even if you tossed the entire Earth into the Sun, the most you’d do is upset Al Gore. Probably a lot.

Stars are always in a balance between their own massive weight that tries to crush their cores, and the heat generated by fusion reactions in the core that pushes all that weight back out. The more the core is crushed, the hotter and denser it gets, and the more the rate of fusion reactions increases (increases the cores rate of “explodingness”), which pushes the bulk of the Star away from the core again. As long as there’s “fuel” in the core, any attempt to crush it will result in the core pushing back.

Young stars burn hydrogen, because hydrogen is the easiest element to fuse and also produces the biggest bang. But hydrogen is the lightest element, which means that older stars end up with a bunch of heavier stuff, like carbon and oxygen and whatnot, cluttering up their cores. But even that isn’t terribly bad news for the star. Those new elements can also fuse and produce enough new energy to keep the core from being crushed. The problem is, when heavier elements fuse they produce less energy than hydrogen did. So more fuel is needed. Generally speaking, the heavier the element, the less bang-for-the-buck.

Iron is where that slows to a stop. Iron collecting in the core is like ash collecting in a fire. It’s not that it somehow actively stops the process, but at the same time: it doesn’t help. Throw wood on a fire, you get more fire. Throw ash on a fire, you get hot ash.

So, iron doesn’t kill stars so much as it is a symptom of a star that’s about to be done. Without fuel, the rest of the star is free to collapse the core without opposition, and generally it does. When there’s a lot of iron being produced in the core, a star probably only has a few hours or seconds left to live.

Of course there are elements heavier than iron, and they can undergo fusion as well. However, rather than producing energy, these elements require additional energy to be created (throwing liquid nitrogen on a fire, maybe?). That extra energy (which is a lot) isn’t generally available until the outer layers of the star come crushing down on the core. The energy of all that falling material drives the fusion rate of the remaining lighter elements way, way, way up (supernovas are super for a reason), and also helps power the creation of the elements that make our lives that much more interesting: gold, silver, uranium, lead, mercury, whatever.

There are more than a hundred known elements, and iron is only #26. Basically, if it’s heavy, it’s from a supernova. Long story short: iron doesn’t kill stars, but right before a (large) star dies, it is full of buckets of iron.

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