Thursday, December 19, 2013

Silver and Gold: Clues to the History of Our Solar System

It's that time of year when many of us are buying gifts for our loved ones.  And there's a song from the old Rudolph the Red-Nosed Reindeer TV special that comes to mind (sung by the character Yukon Cornelius):  "Silver and gold, silver and gold.  How many wonders can one cavern hold?..."    But have you ever stopped to think about where your jewelery came from?  I don't mean how your silver and gold was mined, but how was it created?  It's much more interesting than anything you've probably imagined!


Gold crystal (image from Wikipedia)

First, let's go back to the birth of the Universe:  BANG!  (That was the Big Bang.)  Sometime between 10 seconds and 20 minutes from now, the first atoms will be formed (this is called Big Bang nucleosynthesis).  These first atoms were of the simplest elements in the periodic table; mostly hydrogen and some helium and lithium.  Even today, the most common elements in the Universe are hydrogen and helium.  A few hundred million years later, clouds of this material will collapse to form the first stars.  This birth is marked by the ignition of nuclear fusion: the smashing of lighter elements together to make heavier ones.  All stars start by smashing 2 hydrogen (H) atoms together to make a helium (He) atom.  Eventually, the star will start to run out of hydrogen and will start to fuse helium to make beryllium (Be) and carbon (C).  The largest stars will continue fusing atoms together until the product element is iron (Fe).  For elements lighter than iron, smashing two lighter elements together to make a heavier element will release some energy.  For elements heavier than iron, breaking an atom into two lighter atoms will release energy: this is called nuclear fission.  So, the heaviest element that any star can normally make during its life is iron.  Heavier elements like silver (Ag), gold (Au), and platinum (Pt) and many other elements can only be created in the most extreme environments such as a supernova explosion that marks the death of a star.  Therefore, the metal in our jewelery is truly a relic of a star that died before our Sun was born.

Our Sun and solar system was born from the supernova explosion of an older star that died and, in doing so, seeded the material that makes up our solar system with heavier elements (this is the nebular hypothesis).  For example, since the Earth is not a star it cannot perform nuclear fusion.  Yet the core of our planet is primarily iron and nickel and had to be made in a previous star.  This is what Carl Sagan meant in his famous quote, "We are all starstuff."


Diamond set into a gold ring (image from Wikipedia)

 While I'm on the topic of the exotic origins of jewelery (and many other things), let me talk a little about why our Sun may turn into a giant diamond.

First off, a diamond is made from carbon which our Sun is capable of making much later in its life (more on this in a little bit).  Carbon is everywhere around us - pretty much every molecule that isn't water in your body has at least one carbon atom in it (this is the definition of an organic compound).  What makes a diamond special from all of the other common forms of carbon is that it has a very specific crystal structure that is only formed under extreme pressure and heat.  (Any mineral that has the same chemical structure but a different crystalline organization is called a polymorph.)  So, the diamonds that are formed naturally on Earth started as a carbon deposit that transformed into a diamond under the extreme pressure and temperatures found beneath the surface of the Earth.

What does this have to do with our Sun?  Well, right now the Sun is busy turning hydrogen into helium.  It will eventually (in about 5 billion years) start making heavier elements, but only much more massive stars ever have enough oomph (enough mass to create the most extreme pressures) to fuse elements all the way up to iron.  Our Sun is not one of those stars (which is good since those massive stars live comparatively short lives).  The Sun will only be able to produce up to carbon and oxygen (O) before fusion stops and the outer layers of the Sun are blown into the interstellar medium to seed future stars and solar systems.  What is left behind is called a white dwarf composed of carbon that has been exposed to intense pressure and temperatures.  That means that the white dwarf our Sun leaves behind may very well be a diamond!

A white dwarf that could be a massive diamond has already been observed and nicknamed "Lucy" (after the Beatles song "Lucy in the Sky With Diamonds").  It is more massive than the white dwarf our Sun will leave behind, but seems to be created by the same processes we expect for our Sun.  All of this is very promising for a diamond to be the memorial for our Sun (and solar system).


Directly, not much.  However, this past August it was confirmed that a short gamma-ray burst was the result of a kilonova, the collision of two neutron stars that releases a massive amount of energy (read more about this here and see my comments towards the end of the article).  Neutron stars are the remnants of dead stars that were much more massive than our Sun (about 10 to 40 times the mass of our Sun) that died in a supernova.  Those past supernovae would have created silver, gold, and everything heavier than iron and seeded the interstellar medium for future stars and solar systems.

These kilonova sources should produce copious amounts of gravitational waves, however, there has never been a short gamma-ray burst (like the one I mentioned above) detected close enough for LIGO to see it.  Once Advanced LIGO is complete (soon!) the closest short gamma-ray burst will be just on the edge of the distance we expect to be able to detect these kinds of gravitational waves.  But the great thing about what we do is that we really don't know how strong the gravitational waves from the actual explosion will be so they could very well be detectable in the future.

Fingers crossed!