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Whence Gold?

 

Neutron Star Merger: NASA Goddard Space Flight CenterBy the time I was in high school in the 1960s, the origin of the chemical elements seemed pretty clear. Hydrogen was created in the Big Bang, and very shortly afterward about one quarter of it fused to make helium with a little bit of lithium. (This process is now called Big Bang nucleosynthesis, and models of it agree very well with astronomical observations of primordial gases in the universe.)

All of the heavier elements, including the carbon, oxygen, and nitrogen which, along with hydrogen, make up our bodies and all other living things on Earth, were made in stars which fused hydrogen into these heavier elements. Eventually, the massive stars fused lighter elements into iron, which cannot be fused further, and collapsed, resulting in a supernova explosion which spewed these heavy elements into space, where they were incorporated into later generations of stars such as the Sun and eventually found their way into planets and you and me. We are stardust. But we are made of these lighter elements—we are not golden.

But, as more detailed investigations into the life and death of stars proceeded, something didn’t add up. Yes, you can make all of the elements up to iron in massive stars, and the abundances found in the universe agree pretty well with the models of the life and death of these stars, but the heavier elements such as gold, lead, and uranium just didn’t compute: they have a large fraction of neutrons in their nuclei (if they didn’t, they’d be radioactive [or more radioactive than they already are] and would have decayed long before we came on the scene to observe them), and the process of a supernova explosion doesn’t seem to have any way to create nuclei with so many neutrons. “Then, a miracle happens” worked in the early days of astrophysics, but once people began to really crunch the numbers, it didn’t cut it any more.

Where could all of those neutrons could have come from, and what could have provided the energy to create these heavy and relatively rare nuclei? Well, if you’re looking for lots of neutrons all in the same place at the same time, there’s no better place than a neutron star, which is a tiny object (radius around 10 km) with a mass greater than that of the Sun, which is entirely made of them. And if it’s energy you’re needing, well how about smashing two of them together at a velocity comparable to the speed of light? (Or, more precisely, the endpoint of the in-spiral of two neutron stars in a close orbit as their orbital energy decays due to emission of gravitational radiation.) Something like this, say.

This was all theory, until today. At 12:41 UTC on 2017-08-17, gravitational wave detectors triggered on an event which turned out to be, after detailed analysis, the strongest gravitational wave ever detected. Because it was simultaneously observed by detectors in the U.S. in Washington state and Louisiana and in Italy, it was possible to localise the region in the sky from which it originated. At almost the same time, NASA and European Space Agency satellites in orbit detected a weak gamma ray burst. Before the day was out, ground-based astronomers found an anomalous source in the relatively nearby (130 million light years away) galaxy NGC 4993, which was subsequently confirmed by instruments on the ground and in space across a wide swath of the electromagnetic spectrum. This was an historic milestone in multi-messenger astronomy: for the first time an event had been observed both by gravitational and electromagnetic radiation: two entirely different channels by which we perceive the universe.

These observations allowed determining the details of the material ejected from the collision. Most of the mass of the two neutron stars went to form a black hole, but a fraction was ejected in a neutron- and energy-rich soup from which stable heavy elements could form. The observations closely agreed with the calculations of theorists who argued that elements heavier than iron that we observe in the universe are mostly formed in collisions of neutron stars.

Think about it. Do you have bit of gold on your finger, or around your neck, or hanging from your ears? Where did it come from? Well, probably it was dug up from beneath the Earth, but before that? To make it, first two massive stars had to form in the early universe, live their profligate lives, then explode in cataclysmic supernova explosions. Then the remnants of these explosions, neutron stars, had to find themselves in a death spiral as the inexorable dissipation of gravitational radiation locked them into a deadly embrace. Finally, they collided, releasing enough energy to light up the universe and jiggle our gravitational wave detectors 130 million years after the event. And then they spewed whole planetary masses of gold, silver, platinum, lead, uranium, and heaven knows how many other elements the news of which has yet to come to Harvard into the interstellar void.

In Saturday Night Science of 2013-07-06 I discussed how relativity explains why gold has that mellow glow. Today, we discovered where gold ultimately comes from. And once again, you can’t explain it without (in this case, general) relativity.

In a way, we’ve got ourselves back to the garden.

Here is a video of the press conference announcing the discovery of the neutron star merger.

