Gold From A Galaxy Far, Far Away

Monday earlier this week the internet exploded with the news from ESO that two neutron stars merged in the galaxy NGC4993 located 130 million light years away. This was measured on August 17 this year and the discovery was therefore named GW170817, which is short for ‘Gravitational Wave on August 17, 2017’ as measured by LIGO in USA and Virgo in Europe.

The discovery is groundbreaking for several reasons:

  • It was the first time gravitational waves were measured from merging neutron stars. Until August 17 gravitational waves had been measured only four times and each time was from merging black holes. Gravitational waves tell us something about the mass of the merging system.
  • Neutron stars emit light (unlike black holes) and this is important, because light carry the information we have about the Universe. Light tells us something about the composition and content of any system in the Universe, including a neutron star merger. The light was also measured on August 17 and for the first time astronomers had access to gravitational waves AND light from the same system.
  • It had long been thought that heavier elements in the periodic system, such as gold or platinum, were created in neutron star mergers to a much higher degree than in supernova explosions. By measuring the spectrum from a neutron star merger, astronomers can directly observe this creation process – and not just have a theory.

Astronomers published more than 60 papers on arXiv after the ESO announcement on Monday. Some 3,000 scientists have worked on this discovery, and the observation data will keep them busy months and maybe even years into the future.

So, while astronomers are working on more details, let us look into how the gold in your ring is created and how it ended up on your finger.

Communication In The Universe

The fastest way something can move is the speed of light (roughly 300,000 km/s). This was something Einstein came up with in his theory of relativity about 100 years ago. So far, all measurements have confirmed this theory and even though there are still some unanswered questions (like what is dark energy?), the theory is consistent with observations. The consequence of this is that nothing moves fast than the speed of light – even gravity obeys this law. When something happens anywhere in the Universe we (on Earth) can not know faster than it takes for the signal to travel to us. If something happens 300,000 km away, we will know of the event 1 second after it happened. If something happens 150 million km away, then we will know after 8 minutes. This is why it takes 8 minutes for the solar light to reach us after it was emitted from the Sun’s surface.

The distances in the Universe are far too big for it to be efficient to measure distances in km. Therefore, astronomers use light minutes or light years. The sun is 8 light minutes away, because it takes 8 minutes for the sun light to reach us. Much easier to work with this unit, and astronomers do not have to spend time writing too many figures on the paper.

Some say the GW170817-source is 130 million light years away – others say it happened 130 million years ago. So who is right? The answer is: Both are right. The merger event happened 130 million years ago, because it took the gravitational wave signal 130 million years to travel to us.

And by the way: you are right! Gravitational waves and light are two very different things. But when it comes to travel speed they both propagate at 300,000 km/s, because both wave types are massless. In fact, the velocity of gravitational waves was confirmed by the GW170817, as the two wave types arrived within less than two seconds of each other.

The gravitational waves from the neutron star merger are ripples in space-time (whoa!) that propagates at the speed of light.

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So we can never know something faster than the signal can travel much the same way, that you can never get faster to work than your legs can walk or your car can drive. Why is it then called the speed of light and not the speed of gravity? For that simple reason that the physics of light was formulated way earlier than the physics of gravity.

Where Do Neutron Stars Come From?

The GW170817 had two neutron stars that merged into one object. Each of the two neutron stars used to be a normal star. This means a regular star to observe on the night sky – just like any other star. Their only special features were their masses, as each of them need to be above the threshold for highmass stars, which is estimated to be around 8 solar masses. A star above that mass will die in a violent supernova. A star with a mass below that threshold (like say our own Sun) will not.

Throughout its life a highmass star will produce up to iron in the periodic table of elements. This happens though nuclear processes in the core of the star, where the temperature and density is high enough for atomic nuclei to fuse. At some point the star will exhaust iron production and can not continue to produce elements with higher atomic number as the nuclear binding energy is too high for it to be favorable. After all, atoms are all about having the lowest possible energy. The star therefore explodes as a result of the contraction when the core fusion comes to a halt. A shock wave is sent out from the core as the star violently dies.

Stars that die in supernova explosions will send out massive amounts of material into space, leaving only a small central object called either a neutron star (as in the case with GW170817) or a black hole. Highmass stars typically live some 10 million years before they explode.

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For the GW170817 this needs to have happened twice as there were two neutron stars. It also needs to have happened sufficiently close for the two neutron stars to start their famous spiral dance. There is time enough. After the neutron star has been formed from a supernova explosion, it will stay this way for the next billions of years – until something happens to it, like for instance a kilonova explosion!

This is the new word in popular astronomy: kilonova. As the word sort of implies, it is similar to a supernova explosion but different.

A kilonova explosion can happen when two neutron stars spiral together and eventually merge into one object. The stars can orbit each other for years and years, slowly approaching one another until finally the inevitable will happen. They merge into one object. This does not go by unnoticed as both objects are very dense. A spoonful of neutron star will weigh as much as Mount Everest! When the Universe produces so extreme objects, then extreme events are bound to happen when two such objects interact.

In the neutron star case, the two stars come together and release a large amount of energy before they turn into a joint object. This could be a new neutron star or even a black hole.

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Gold Production From Kilonova

The kilonova is different from the supernova, because the origin of the explosion differs. A supernova originates from a regular star composed of hydrogen, helium and a few heavier elements up to iron. When the star explodes the production of new elements depends on which elements were there to begin with. In this perspective it should not be hard to imagine that when two gigantic balls of neutrons splash together the explosion will be different and so will the element production.

Is it exactly this difference that is thought to account for production of elements like gold or platinum. As material is shot in all directions during the explosion, new elements are formed in the turbulent and energetic cloud of excess energy and material. All the material that does not go into the new object (that is a neutron star or black hole) will be distributed in space with very high energy. It is in this process that new elements like gold is believed to be formed.

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Getting The Space Gold On Earth

The explosion is violent and short. The light from the kilonova is measured over a few days, before it disappears on the night sky. The explosion is over. Done. But all the material that was sent into space during the explosion is still hanging there in the space between stars: the interstellar space. The material is mostly gas mixed with dust and is called ‘the interstellar medium’ as it is large clouds of gas and dust distributed in the galaxy between the normal burning stars. Examples of such regions are The Pillars Of Creation or the Crab Nebula.

The cloud will contain all the atoms that were created in the kilonova explosion – likely mixed with gas and dust that were already present. These clouds are called nebulae or molecular clouds as they contain a broad variety of atoms and molecules. From hydrogen to gold, from water to alcohol.

The cloud is turbulent because everything in space continues to move until something stops it. At some point regions in the cloud can become so dense, that they start to collapse under their own weight. This is how stars are formed. This is how our Sun was formed. Most material from the region will go onto the star, but the leftovers – the material that did not make it onto the surface of the newly born star – will go into planet formation. So all the gold and platinum along with all other atoms that were produced with the kilonova explosion and eventually left in the nebula, will now be swept up by the clumps of rocks that will eventually become planets like the ones in our own Solar system.

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