Au-some!

This past month there have been numerous stories about how all the gold in the universe was created.  Just google "gold neutron star", and you'll see what I mean. So being me, I wanted to read the original research: An R-Process Kilonova Associated with the Short-Hard GRB 130603B, E. Berger, W. Fong, and R. Chornock of the Harvard-Smithsonian Center for Astrophysics. (1)

I first gave it a cursory read. I couldn't find a mention of the creation of gold anywhere, so I took advantage of technology and used the Find command in my browser. Nothing there, but then I realized why. Someone at Harvard must have written a press release, and in order to get the media interested in the research - which is legitimately important - they  made it sexy and what is sexier than gold.

So I sat down to read the kilonova paper carefully. I didn't have to read far before I read the phrase r-process and Justin Huang's graduate level nuclear physics class came to mind. [Although r-process is in the title, my mind didn't click in until later.]

First some background for those of you not nuclear or astrophysicists. After a microsecond, protons and neutrons formed and couple of minutes or so later, the universe had cooled enough for primordial deuterium, and helium nuclei to form.  Many years later, 379,000 actually, electrons combined with these nuclei to form atoms. It wasn't until a hundred or so million years that stars began to shine.

CREDIT: Atmospheric Imaging Assembly (AIA 304)
of NASA's Solar Dynamics Observatory (SDO)

Fred Hoyle and his colleagues in a classic paper (2) showed how the elements other than hydrogen and the primordial helium were created. I won't go into a long explanation of stellar evolution, but the elements up to iron were created by nuclear fusion. The Sun is fusing two hydrogen nuclei into a helium nucleus and this process releases a lot of energy (we call it sunshine). I'm oversimplifying here, but when the hydrogen in our Sun is depleted in about 5 billion years, it will start to fuse helium. When the helium is gone, our star will be a very hot ball of carbon and oxygen. The Sun will be a white dwarf - basically a very large and very hot diamond in the sky. 

Stars more massive than the Sun will keep fusing and shining up to the point where iron is formed. The process stops there because to fuse iron and elements heavier than it requires an input of energy.

If you now look at a periodic table, you should wonder where did all the other elements like gold come from. There are two processes that are responsible for these heavy elements: the r (rapid) and s (slow) processes. The s-process cannot create elements heavier than lead and this method occurs in old giant stars. 

The Periodic Table of the Elements with hydrogen, helium, iron, and gold highlighted.

Hoyle et al. suggested that the r-process occurred in type II supernovae. This sort of supernova comes about when the thermal motion of the nuclei is not enough to counteract the effect of the star's self-gravity. The core of the star collapses and the outer layers are violently ejected. Depending on the initial mass of the star, the result of the core collapse is either a neutron star or a black hole.

The problem with this hypothesis is that given the abundance of the elements either most supernovae did not expel much matter or that supernovae ejects only a small amount of the material. Given the intensity of a type II supernova, either possibility seems unlikely.

The Crab Nebula, the aftermath of a supernova.
CREDIT: NASA/ESA

One possibility was the collision of neutron stars (3), and here we're back to the kilonova paper. Berger and his co-workers present evidence of "the presence of excess near-IR emission matching the expected brightness and color of an r-process powered transient (a kilonova)" and the inferred mass ejected during the collision "matches the expectations from numerical [neutron star] merger simulations."

My head exploding.
CREDIT: Gil Thorpe

Look at that ring on your finger or the chain around your neck. Then imagine two neutron stars colliding. 

References:

(1) An R-Process Kilonova Associated with the Short-Hard GRB 130603B, E. Berger, W. Fong, and R. Chornock, http://arxiv.org/abs/1306.3960v2.

(2) Synthesis of the Elements in Stars, E.M. Burbridge, G.R. Burbidge, W.A. Fowler, and F. Hoyle, Reviews of Modern Physics, Vol. 29, No. 4, pp. 547 - 650 (1957).

(3) R-Process in Neutron Star Mergers, C. Freiburghaus, S. Rosswog, and F.K. Thielemann, The Astrophysical Journal, Vol. 525, No. 2, pp. L121-L124 (1999)

The United States Bullion Depository, Fort Knox, KY, shown here in a scene from the James Bond movie Goldfinger.
CREDIT: United Artists/Eon Productions