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HET 611 - The Story of Gold
Paul Kohlmiller
Golden Opportunities
One of my favorite astronomy-themed t-shirts says “You are star dust”. It makes the point that we are
all made from long dead stars. Of course, everything around us is also made of star stuff. This even
includes rare elements such as gold. This paper discusses the processes of gold formation.
Most main sequence stars are in the business of making helium from hydrogen. Other elements can be
created through fusion (Chaisson & McMillan, 2002). However, this only works for elements as heavy as
iron or less. That’s because creation of elements heavier than iron requires additional energy rather
than producing energy.
For this reason we don’t expect to have stars creating the heavier elements in a typical situation.
Instead, some large amount of energy must be available to create these heavier elements. The process
of building up atomic nuclei is called nucleosynthesis (Bolte, et. al. 2000).
How Gold is Made
There is no process of nucleosynthesis that is unique to gold. Rather, all of the heavy elements are made
in roughly the same manner. The process involves building up the number of protons in the nucleus of
an atom. This means we need a supply of protons (or more likely neutrons as shown below) and the
energy to modify the atom.
What are Neutron-capture elements
Neutron capture is the process where a neutron is captured by the nucleus of an atom. This produces a
heavier isotope of the same element. This isotope is not stable and a neutron is likely to turn into a
proton in a process called beta decay. This changes the atom into a different element. Neutrons can be
captured more easily than protons because the charge on the proton causes it to be repelled by the
protons already in the nucleus of an atom.
s-process and r-process
There are two ways for neutron capture to occur. The two processes are called the s-process and the rprocess. The s-process is the slow process and the r-process is the rapid process. These processes were
largely demonstrated in a seminal paper by Burbidge et. al. (1957). This paper is so famous it is often
referred to simply as B2FH (the last initials of the authors).
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The s-process can produce some of the heavier elements but s-process elements tend to be much
lighter than gold. The s-process can occur during a star’s carbon burning phases (The et. al., 2007).
In the r-process, an iron (or heavier element) nucleus is bombarded with neutrons. A neutron is
captured but decays in less than 100 milliseconds. This beta decay results in a neutron being converted
into a proton. This creates a new element (NSCL Website, 2002).
Here is an example:
58
Fe + n
59
Fe
59
Co + e- +
(Sheffield University Website)
An iron atom captures a neutron and briefly becomes a different isotope of iron. Then a neutron goes
through beta minus decay (referred to simply as beta decay in this paper). This changes the neutron to a
proton plus an electron and an anti-neutrino.
Our solar system has been the benefactor of the r-process. Many rare earths including gold show that to
be the case. (See the list of s-process and r-process elements in Appendix A.) The evidence is that our
solar system was metal enriched by more than one supernova. Stirling et. al. (2006) showed that the
abundance of r-process elements can’t be explained by one r-process event.
Nickel-78 as a bottleneck in the r-process (but maybe not as much a
bottleneck as was thought).
One key problem has to do with Nickel-78. This element is thought to be key in the r-process. It was
originally thought to take longer to decay which would put a brake on the creation of heavier elements.
But more recently it has been shown that the decay of Nickel -78 is just 110 milliseconds (Dume, 2005).
Peak Abundances
The measured abundance of the r-process elements doesn’t always match up with the theoretical
rates(Muir, 2001). The solar system abundances of r-process elements has three peaks which
correspond to elements Selenium, Tellurium and Platinum. These results give “a great challenge to
supernova models” (Meyer & Brown, 1997). Also, these peaks do not correspond to elements that have
full electron shells. However, the elements produced in the r-process are not directly created by fusion
but rather involve decay. So the 3 peaks are equally distant and just below elements that have fully
populated electron rings (Sheffield University Website).
Supernovae or Neutron Stars
Despite knowing how the r-process should work, it is not certain when the r-process does work. It seems
certain that the r-process is related to supernovae, but how exactly? There are some candidates:
a) Magnetically controlled jets from massive stars.
b) The remnant star from a supernova can be a neutron star. The wind from the new neutron star
can increase the likelihood of a successful r-process.
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c) A pair of neutron stars will merge about once per 100,000 years in the Milky Way galaxy. The
effects of this rare occurrence are not completely known. The production of r-process elements
might be most efficiently done during a neutron star merger.
d) Another way to synthesize r-process elements is when a black hole is created in a collapsar.
e) The supernova itself. The energies unleashed in such an explosion are sufficient but there is
some doubt that supernovae can account for the abundances of r-process elements.