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Published in Science & Technology
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There are 36 comments.

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  1. Member

    I was just reading about this on another site. Good stuff.

    • #1
    • October 16, 2017 at 3:52 pm
    • 1 like
  2. Member

    John Walker: we are not golden.

    You sure busted my bubble.

    • #2
    • October 16, 2017 at 4:02 pm
    • 2 likes
  3. Coolidge

    The detected gamma rays and gravitational waves are experimental data. They presumably confirm somebody’s theories. Is that somebody now the favorite for the 2018 Nobel Prize in Physics? Have you already placed your bet via Irish gaming houses?

    • #3
    • October 16, 2017 at 4:02 pm
    • 1 like
  4. Contributor
    John Walker Post author

    ctlaw (View Comment):
    The detected gamma rays and gravitational waves are experimental data. They presumably confirm somebody’s theories. Is that somebody now the favorite for the 2018 Nobel Prize in Physics? Have you already placed your bet via Irish gaming houses?

    This is a prediction with many parents who have predicted various aspects of the phenomenon. If things go true to form (I may be listening too much to cynical theorists with whom I hang out), the Nobel will probably be split three ways, among an experimentalist who analysed the gravitational waveform and found it consistent with a neutron star merger, an astronomer who observed the ejecta from the event and confirmed the presence of heavy elements, and one of the theorists who predicted all of this many years before.

    The structure of Nobel prizes is increasingly superseded by the way science is done today. The main discovery papers for this event are expected to have on the order of 4,600 authors. The requirement to divide the Nobel prize among at most three living authors isn’t congruent with modern science.

    • #4
    • October 16, 2017 at 4:18 pm
    • 5 likes
  5. Reagan

    It’s so fascinating! Thanks for this great post, John. We are so fortunate to live in an era when these incredible events are detectable and the great mysteries are revealed.

    • #5
    • October 16, 2017 at 6:25 pm
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  6. Member

    As they say,”now for the rest of the story.” I saw a 30 second blurb on TV this afternoon about two stars collidin 130 million years ago but little information. Thanks for the rest of the story John.

    • #6
    • October 16, 2017 at 6:50 pm
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  7. Member

    Amazingly cool.

    • #7
    • October 16, 2017 at 8:19 pm
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  8. Member

    Does the theory include this being a commonplace enough phenomenon to account for the mass of these elements that are found in the universe? It doesn’t seem like something that would happen all that often.

    • #8
    • October 16, 2017 at 8:23 pm
    • 2 likes
  9. Member

    I really dug alpha decay when it was pointed out to me that, say, a U-238 nucleus has a He-4 nucleus inherently within. It bounces around in there with the speed of a particle of 4 amu and temperature 4.5 MeV. All the He-4 nucleus needs is a chance (a small one in this case) to exist (tunnel) slightly beyond the “glue” of the nuclear force and <pop> out it goes.

    Well, with that, a neutron star (or momentarily a supernova) can be seen in my mind to be just a really heavy nucleus (of ~10^56 nucleons and beyond) with all the isotopes to the right of the line of stability (arranged with downward concavity) racing around trying to exist outside the forces holding the star/nucleus together.

    I don’t need confluence of two neutron stars to have this happen. Though it’s cool that this apparently occasionally happens, “regular” supernovae (of later generation stars) are adequate to explain trans-ferrous elements.

    • #9
    • October 16, 2017 at 8:56 pm
    • 1 like
  10. Member
    LC

    My boyfriend is a member of the Dark Energy Survey and has been hinting at me for about two weeks now that a big announcement is coming. I had a feeling LIGO had detected something big. Exciting news.

    • #10
    • October 16, 2017 at 10:44 pm
    • 5 likes
  11. Member

    Martin Gaskell has been indicating this was forthcoming for a couple of weeks. Thanks for the detailed summary.

    • #11
    • October 16, 2017 at 11:49 pm
    • Like
  12. Contributor
    John Walker Post author

    Reese (View Comment):
    I don’t need confluence of two neutron stars to have this happen. Though it’s cool that this apparently occasionally happens, “regular” supernovae (of later generation stars) are adequate to explain trans-ferrous elements.

