Date: July 25th, 2012
Title: All the way up to Iron
Description: We all know that Stars fuse Hydrogen into all the other elements in the Universe. But most descriptions of this process just mention a few elements like Helium, Oxygen, Carbon, Neon and Silicon and then jump to “all the way up to Iron”.
If you’re like me you’ve wondered just what ALL the steps are in between Hydrogen and the all important Iron.
So I broke my brain researching in turn how each element is manufactured, so that you don’t have to.
Bio: Proud owner of a VMT (Very Medium Telescope) with an interest in all things Astronomical. I live in Adelaide, South Australia where if there’s anything interesting to look at in the sky, you can guarantee it will be cloudy. I’ve contributed readings to Steve Nerlich’s CheapAstro podcasts and thought I’d give it a bit of a whirl myself.
Today’s Sponsor: This episode of “365 days of Astronomy” is sponsored by iTelescope.net – Expanding your horizons in astronomy today. The premier on-demand telescope network, at dark sky sites in Spain, New Mexico and Siding Spring, Australia.
Hi, my name is Julia and this is my second contribution to the 365 Days of Astronomy Podcast.
Instead of choosing a subject for my podcast that I already know and want to share – I have instead picked a subject that I’m curious about but otherwise know absolutely nothing.
This means I’m likely to break my brain, but at least I’ll learn something and I hope you all do too.
Last time I tried to figure out what objects in the Solar System you can jump off (if you missed that one, it was back on December 26, 2011) and this time I wanted to find out why everyone out there who explains how the Stars make the elements go into great detail about Hydrogen fusing to Helium, but then just skip to Carbon and Oxygen, then to Silicon and finally “all the way up to Iron” missing all the steps in-between.
Don’t believe me? Have a listen: (“All the way up to Iron” clips from other podcasts).
So . . . if you’ve been listening or watching the same podcasts and TV documentaries as I have, you’ll know that Stars are element factories. They take a great big wad of Hydrogen (the single most popular element in the Universe) and by sheer force of mass, gravity and heat fuse it into the other elements.
Most stars like our Sun or smaller just do good old basic Hydrogen burning. And that’s pretty much all they do, they just aren’t massive enough to go much further.
But the bigger stars – they go “all the way up to Iron” and then if you get a nice Supernova going, you get the really kewl stuff beyond Iron, like Gold. Shiny!
So . . . why does everyone skip most of the juicy stuff in-between Hydrogen and Iron? Is it too boring? Like singing the this bone connected to that bone song? Or does it just get a bit too complicated?
Being someone who likes to torture themselves with science that’ll make your head spin, I went off to do some research.
Well – it’s in the “complicated” camp. It’s not just a plain linear path, there are multiple off-shoots and back-flips and it looks more like a family tree than a straight hierarchy. And a family tree where a lot of inbreeding and polygamy has taken place to boot. Also, there’s usually more than one, and sometimes several ways to make each element.
I’ll give it a bit of a go anyway. To keep things simple I’m going to concentrate on the main stable “isotope” of each element and give the most common (or the easiest to find) method of production. Some elements are a mystery and some seem to be a bit contentious – so for those I just went with what I could pry out of the multitude of extremely hard to read scientific papers I managed to get my hands on.
First up is Hydrogen, which is basically 1 Proton and 1 Electron. All the Hydrogen in the Universe was formed in the first three minutes after the Big Bang.
Hydrogen then fuses to form Helium. Well first it fuses into Deuterium and then again to Helium3 and again to Helium4 (which is the first stable/most common Isotope we’re after). But what I was surprised to learn is that in actual fact MOST of the Helium4 in the Universe was also formed in the first three minutes after the big bang. Sure, stars do a bit of that too – but the vast majority of ALL the Helium? Big Bang.
Which leads us to the next element – Lithium. Lithium7 being the most common stable Isotope. This is formed by a Helium3 and a Helium4 getting together. A small amount of Lithium7 is produced in stars but it is burned up and used as fuel about as fast as it is formed. So, most of the Lithium7 in the Universe was also formed just after the Big Bang. And really only because the process stopped after those magic three minutes, if it hadn’t then it would have been burned up then too.