MHD Jets
A tightly focused jet of neutrons might create r-process elements. Calculations based on
magnetohydrodynamic jets were done for a star with 13 solar masses. In this scenario a supernova
explosion does not create a simple spherical outward shock wave but instead a magnetically focused jet
passes through oxygen rich layers which kicks off the r-process. The simulations agree with the
abundance of r-process elements (Nishimura, et. al., 2006).
Proto Neutron Stars
The idea that a neutron star is a good place for the r-process makes a good deal of common sense. If we
are going to capture a neutron, it has to be easier when a lot of neutrons are available.
Normally we think of Neutron stars as the end result of a supernova. But another kind of nova, called a
quark-nova, could create the greatest explosions in the universe. It’s possible that quark-novae are the
source of gamma-ray bursts (Space Daily Website, 2006).
For r-process elements, the key point about a quark-nova is that it is “a violent nuclear-quark phase
transition in the core of a neutron star that ejects neutron-rich matter at the surface” (Jaikumar et. al.,
2006). This matter decompresses creating a neutrino-driven wind. The resulting wave increases the
likelihood of r-process nucleosynthesis as shown by Metzger, et. al. (2006).
Neutron Star Merger
Neutron star mergers are rare. Two nearby stars have to both go supernova without one star gaining
enough mass from the other to form a black hole. Then the neutron stars revolve around a common
center close enough that they eventually merge. The result might create a black hole but a lot of
neutrons are available for capture (Freiburghaus, et. al. 1999). As in the Proto Neutron Star scenario, the
ejecta from the neutron star material and the expected abundances match what we see in the galaxy.
Collapsar
Heavy elements can be synthesized in collapsars. A collapsar is formed when a black hole is formed in a
specific way. The original massive star is rotating when it explodes but the explosion is incomplete
(Woosley, et. al. 1999). Collapsars (like the quark-novae described above) are a leading candidate for
gamma-ray bursts (Dooling, 1999). Fujimoto, et. al. (2007) found that heavy elements can be produced
in significant amounts in a collapsar. The total amount can be 1% of a solar mass. Some heavy elements
are created in a collapsar much more easily than in a supernova.
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Supernovae
All of the above involve the aftermath of a supernova. So the simplest answer to “How is gold made?” is
still supernovae. It has been reported that the dominant source of the r-process elements are Type II
Supernovae with progenitor masses between 20 and 40 solar masses (Chen, et. al. 2006).
But it is not certain if one or more of the above processes are necessary or if the supernova itself creates
the r-process elements. If a supernova explodes immediately it is called a prompt supernova explosion.
Is a prompt supernova explosion sufficient?
One model of a prompt supernova explosion describes a neutrino-heated bubble. Meyer et.al. (1992)
found that the neutrino-heated bubble model was hot enough to enable the r-process and to account
for the abundances of r-process elements that we see in the solar system.
There are a couple of specific supernovae scenarios that can create r-process elements. One case
involves a star with about 10 solar masses at the time of the supernova. Such a supernova gets two
chances to create r-process elements: in the prompt supernova explosion or in a neutrino-driven wind
that follows (Wheeler, et. al., 1998).
Another supernova study involved the explosion of an 11 solar mass star. This time the prompt
supernova explosion is the cause (Sumiyoshi, K., et. al. 2001).
Conclusion
The exact process that creates gold is not known. At a minimum, a star has to explode as a supernova.
The r-process elements are then created either in the supernova itself, in the ejecta from the resulting
neutron star or perhaps during a merger of two neutron stars. But each r-process scenario has some
disadvantage.
a) MHD Jets – “Although the solar abundance distribution is reproduced well, the results depend
on the degree of neutronization, which is very uncertain for multi-dimensional calculations”
(Fujimoto, et. al, 2007). Neutronization is the change of protons into neutrons – in this case
caused by neutrinos.
b) Neutrino-driven winds – requires an improbably heavy neutron star and the winds may not be
sufficient for the heavier r-process elements such as gold (Fujimoto, et. al, 2007). Also, neutrinodriven winds have an inhibitory effect on the r-process (Meyer, et. al., 1998).
c) Neutron star merger – the abundances of r-process elements in metal poor stars is inconsistent
with the number of Neutron star mergers that we expect to occur (Fujimoto, et. al., 2007). But
neutron star mergers are rare with only one such merger within a 60 million light year radius
every 1 to 20 years (Britt, 2003) or once in the Milky Way Galaxy per million years (Jaikumar, et.
al.) or once per 100,000 years (Lee, et. al., 2006).