    Here is the paper from Nature, published yesterday, “Origin of the heavy elements in binary neutron-star mergers from a gravitational wave event” which, based on observations of the ejecta from the event, concludes:

    The cosmic origin of the elements heavier than iron has long been uncertain. Theoretical modelling shows that the matter that is expelled in the violent merger of two neutron stars can assemble into heavy elements such as gold and platinum in a process known as rapid neutron capture (r-process) nucleosynthesis. The radioactive decay of isotopes of the heavy elements is predicted to power a distinctive thermal glow (a ‘kilonova’). The discovery of an electromagnetic counterpart to the gravitational-wave source GW170817 represents the first opportunity to detect and scrutinize a sample of freshly synthesized r-process elements. Here we report models that predict the detailed electromagnetic emission of kilonovae and enable the mass, velocity and composition of ejecta to be derived from the observations. We compare the models to the optical and infrared radiation associated with GW170817 event to argue that the observed source is a kilonova. We infer the presence of two distinct components of ejecta, one composed primarily of light (atomic mass number less than 140) and one of heavy (atomic mass number greater than 140) r-process elements. Inferring the ejected mass and a merger rate from GW170817 implies that such mergers are a dominant mode of r-process production in the Universe.

    (emphasis mine).

    The last paragraph of the main paper states, “In the past, the uncertain origin of the heaviest elements was studied indirectly, by analysing fossil traces of these species in the surfaces of old stars. With AT 2017gfo we have now directly glimpsed and spectroscopically dissected a sample of pure r-process matter, big enough to enrich a million such stars.” Neutron star mergers can be rare events, but they eject such a large volume of heavy elements and with such velocity that “the accumulated nucleosynthesis from mergers could account for all of the gold, platinum and many other heavy elements around us.”

    • #12
    • October 17, 2017 at 5:14 am
    • 2 likes
  13. Member

    “There are more things in heaven and Earth, Horatio, / Than are dreamt of in your philosophy”

    Keep dreaming humanity.

    • #13
    • October 17, 2017 at 5:22 am
    • 1 like
  14. Member

    Thank you for a splendid clear explanation. Best thing I’ve read on the subject so far. And you snuck in a Tom Lehrer riff! Sweet.

    • #14
    • October 17, 2017 at 7:14 am
    • 3 likes
  15. Contributor
    John Walker Post author

    Here is the main scientific paper reporting the gravitational wave observation, “GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral.” There are more than 1,000 authors.

    The paper which reports the combined gravitational and electromagnetic observations from gamma ray to radio frequencies is “Multi-messenger Observations of a Binary Neutron Star Merger.” This one has more than 3,500 authors.

    • #15
    • October 17, 2017 at 7:25 am
    • Like
  16. Member

    Tom Lehrer and Crosby, Stills & Nash in one science post… impressive. Now I’ve got two songs running through my head.

    I will be forever grateful to Lehrer for helping me ace a high school Earth Science test, where we were asked to name as many elements in the Periodic Table as we could. I was the only one who got them all.

    • #16
    • October 17, 2017 at 7:27 am
    • 3 likes
  17. Member

    I haven’t looked at any of the references, but it sounds as though people had a heads-up a couple of weeks ahead of time that something like this was coming. How is that?

    • #17
    • October 17, 2017 at 7:30 am
    • Like
  18. Member

    The Reticulator (View Comment):
    I haven’t looked at any of the references, but it sounds as though people had a heads-up a couple of weeks ahead of time that something like this was coming. How is that?

    Not coming, the observation happened in August. People knew the paper was coming out.

    • #18
    • October 17, 2017 at 7:42 am
    • Like
  19. Member

    A recent episode of The Big Bang Theory had as a story line the boys getting depressed and complaining that physics hadn’t produced anything new in a long time. The were mostly complaining that the Large Hadron Collider hasn’t produced anything amazing since the confirmation of the Higg’s Boson. I found it ironic because of the successes of LIGO and gravitational wave detectors. If they invest as much money in gravitational wavedetectors as they did the LHC then there will be new observations daily. If they can ever get the Extremely Large Telescope going, the next 50 years of physics breakthroughs will be dominated astronomy.

    • #19
    • October 17, 2017 at 7:49 am
    • 3 likes
  20. Member

    Z in MT (View Comment):

    The Reticulator (View Comment):
    I haven’t looked at any of the references, but it sounds as though people had a heads-up a couple of weeks ahead of time that something like this was coming. How is that?

