Iron vs. Cold Iron?

[LokiDR]

Actually, as I read it, the heavier elements are byproducts of having insufficient hydrogen to fuse into helium. It isn't an orderly process, the hydrogen just undergoes fussion much easier.

 

[MerakSpielman]

Something like that.

I seem to remember it being possible for a star to have multiple layers, each using a different element to fuel the fusion reaction there. The visible part of a star is generally fusing hydrogen, because it requres the least amount of energy to ingnite. As you get deeper into the star, the hydrogen has all fused into other elements, which, if possible at the density/temperature, have also ignited. Lots of stars end up with an iron core.

Of couse, nobody has actually managed to get core samples of a star, so I have no idea where scientists come up with this stuff, or even if I'm understanding it even close to correctly. I'm an English major, not a astrophysicist.

 

[Darklone]

Fusion may create all kinds of elements till a certain mass number, it's just statistically distributed and this distribution is influenced by temperature and pressure in the star...

 

[Spatzimaus]

The visible part of a star is generally fusing hydrogen, because it requres the least amount of energy to ingnite. As you get deeper into the star, the hydrogen has all fused into other elements, which, if possible at the density/temperature, have also ignited. Lots of stars end up with an iron core.

Almost. I'll try an explanation; let's see if I remember it all correctly.

When stars first form, they're what we refer to as "Main Sequence" stars. They burn hydrogen in their centers, forming helium which sinks to the core; they stay the same size and brightness (more or less) throughout this entire time. The outer part of any star is never actually part of the fusion process; it's just the innermost parts.
The bigger the star, the shorter the lifespan. Our sun might live for another five or six billion years, but the biggest stars last only millions. The reason is, the center is hotter (more pressure pushing down), so it burns much faster. Eventually, you'll reach a point where the center is all helium. The outer areas will still be hydrogen; for our sun, it's estimated that when it dies it'll still be something like 60% hydrogen.
At that point, the star starts expanding, turning into a red giant star. One of two things will happen:
1> If the star is big enough, the temperature at the center will become high enough to fuse helium into carbon (it usually skips over lithium and boron, and only uses beryllium as a short intermediate step in the process). You see something referred to as a "helium flash", and the star shrinks back down to something approaching its normal size.
2> If the star isn't big enough (ours isn't), a certain type of supernova occurs, most of the remaining hydrogen is blown off, and you're left with a slowly-cooling ball of gas called a "white dwarf". It still glows because it has a lot of energy stored inside, but it's not actually undergoing fusion.

Let's assume #1 occurs. The star starts burning helium in the center, and probably starts fusing hydrogen into helium in a "shell" around the core. If the star is only kinda big, it'll stop after that, make a supernova of its own, the end.

If it's REALLY big, it'll then start turning that carbon into nitrogen, oxygen, etc. right up until it hits iron (at which point it'll probably be burning seven different elements in multiple layers, what's referred to as the "onion-skin" effect). Iron is the largest element that can be formed through nuclear fusion. The reason has to do with the size of its nucleus, but basically it's the last element where you can gain energy by fusion; after that point, it costs you too much energy to add another particle to the nucleus. So, once the core is full of iron it can't fuse any more, and eventually explodes in a really big supernova (then turns into a dwarf, black hole, or neutron star), spewing metals all over the place.

So, iron (mostly) comes from the fusion processes of the biggest stars. Since the biggest stars have the shortest lifespans, they can still explode while other stars are beginning to form. In our case, a large star must have gone supernova nearby while our sun was still forming, dumping a large amount of metal into the gas cloud our solar system formed from. As a result, our sun is unusually rich in metals (about 2% of its mass is heavier than helium, which is much higher than normal).

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As to the original question: in Real Life, "cold iron" was just any iron worked cold (shaped by brute force), without using a forge to warm the metal up to where it was easily malleable. In D&D 3.5E, cold iron is made from a rare variety of iron ore, which can only be worked cold. Not quite the same thing.

 

[CRGreathouse]

IIRC, from my Astronomy 101 class (read: not an expert), the fusion process in stars "burns" Hydrogen (atomic number 1) to create helium (atomic number 2). A higher temperature (greater starting mass) is requred to ignite helium to get lithium (atomic number 3). Lithium, in turn, can be burned to get Beryllium (atomic number 4). And so on and so on until you reach iron (atomic number 26). The regular fusion reaction of a star cannot create elements heavier than Iron. All elements heavier than iron in the entire universe are created in (super)novae, when the heat and pressure reach, momentarily, the point needed to fuse fundamental particles into such large atoms.

More or less... as Spatzimaus said, many elements are often skipped. Also, fusion can create elements heavier than iron, but there's a probem. Iron is the saddlepoint of energy (not sure how to say this), so making anything heavier with fusion, or anything lighter with fission, takes more energy than it creates.