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d) Collapsar – the collapsar scenario is a new idea and hasn’t had enough time to be attacked.
Previous studies showed that collapsars with slower rotations would not produce the heaviest rprocess elements.
e) Supernovae – “It is conceivable that only a limited fraction of supernovae create the hot spot …
This can be a reasonable explanation in that only a limited fraction of core-collapse supernovae
undergo the r-process nucleosynthesis, which is inferred from the spectroscopic analysis of
extremely metal-poor stars and the Galactic chemical evolution study” (Wanajo, 2006). Also, the
precise type of supernova is not known.
Gold is the result of supernovae. But the exact mechanism is unknown. It may be one or more of the
processes described here or perhaps another mechanism not yet studied.
References
Baruah, R., Duorah, H., Duorah, K., 2006, “The r-process nucleosynthesis in an expanding hot bubble in
supernovae explosion”, Supernovae: One Millennium After SN1006, 26th meeting of the IAU
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http://www.ucolick.org/~bolte/AY4_00/week8/nucleosynthesis.html (seen March 31, 2007)
Britt, R., “Neutron Star Collisions Common, Study Suggests”,
http://www.space.com/scienceastronomy/neutron_stars_031203.html (seen April 7, 2007)
Burbidge, E., Burbidge, G., Fowler, W. and Hoyle, F., 1957, “Synthesis of the Elements in Stars”, Rev.
Mod. Phys. Vol. 29
Chaisson, E. & McMillan, S., 2002, Astronomy Today, 4th Edition, Prentice-Hall, pp. 545-546
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Evolution of the Galaxy”, Astrophysics and Space Science, Vol, 306, pp. 33-39
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http://science.nasa.gov/newhome/headlines/ast02nov99_1.htm
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http://physicsweb.org/articles/news/9/4/6/1 (seen March 31, 2007)
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Jaikumar, P., Meyer, B., Otsuki, K., Ouyed, R., 2006, “Nucleosynthesis in decompressing neutron star
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Prompt Supernova Explosions Revisited”, ApJ, Vol. 562, pp. 880-886
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The, L.; El Eid, M.; Meyer, B., 2007, “s-Process Nucleosynthesis in Advanced Burning Phases of Massive
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Appendix A – s-process and r-process elements
The following table lists the primary elements created in the s- and r- process. One source is
http://www.astronomy.com/ASY/CS/forums/263967/PrintPost.aspx
Elements made in the s-process
Elements made in the r-process
Strontium – 38
Arsenic – 33
Yttrium - 39
Bromine – 35
Zirconium – 40
Ruthenium – 44
Niobium – 41
Silver – 47
Barium - 56
Tellurium – 52
Lanthanum – 57
Iodine – 53
Cerium - 58
Xenon – 54
Lead - 82
Cesium – 55
Europium - 63
Gadolinium – 64
Terbium – 65
Dysprosium – 66
Holmium – 67
Erbium – 68
Thulium - 69
Luretium – 71
Rhenium – 75
Osmium – 76
Iridium – 77
Platinum – 78
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Gold – 79
Thorium – 90
Uranium - 92
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Appendix B – Notes not otherwise used
Why elements with an even number of protons are synthesized at a higher
rate
Elements that have both an even number of protons and an even number of neutrons make up 90% of
the Earth’s crust. (Walter, 2002) The CRC Handbook1 lists the abundance of each element in the earth’s
crust. Look at the abundance of each odd number element from 57-79 compared to the even numbered
elements on either side of that element. In other words, we compare the abundance of element 57
(Lanthanum) to elements 56 (Barium) and 58(Cerium). In the 12 odd-numbered elements from
Lanthanum (57) to Gold (79), all but one is more rare than the elements on either side of it.
1
The CRC Handbook statistics were found at
http://en.wikipedia.org/wiki/Abundances_of_the_elements_%28data_page%29
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