    Not coming, the observation happened in August. People knew the paper was coming out.

    Ah. I saw 8-17 and thought That’s Today! Two mistakes in one.

    • #20
    • October 17, 2017 at 7:56 am
    • 1 like
  21. Member

    Randy Webster (View Comment):

    John Walker: we are not golden.

    You sure busted my bubble.

    He speaks only of those of you who are not physicians, obviously…

    • #21
    • October 17, 2017 at 8:48 am
    • 2 likes
  22. Member

    That is fascinating! Is this event rare because we’ve not been able to observe it before or because it is a one in a million? What are the odds (literally) that even if it all generated just right, that on our little planet with the sun and moon at just the right angles (an inch one way or another would mean curtains) to provide and sustain life? Do you ever work or are you affiliated with the Hydrogen Collider in Switzerland and will this finding change their research? Lots of unusual celestial activity this year!

    • #22
    • October 17, 2017 at 9:03 am
    • Like
  23. Thatcher

    I nominate John Walker for the Nobel Prize in Explanation of Cosmic Phenomena to Reasonably Intelligent People.

    We here at Ricochet are SO fortunate to have @johnwalker as a member.

    • #23
    • October 17, 2017 at 11:45 am
    • 6 likes
  24. Member

    John Walker: Most of the mass of the two neutron stars went to form a black hole, but a fraction was ejected in a neutron- and energy-rich soup from which stable heavy elements could form.

    What’s the next step? Are there a bunch of free neutrons that find each other and fuse? Or are there huge chunks of pure neutron-stuff that decay or break up into smaller and smaller chunks, eventually the size of nuclei? Then some of the neutrons have to decay into protons and electrons. Do we have any insight on this process?

    • #24
    • October 17, 2017 at 11:53 am
    • Like
  25. Contributor
    John Walker Post author

    Judge Mental (View Comment):
    Does the theory include this being a commonplace enough phenomenon to account for the mass of these elements that are found in the universe? It doesn’t seem like something that would happen all that often.

    The estimate (which will be refined as more events of this type are observed) is that neutron star mergers happen around once every million years in a galaxy like the Milky Way. The only reason we have any chance of observing them is that LIGO, at its current sensitivity, is able to detect such events in a volume of space which contains around a million galaxies, so it should see about one a year. LIGO is currently in a one-year shutdown for instrument upgrades which are expected to double its sensitivity. This will increase the volume of space in which it can detect these mergers by a factor of 8 (2³), so detections can be expected every few months.

    Although the events are rare, on the cosmic scale there are plenty of them: the Sun is about five billion years old, so there have been around 5,000 of them in our Galaxy since the Sun started to shine. Each one creates tens to hundreds of Earth masses of each heavy element (a rough estimate of this event is that it produced 50 Earths’ masses of silver, 100 of gold, and 500 of platinum), and they are ejected at a substantial fraction of the speed of light and merge into the interstellar medium and are dispersed over time around the galaxy, which is an efficient mixmaster. They are then swept up when new stars and planets form. Calculations show that the event rate, time span, and amount produced in each merger are sufficient to account for most of gold, silver, and platinum and all of the uranium found on the Earth. (The rest of these heavy elements are believed to be produced in dying low mass stars.)

    Here is a NASA chart (public domain) which shows estimates of the origin of elements found in the solar system.

    Origin of the Solar System Elements (NASA)

    • #25
    • October 17, 2017 at 12:05 pm
    • 5 likes
  26. Contributor
    John Walker Post author

    Mark Wilson (View Comment):

    John Walker: Most of the mass of the two neutron stars went to form a black hole, but a fraction was ejected in a neutron- and energy-rich soup from which stable heavy elements could form.

    What’s the next step? Are there a bunch of free neutrons that find each other and fuse? Or are there huge chunks of pure neutron-stuff that decay or break up into smaller and smaller chunks, eventually the size of nuclei? Then some of the neutrons have to decay into protons and electrons. Do we have any insight on this process?

