Answers for Authors: Where do planet forming elements come from?

Above: A picture. Worth a thousand words? I don't know, but it's a galaxy, so it could be home to a lot of space aliens

They say a picture is worth a thousand words.

I’d argue that a picture of say, a horse, is worth much less than a thousand words of accurate advice on undetectably rigging the lottery. 

But that’s just me being contrary, as the picture we need to look at is actually a map(s) of our galaxy:

True, it looks like a Frisbee made of glowing mist, but that's actually over two hundred billion stars (one of which is our Sun), each with a complex relationship to all the others. We can simplify it down to three bits however: 

• The central bulge - very densely packed stars. 
  
• The disc/arms - sparser stars, of which our Sun is one. 
• The wispy halo - very sparse stars, but with occasional dense clusters. 

The bright clusters in the halo are things called globular clusters: Huge, round, families of old stars often separated by less than the width of our solar system. The central bulge also has a lot of old stars, and everything is very close packed, but with some very active areas of new star growth. The disk is mostly made of newer stars. 

Where’re the heavier, planet forming, elements concentrated on our map? 

Mostly in the central bulge, amongst the super jam packed stars there. There’s a fair bit scattered throughout the disk, but very little in the halo of globular clusters.  

Why?

That’s down to how elements heavier than helium were made: Our Universe formed (we think) with a lot of hydrogen, a fair bit of helium, and a tiny trace of heavier atoms – not nearly enough to form lots of planets, and our galaxy has around a hundred billion of those. 

The heavier atoms were mostly formed by hydrogen and helium atoms smashing into each other and fusing together – something that only happens under very hot and dense conditions, like a star’s core. The fusing together of these light elements to produce heavier ones gives off energy, which is how the star shines. Eventually it dies – which usually mean it blows off its outer layers - and the heavy atoms it’s forged get spread through the local dust an gas clouds. Those eventually form new stars, with the heavier elements mixed into them. 

It’s the circle of life! Except much bigger, and with stars instead of lions. 

"So, Mufasa, will you be eating me at the end of the song or do I make it to the second act? I am holding you're son, sire."

And no singing warthogs, because no one needs singing warthogs on that scale. 

Where it can, the process repeats – producing stars and nebula clouds with even more heavy elements, and eventually the concentrations can get high enough for rocky planets like Earth to form. If we look at it like that, it’s obvious the heavier elements will be found where there have been more generations of stars - and our galaxy started birthing stars in the central bulge, about 13.5 billion years ago.

Pwooooaaaaarrrr, check out that central bulge!

Then there was a mysterious pause, and then the stars began forming in the disk about 5 billion years later. That’s a pause longer than the age of Earth, so astronomers clearly still have some work to do here, but the point is:

• The galactic bulge is the oldest part of our galaxy where star birth is still going on, so there’s been lots of time for heavy elements to build up there – typically 2-3 times the concentration of heavy elements found in our Sun. 
• While the disk hasn’t seen as many generations of star growth, it’s seen enough for a reasonable amount of heavy elements to build up – planets are common enough, stars generally have 0.5 to 1 times the Sun’s metal content. 
• The wispy halo of globular clusters is the most heavy element poor bit of the galaxy – they’re incredibly ancient places**, but they haven’t seen much in the way of new star growth during their entire lifespan, so there’s not much built up there. 

But there’s a problem with that explanation: Stars make heavy elements by fusing lighter elements to make energy. So far, so cosmic. But iron, and elements heavier than iron, don’t produce any energy when they fuse: A big, hot, star that’s fused everything up to the point of iron is.... dead. It won’t get any energy from fusing iron, so it won’t make anything heavier, like say gold, lead, or plutonium. So where do these things – especially the really heavy radioactive elements - come from? 

They're actually ash from the most violent events in the universe: Supernova.

Above: The Crab Nebula, a huge debris cloud from a supernova.

Only the biggest and hottest stars can even get their fusing as far as iron. Big, super hot, stars only lead relatively short lives of a few hundreds of millions of years or less – so unlike most stars they never travel very far from the nebula where they’re born. When they die the supernova floods the inert iron 'ash' in the core with neutrons, which combine with the nuclei of the iron atoms to make heavier elements. 

For that reason supernova sites, like the Crab Nebula above, are hugely enriched in  radioactive elements (heavy atomic nuclei are much more likely to be unstable), and the gas clouds around the supernova all get a hefty dose too. 
Normals sized, long lived, stars and their solar systems form from those clouds, and drift away from the heavy element forming hotbeds. Over time their radioactive elements decay - that’s why our solar system originally had a lot more radioactivity – enough for things like the naturally occurring Oklo nuclear reactor to run 2 billion years ago. 

So that’s the general outline: Our galaxy has a heavy element rich middle bulge (which is a mix of very ancient metal poor stars and very metal rich young stars), a disk moderately well supplied with heavier elements, and a halo globular clusters made of very heavy-element-poor ancient stars. Scattered throughout both the core and the disk are star forming regions, which are the source of the unstable, radioactive, super heavy elements. 

If you’re looking for more specific locations where huge concentrations of super-heavy elements could be found though, here are some suggestions:

Neutron star planets: 
Whole worlds can be born from death on a cosmic scale - or, to put it a bit less melodramatically, from the remains of a supernova: When a central remnant, usually a neutron star, is formed it can hold a swirling disk of debris from the old stars core around itself. That disk can then undergo the same kind of transformation that the protoplanetary disks around young stars undergo, and produce planets. Massively rich in radioactive elements, and bathed in high energy radiation they're orbiting the superdense corpse of a dead star. They rejoice in names like Draugr, Poltergeist and Phobetor - because astronomers love an opportunity to dredge up the names of old, dark entities and put them to use.

Above: A map of a neutron star solar system.

These planets wouldn’t be black as night though: As well as the constant volcanic eruptions caused by radioactive heat, the neutron star itself would retain the heat from the supernova for a long time, illuminating them as an incredibly intense pinprick of very slowly fading light over millions of years. 

Neutron star collision sites: 
It’s not unknown for two supergiant stars to form as a binary system, then die and leave behind two orbiting neutron stars. Over time those neutron stars lose their orbital energy, and spiral in towards each other. The two city sized objects, each so dense that a pinhead of their matter would weigh more than a supertanker, spin around each other at nearly the speed of light, until they merge together. A neutron star can be thought of as a 20km wide atomic nucleus and the debris from two giant atomic nuclei colliding at nearly lightspeed is, unsurprisingly, incredibly rich in heavy elements. In fact if you’re wearing a piece of gold right now the odds are good that it was made in one of these insanely overpowered events.

Above: A simulation of colliding neutron stars, courtesy of NASA. Because what else is NASA going to do with a super computer?

Personally I'd love get a chance to learn more about these places... although, given the levels of radiation, hard vacuum, incredible heat, and general death... probably not by visiting.

* It’s not great place to look for aliens, because the jam packed stars make a stable planetary system hard to form, and the constant barrage of radiation makes it hard for complex life to develop, but it’s sure got a lot of heavy elements. 

**We’re not clear on why, or why they never made any new stars, but at least that’s keeping astronomers from roaming the streets in packs.

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