Iron: The Most Dangerous Element

What's the most dangerous element in the universe? You'd think it would be something radioactive like plutonium or polonium, or some kind of poison or neurotoxin. You might even argue it's Botox, which is a minute dose of the most deadly kind of food poisoning.

Radiation can destroy life in a wide area, and some toxins can kill even in tiny quantities. But strangely enough, it is a very ordinary substance, iron, which can kill a star.

In order to understand why iron is such a dangerous element, we first have to understand how stars work. They are a true miracle: natural nuclear reactors so massive that millions of Earths could fit inside them. However, unlike our nuclear reactors, which use the energy released by the breakdown of unstable elements (fission), stars work by nuclear fusion. Their immense gravity and heat force atoms to fuse together. When two atoms merge, a burst of energy is given off. The energy given off during the fusion process is an explosion of heat and radiation. These explosions keep the star from collapsing under its own gravity.

You can tell just how powerful these explosions are when you step outside on a sunny day. The sun is about 93 million miles away, and we are shielded by earth's ozone layer. Even so, you can feel the sun's heat on your face, and its radiation can burn your skin! That's one powerful space heater. Nonetheless, the sun is rather puny, compared to many stars. (It would be very hard for life to survive in the neighborhood of a massive star).

Stars begin their lives as a cloud of hydrogen and helium. These are the simplest elements in the universe. It's the hydrogen that serves as the fuel for stellar fusion: hydrogen atoms combine to form helium atoms. Eventually, all the hydrogen in the core of a star fuses into helium. With extremely small stars, the process stops here; they have insufficient heat and gravity to fuse helium atoms. They fade away as red dwarfs. But larger stars have a more spectacular finale.

With Sun-sized stars, when the core has fused into helium, there is enough mass and gravity for the core to compress further. A compressed stellar core heats up even more, just as refrigerated dough warms up when you knead it. The heat from this compression causes the outer layers of the aging star to expand outwards, forming a red giant. The core of a larger star fuses into helium first; the outer layers, still containing hydrogen, keep on burning. As the outer layers fuse into helium, this heavier element is drawn by gravity into the core of the star, adding more and more mass to it. There is now enough mass in the core for gravity to fuse helium atoms into heavier elements, carbon and oxygen.

The energy released by helium fusion is greater than that of hydrogen fusion, and the resulting explosions are spectacular. "Helium flashes" throw off bits of the star's outer layers in lovely planetary nebula. A sequence of helium flashes slowly send the star's outer layers flying out into space, until all that is left is the core, a white dwarf made of carbon and oxygen about the size of Earth. Fusion ceases, as there is not enough mass to fuse these elements. Slowly, the white dwarf burns, cools and dies.

Wait, what about iron? This dangerous element does not form in ordinary stars like the Sun. However, massive stars have a truly cataclysmic ending.

These monsters have much more mass. Therefore, the force of gravity at the core is so strong that they do not stop with carbon and oxygen. The next stage is to fuse into neon, sulfur, magnesium, silicon, phosphorous, and then iron. However, there's a problem with iron. It just so happens that the amount of energy needed to fuse elements into iron exceeds the amount of energy released by the fusion itself. What this means is that iron acts like a stellar vampire. Iron fusion absorbs energy instead of radiating it. The star starts consuming itself from the inside out. The heat and radiation emitted by fusion in the outer layers can no longer counteract the force of gravity, and the star collapses.

Bizarrely, iron can halt the collapse, but at a high price. Usually, the outer layers simply do not have enough mass to penetrate the dense iron-hard core, which is about the size of Earth. The implosion has nowhere to go. It reverses course and blows outward, instead! This is a supernova. The resulting shock waves are so powerful they can fuse atoms into elements heavier than iron. This is where all the other elements, like gold and copper and silver, come from, and it's why they're so rare. What's left behind is a neutron star: a city-size mass of stuff as dense as "cram[ming] all of humanity into a volume the size of a sugar cube." (Dr. M Coleman Miller, "Introduction to Neutron Stars")

If the star's mass is greater than five times that of our Sun, an even more terrifying thing happens: nothing can stop the implosion, and the star collapses into a black hole, from which no matter, energy, or even light can escape.

Either way, it's the formation of iron which sounds the death knell of massive stars.