Beryllium9 is our first odd one. It wasn’t created just after the Big Bang and it’s not produced in stars, at least if it is it doesn’t last very long at all. It’s actually produced via “cosmic ray spallation” and whilst that sounds like a medical procedure you’d best to avoid, it’s actually where cosmic-ray particles hit interstellar atoms and break fragments off, making a lighter element. So once you get Carbon and Oxygen out there in the Universe, they get whacked and produce Beryllium. Hence this element is very rare.
A similar process is what gives us Boron11. It’s basically a Carbon12 atom with a nucleon knocked off, which happens in the atmosphere of a star as it goes nova. Type II supernova give off loads of neutrinos which do the trick.
And now we’re up to Carbon12. Which is made with 3 Helium4s. This is known as the Triple Alpha Process, as Helium is also referred to as an alpha particle. Actually the process is a bit more complicated and includes a Beryllium8 popping into and out of existence in the middle, but it’s still essentially 3 Helium4s.
Next up is Nitrogen14. A Carbon12 fuses with Hydrogen and creates Nitrogen13, but this is unstable and decays to Carbon13, then that Carbon13 in turn fuses with Hydrogen and thus creates the stable Nitrogen14.
Oxygen! This one is nice and simple. A Carbon12 fuses with a Helium4 and we get Oxygen16.
Which is good because the next one broke my brain.
Fluorine. I really like the one theory that Fluorine19 is produced by a Neon20 getting hit by a neutrino, and knocking off a proton leaving Fluorine19. But that idea whilst lovely and simple to describe apparently is lacking firm evidence to support it. Instead it goes something like this (Note: I had to get a real live Astronomer to help me with this one – thanks Ken!):
A Carbon13 fuses with Helium4 creating Oxygen16 plus a neutron.
That neutron impacts Nitrogen14 to create Carbon14 plus a proton.
Remember that proton, it joins the party again in a moment.
Meanwhile the Nitrogen14 fuses with Helium4 to create Fluorine18 which is unstable and decays into Oxygen18 plus a positron.
Then the Oxygen18 invites that proton back to the party and it transforms the Oxygen18 to a Nitrogen15 plus a Helium4.
The Nitrogen15 and Helium4 get back together and make Fluorine19.
Okay, onwards now to Neon. Oh goodly another easy one. Oxygen16 fuses with a Helium4 and gives us Neon20.
By now you might be noticing that good old Helium4 is quite the player . . . and you might also be noticing that therefore the nice, simple stable isotopes have a isotope number in increments of 4. You’d be right.
Sodium23 is produced via 2 Carbon12s fusing, producing a Sodium23 plus Hydrogen. You can also get to Sodium23 via Neon22 fusing with Hydrogen, though only about 10% is produced that way.
Next up is Magnesium24 (Yah – a four!). Again nice and simple – this is formed via a Neon20 and a Helium4.
Aluminium’s stable isotope is 27 and this is produced via Magnesium26 fusing with Hydrogen.
Of course first you have to get to Magnesium26 and that’s another story, it goes something like this:
Carbon12 + Carbon12 creates Neon20 plus Helium4. That alpha particle then gets it on with a Neon22 producing Magnesium25 and a neutron, the Magnesium25 then captures that neutron and becomes Magnesium26.
So you see, we’re only half way there and already it’s getting quite complicated. Once you get past Carbon you have all sorts of reactions taking place. Knocking of neutrons and protons left right and centre (and of course not forgetting that all these reactions also give off all important energy), then recombining those into different isotopes of each element which again combine into others. If you’re interested in the specifics I’d recommend Donald Claytons “Isotopes in the Cosmos” handbook, but in order to avoid getting too bogged down here, I’m just going to move on and hope you’ll forgive me for not going over how each and EVERY isotope of each and every element is created. Let’s just assume that once we get past each element all the other versions of it happen too, otherwise we would literally be here all day.
Moving on – number 14 on the table is Silicon28. It’s a nice, simple combination of Magnesium24 + Helium4. Or, since we have a bunch of stuff to play around with by now I also heard a rumour that Carbon12 + Oxygen16 can also produce Silicon28, as well as 2 x Oxygen16s producing a Silicon28 + a Helium4.