    The means by which these heavy elements are produced is called the “r-process”. The basic mechanism is that in an extremely neutron-rich environment, a nucleus can capture additional neutrons faster than it can radioactively decay, which allows it to pass through unstable configurations of protons and neutrons on the way to a stable or long-lived mix. This is believed to happen in core collapse supernovæ, and has been observed to a minor extent in thermonuclear explosions (the actinide elements einsteinium [98] and fermium [100] were discovered this way). Elements can end up with too many neutrons for stability through this process and, when ejected from the explosion, will beta decay into elements with more protons (a higher atomic number).

    This has been extensively modeled in the case of supernovæ, but less so in the case of a neutron star merger. The conditions are very different, in that when starting from neutron stars, all you have going in are neutrons. Do they make balls of all neutrons which then beta decay into stable isotopes, or do some neutrons decay individually and then accrete others through the r-process? I doubt anybody knows with any confidence.

    • #26
    • October 17, 2017 at 12:20 pm
    • 5 likes
  27. Moderator

    Given the volumes of these heavy elements here in Earth, is there any way to guess backwards at how recently and how close such a neutron star collision occurred? If our own sun is roughly 5 billion years old, would we be talking somewhere also roughly 5 billion years ago? How rapidly and how far do these clouds of heavy elements disperse, and should we perhaps expect to see similar distributions in nearby star systems? Would the resultant black hole still be about?

    The number of questions this discovery raises is boggling, and I eagerly await what comes next.

    • #27
    • October 19, 2017 at 4:26 pm
    • 2 likes
  28. Contributor
    John Walker Post author

    SkipSul (View Comment):
    Given the volumes of these heavy elements here in Earth, is there any way to guess backwards at how recently and how close such a neutron star collision occurred? If our own sun is roughly 5 billion years old, would we be talking somewhere also roughly 5 billion years ago? How rapidly and how far do these clouds of heavy elements disperse, and should we perhaps expect to see similar distributions in nearby star systems? Would the resultant black hole still be about?

    The heavy elements created by one of these mergers disperse through the galaxy pretty rapidly because they’re ejected at a substantial fraction of the speed of light and then encounter the turbulent flow of gases in the galaxy. Since these events only occur around once every million years in a galaxy like the Milky Way, there are around 250 for every rotation of the galaxy at the Sun’s radius, so there is plenty of time for the products to be evenly dispersed.

    If the theory of heavy element creation by neutron star mergers is correct, we’d expect to find a linear relationship between the age of stars and the amount of heavy elements they contain and, within the very rough degree we can measure such things, this seems to be the case.

    There is no hope of finding the black hole which created the heavy elements we observe. It had around five billion years before the Sun formed to create them, and orbits around the galaxy completely randomised its position long before we came upon the scene.

    We may be able to calculate how long ago the neutron star merger which created the heavy elements in the Solar system occurred by the ratio of uranium to lead. The longer ago, the more uranium will have decayed into lead. I don’t recall reading a paper where somebody calculated this, and any attempt to do so is complicated by the likelihood that the solar system was formed from a mixture of gas enriched by multiple neutron star mergers.

    • #28
    • October 19, 2017 at 4:56 pm
    • 4 likes
  29. Member

    John Walker (View Comment):

    We may be able to calculate how long ago the neutron star merger which created the heavy elements in the Solar system occurred by the ratio of uranium to lead. The longer ago, the more uranium will have decayed into lead. I don’t recall reading a paper where somebody calculated this, and any attempt to do so is complicated by the likelihood that the solar system was formed from a mixture of gas enriched by multiple neutron star mergers.

    It seems like we’d need a reasonably accurate estimate of the expected ratio of uranium to lead yielded by a neutron star collision, which would probably depend on their masses. There are a lot of unknown variables.

    • #29
    • October 19, 2017 at 5:01 pm
    • 2 likes
  30. Contributor
    John Walker Post author

    Mark Wilson (View Comment):
    It seems like we’d need a reasonably accurate estimate of the expected ratio of uranium to lead yielded by a neutron star collision, which would probably depend on their masses. There are a lot of unknown variables.

    There are indeed many unknown variables, but the ratio of uranium to lead created in a neutron star merger can be estimated roughly from nuclear physics, and can be checked by measuring the relative abundance of uranium and lead in stars of various ages by spectroscopy.

    We’re not talking about many decimal places of precision here, but just rough estimates.

    • #30
    • October 19, 2017 at 5:17 pm
    • 3 likes
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