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Iron: The Most Dangerous Element

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Supernova: The Death of a Star

Illustration: NASA/CXC/M.Weiss

Illustration: NASA/CXC/M.Weiss

What Do Stars and Faeries Have in Common?

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.

How Stars Work: Nuclear Fusion

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).

The Fate of Our Sun

The Cat's Eye Nebula is a beautiful example of a so-called "planetary nebula," thought by early astronomers to resemble a planet, but now known to be the death throes of a Sun-sized star.

The Cat's Eye Nebula is a beautiful example of a so-called "planetary nebula," thought by early astronomers to resemble a planet, but now known to be the death throes of a Sun-sized star.

Stellar Evolution: A Star's Life

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.

Supernova: "After" & "Before"

Supernova 1987a was the most massive explosion observed from Earth in 400 years.

Supernova 1987a was the most massive explosion observed from Earth in 400 years.

The Death of Massive Stars

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, phosphorus, 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.

NOVA: An Introduction to Stellar Fusion (Narrated by astrophysicist DeGrasse Tyson)

My Favorite Astronomy Book


Blackspaniel1 on January 08, 2015:

The problem with iron is it cannot undergo additional fusion and release energy. Past iron in the periodic table the elements release energy by fission. So, when a star reaches the iron stage, it runs out of fuel. I read above about impacts from meteors, which are not a problem as long as there is adequate fuel of lighter elements to undergo fusion.

James on July 23, 2014:

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Greekgeek has it right (not guessing at all). It is not the presence of iron which induces the supernova. It is the absence of any significant quantity of anything else to fuse together. Something else to fuse together means something that would produce more energy than it consumes. One must speculate (though I have not done the math) that if fusing two Aluminum atoms into Iron produces less energy than it consumes, that fusing three Aluminum atoms into Yttrium also requires more energy than it produces. Even adding a Helium nucleus to an existing Iron atom (producing Nickel) would have to produce less energy than it consumes. Only problem is I've seen stellar reaction equations that produce Nickel, so my suspicion is that the Iron story is just an approximation to the point where fusion can no longer produce more energy than it consumes.

MegannNicole from Florida on December 17, 2012:

Very interesting hub!

soundtrack junkie from New Hampshire on July 19, 2012:

great hub

Ellen (author) from California on July 09, 2012:

Kate Sisko -- unfortunately, I am not a stellar physicist, so I'm unable to follow your comment entirely. Perhaps some of my readers will be able to figure it out. Or perhaps you might be able to explain in laymen's terms? That would be great!

This page's audience is the general public, a bit like a Discovery Channel special or a planetarium show. My own mostly self-taught knowledge is the equivalent of astronomy 101, so I'm unfamiliar with the theorists you're disputing or supporting.

katesisco on July 08, 2012:

Current theory says that iron kills nuclear reactions so O Manuel's evidence is worthless: that Sol has ordered its elements in a controlled collapse, and that nuclear reactions take place on the surface as in W Thornhill's Electric Universe. Another rejected theory. The Earth is supposed to have a 4 sided iron core crystal. Since we do not have the energy to experiment with pressure we do not know for sure how iron is manipulated by temperature and pressure.

Bear in mind the current compression of our heliosphere in underway and NASA has even turned off the satellite that monitors such. You will find very little referencing compression except referencing our travels into a cloud of lighter or denser gas.

Ellen (author) from California on May 13, 2012:

That is an incredibly good question, and I should probably submit it to a planetarium (alas my grandma is no longer alive for me to ask).

My guess is that it's not really the iron itself that's the problem, but rather, it's the energy cost of the fusion reactions to make iron. The fusion reactions creating lighter elements like carbon release more energy than it costs to create them from fusing still lighter elements, whereas the reaction to make iron costs more energy than is released. A star is in constant state of collapse from its huge mass, but the equally huge explosions caused by fusion reactions keeps it superheated, counteracting gravity and pushing its layers out. Once it's fusing atoms to make iron, it's consuming energy instead of generating it, and not only do the fusion reactions to make the iron cause it to burn up faster and faster, but the heat and explosive power of the core stops being strong enough to counteract the force of gravity, and the star implodes.

Whereas if an iron meteorite falls into the sun, the sun didn't have to do any fusion reactions to create that iron, so I guess it doesn't harm the star. But I do wonder where that iron ends up. You're right... Shouldn't there be heavy elements in the core of the star?

Josh Barfield on May 13, 2012:

Iron meteorites posses the element iron. Our star, the Sun, has a mass that is 330000 times that of Earth. Tons of iron meteorites have impacted the Earth. The gravity of the Sun dwarfs that of the Earths.

How does the Sun (or any star) survive the colossal onslaught of iron raining down upon it?

It would seem that the surface of the Sun would posses the element iron and that the immense gravity of the Sun would pull the heavy element ever closer to its core. What prevents this?

Judy Specht from California on February 28, 2012:

Enjoyed your hub. The title is a great hook. Very interesting.

fordie on November 20, 2011:

Excellent story, well told. Now following

Ellen (author) from California on August 27, 2011:

Thanks for letting me know. I only had the information available on the video, which did not include the narrator credit. Adding now.

eregouf from Salem MA on August 27, 2011:

The voice on that film is that of Neil DeGrasse Tyson, a very smart astrophysicist and the author of the"Nova Science Now" television series as well as many books.

It's a shame that he not given credit!!

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