How We Found The Mother Lode - The Ultimate Source of Gold

A physics-based visualization of the effects of gravitational waves produced by a pair of neutron stars spiraling into and merging with each other. (Image from the Max Planck Institute for Theoretical Physics.)

Those little particles we are made of, and the world around us is made of - you know, all those atoms - where do they come from?

Thanks to our recently developed ability to measure gravitational waves, we are a big step closer to pinning down the origins of all the atomic elements.

I find that amazing, and satisfying.

I always want to know how things came to be as they are - how they originated and how the universe has evolved to the state it's at now. It almost seems serendipitous, like we got lucky, in discovering this new evidence of where the heavy atoms in the universe come from - but luck, as they say, comes to those who are most well prepared.

It all started with gravitational waves from a neutron star collision being picked up here on Earth. This is the first time a gravitational wave observatories have been able to combine forces with optical telescope observatories to track a single event, marking big advances in the sensitivity and utility of gravitational wave observatories while confirming theories for which we previously did not have such direct evidence.

Artist's rendition of two neutron stars colliding. (From Cosmos magazine.)

Neutron stars are the left-over cores of massive stars that have undergone a supernova and blown away their outer parts. They consist of pretty much nothing but neutrons and are very dense. Given how common it is for stars to occur in pairs that orbit around each other, it is not surprising there would be pairs of neutron stars in mutual orbit around each other. Because kinetic energy and angular momentum tend to be removed from the orbits of binary stars - including energy lost in the tides they cause in each other, and ultimately, as the two stars inspiral closer together and hurl around each other faster, the energy transferred to gravitational waves - it is likely a common fate for paired stars to have their orbits decay until they finally fall into each other.

The creation of heavy elements as a result of pairs of neutron stars collapsing into each other has been suspected as a possibility for years, but instead of just being theoretical, the creation of heavy elements has now been observed as it happens, starting with the neutron stars colliding with each other and merging.

On August 17, 2017, three separate gravity-wave observatories detected the merger of two neutron stars in another galaxy. Because the waves were measured by the two LIGO instruments in the United States and the Virgo instrument in Italy, each in a different place and in a different orientation relative to the incoming waves, scientists could narrow down the direction in space that the waves were coming from.

This shows the "chirp", the curve of rising frequency (waves per second) of gravitational waves coming from a pair of neutron stars as they spiral into each other, orbiting each other faster and faster as they get closer together. (From Abbott and others, 2017).

Astronomers around the world were quickly notified of this event, enabling them to focus their telescopes on that part of the sky in time to pinpoint a radiation source corresponding to the neutron star merger. They found a signal of light just like that predicted for the merger of two neutron stars, its pattern of spectral signals tracking the creation of heavy elements.

Data showing "light curves" at different wavelength bands (shown as different colors) observed by telescopes in the days after the neutron star merger. The dots are observed data. The curves are calculations for a theoretical kilonova. (From Arcavi and others, 2017)

The spectral signal of the light is that of a kilonova. A kilonova begins with a burst of super-powerful gamma rays and then emits a quickly peaking, gradually fading glow. Many of the atoms created during the event are unstable, and the fading glow results from the radiation they release as they turn into stable atoms. Most of the radioactive atoms have such a short life-span that the glow produced by their combined radioactivity fades away as the hours and days go by.

The details of the strength of different wavelengths in the kilonova's light, and how the light fades over time, can be used to fingerprint which types of atoms were produced by the neutron star collision, and how much of each type of atom was produced.

This was one of the biggest scientific breakthroughs of 2017.  See my blog post on the Nobel prize-winning breakthrough of gravitational waves being detected for the first time, waves that originated from the merger of two black holes. Think of what our ability to sense and study gravitational waves has now confirmed:

1. Einstein was correct about the existence of gravitational waves, a fundamental prediction of his general theory of relativity
2. black holes are real, and pairs of black holes occasionally collide
3. neutron stars exist, and pairs of neutron stars occasionally collide
4. a kilonova is the result of two neutron stars colliding
5. many of the heavy elements, including gold, originate in neutron star collisions

The merger of two neutron stars is now thought to be the main process that fuses lighter, smaller atoms into atoms that are bigger and heavier than iron, copper, and zinc. These heavier elements include the precious metals gold and platinum, along with other heavy hitters such as lead, uranium, and the rare earth elements.

Schematic of heavy elements created from  a single star exploding on the one hand (top sequence), and two neutron stars colliding on the other hand (bottom sequence). (From Quanta magazine.)

Because the heavy elements include gold, and gold gets people's attention more than an element as mundane as lead, or one as little-known as bismuth, this breakthrough was publicized in the media as the discovery of the origin of gold. It has been calculated that about 10 Earth masses of gold and platinum were created in the neutron star merger, according to a quote from LiveScience.

Supernovas, in which massive stars collapse explosively into themselves and spew material out into their surroundings, also create some of the heavier elements and seed space with them. Many observations have been made of supernovas and their evolving afterglows, confirming their role in seeding interstellar space with heavy elements.

For a few decades starting in the 1950s, it was thought that supernovas were the one big source of all the heavy elements. That is what I learned in my astronomy classes, a few light-years ago (OK, light years are a distance unit, not a time unit, so make that a few years ago).

The Crab Nebula, an expanding nebula created by a supernova that was observed by people on Earth in 1054 AD. At the center of the nebula is a pulsar, a rapidly spinning neutron star, the leftover core of the star that exploded. (Image from Wikipedia article, "Supernova")

However, as the theoretical models and the power of computers advanced, it became clear that supernovas of massive stars could not produce all the heavy elements. Another source for most of the heaviest elements was needed, a source producing huge amounts of neutrons mixing with not-quite-so-heavy atoms to forge the heaviest atoms, in a process having the right range of particle flux and temperatures.

Theoretical models suggested the merger of a pair of neutron stars would fit the bill. But until now, this process has never been clearly observed and shown to happen as predicted.

Based on the new observations, Jennifer Johnson at NASA re-did the calculations to show the updated results on how much each type of atom comes from the merger of neutron stars, along with the contributions of other astronomical processes. Her results are shown in the periodic table below.

I have heard that some people find the periodic table inscrutable and off-putting, tainted by bad memories of having to know how to decode it for a chemistry test that they did not love the idea of, and I can empathize.

However, in this case the periodic table is being used to show the origins of the elements. I think it's wonderful to be able to just look at the colors in the table to see how each element was made.

The orange in the periodic table shows how much of the heavy elements have been forged in neutron star collisions. That's a lot of orange!

From the Sloan Digital Sky Survey blog: http://blog.sdss.org/wp-content/uploads/2017/01/periodic_table.png

The creation of the chemical elements all started with the lightest elements - hydrogen, most of the helium, and some of the lithium - coming into existence at the known universe's beginning, when energy from the big bang condensed into matter.

It has continued since then mainly from stellar processes, including fusion of atoms in the cores of stars, the stately demise of low-mass stars like our Sun, the collapse and explosion of massive stars in much-more-powerful supernovas, the mergers of pairs of neutron stars, and so on.

Over eons of time, the heavy atoms created by astronomical processes have spread out and become part of the interstellar medium. This is the material that clumps and condenses to form new stars and planets. That is why younger stars and planets have significant amounts of the heavy elements. Enough to make rocky and metallic planets. And enough to make wedding rings of gold or, as has been somewhat fashionable lately, platinum.