Next up is Phosphorus31. Which is the only isotope of Phosphorus stable or otherwise. I had trouble finding details about this one, but I did eventually find a reference to it being formed from either:
Sulphur32 is formed via the process of two Oxygen16s forming a Silicon28 + a Helium4 which then re-combine to form Sulphur32.
Chlorine35 relies on the previous process for Sulphur32 which gets together with a couple of Hydrogen buddies for a night on the town. They wake up the next morning in a cheap motel room as Sulphur34 which then calls another Hydrogen buddy to come and pick them up and you get Chlorine35.
Argon36 avoids the cheap motel room altogether as it’s simply formed from Sulphur32 and a Helium4. No night on the town necessary.
Potassium39 (whose symbol is K, which is a bit confusing seeing as Potassium starts with a P and doesn’t even have a silent K in front. Apparently it’s from the Latin word for Alkali, being Kalium. Which makes me wonder why they didn’t just call it Kalium in the first place). Anyhoo this is produced via 2 x Neon20s forming Potassium39 plus a proton.
Calcium40. Gosh there seems to be dozens of different ways of getting there. Here’s a few:
Scandium45. This was the first of a few elements that made me tear my hair out reading those Scientific papers. I won’t bore you by reading out those passages, but let’s just say that in between the squiggles and parenthesis I managed to pry out only a small amount of decipherable English. So, Scandium45 is produced via a Calcium40 + a Helium4 producing Titanium44, which is unstable so it grabs itself a proton to steady itself and in the process becomes Scandium45.
Oh, did I mention Titanium? Well Titanium48 is next on the list. That same Calcium40 + Helium4 fusing into Titanium44 happens, but this time the unstable Titanium44 grabs a Helium4 becoming Chromium48 which is also unstable, it then undergoes two beta decays after ejection from the core and becomes Titanium48.
Vanadium51. This and the upcoming Manganese55 gave me all kinds of trouble. Both are formed via beta decays of heavier elements. In fact all I could initially find was that Vanadium51 is a daughter of Manganese51 after decay, but nowhere did the authors seem to think it might be important to include how you get to Manganese51 in the first place. Anyhoo, near as I can gather either Chromium50 captures a proton to become Manganese51 which decays to Vanadium51 or it’s a rather long-winded process starting with Chlorine51 and proceeding via decay from each element51 in turn until you get to Vanadium. In any case, it was definitely here where I ALMOST decided to give up and just say “all the way up to Iron”, but since I’m SO close – onwards I go.
Chromium52. This again starts with that process of Calcium40 + a Helium4 producing unstable Titanium44, and again the Titanium44 grabs a Helium4 and does a do-si-do into Chromium48, but this time instead of decaying and becoming Titanium48 it grabs another Helium4 and becomes . . . wait for it . . . Iron52. So, um, we seem to be up to Iron. Don’t we stop here? Well actually no – it’s STABLE Iron that chokes the fusing process and that’s Iron56, we’re only at Iron52 here so we can still keep going. Iron52 is unstable and just happily decays to Chromium52. So no, we’re not quite there yet.
But we are at the penultimate element – Manganese55. As with Vanadium51 all I could find is that it’s the daughter of Cobalt55, but nothing about how to get to Cobalt55 (especially since it’s the next element after Iron and therefore the decay process goes through Iron55 before getting to Manganese55). I did manage to find a nice little chart that starts at Potassium55 and likewise proceeds through all the element55s to Manganese, so I’m afraid that will have to do.
And so here we are – all the way up to Iron. And as with the previous few elements we have to go above Iron to get back down to stable Iron and it goes something like this:
Chromium48 + Helium4 = Iron52 which is unstable, this time however the Iron52 grabs its own Helium4 and becomes Nickel56 which is also unstable. The Nickel56 decays to Cobalt56 which again is unstable and so it in turn decays to Iron56 which is stable.
And there you have it. Once you get to Iron56 no more fusion takes place and you get core collapse and then all sorts of explody supernova stuff, and only then do you end up with the stable isotopes of the heavier elements.
So next time you listen to a podcast or read an article where the author says “all the way up to Iron” you can smile smugly, nod and say to yourself (or to anyone who’ll listen) – “yeah, I know how that goes”.
End of podcast:
365 Days of Astronomy
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