Why the Neutron Star Collision was an Important Observation

I am going to take a moment to actually talk about astrophysics (I know, it's a shock! To actually talk about what I do).

Back on August 17th the LIGO gravity wave observatory detected gravity waves from the collision of two neutron stars. This was quickly followed by the detection of a gamma ray burst by the Fermi space telescope, and then a host of other observations from other telescopes. This event quickly became the most heavily observed single event in astronomy. There are several good general reviews of what happened that are very accessible to the average reader (NY Times, NPR, Veritasium, and one more in depth from Phys.org, there is also a whole webpage about the detection with links to many papers).

So I won't go over the basics because you can get that from other sources, but I will talk about some of the more technical implications to the detection.

First, gold. Gold is very important in astronomy because it is very heavy and hard to make. Usually astronomers ignore the different types of elements, we are usually only concerned about hydrogen and helium. There is a joke in astronomy that the astronomer's periodic table of elements is the simplest one since we only have three elements, hydrogen, helium, and "metals". We refer to everything that is not hydrogen and helium and metals (that includes decidedly non-metallic elements like nitrogen, oxygen, carbon, and neon). It keeps things simple.

But when it comes to metals we are concerned with what we call metallicity, that is the relative amount of metals compared to hydrogen and helium. Because hydrogen and helium make up a combined 98% of the mass of the universe, on a cosmological scale everything else is just a rounding error. But on smaller scales (small, as in the size of a cluster of galaxies) the amount of metals becomes important. Except for a tiny amount of lithium, everything that is not hydrogen and helium was produced inside stars, one way or another.

When a star goes through its life cycle it will return a significant amount of mass back into the interstellar medium in the form of stellar winds. For a large star with an initial mass of 10-20 times the mass of the sun, the star may return 80-90% of its mass to the interstellar medium in the form of stellar winds, or a nova or even a supernova. So while a star may start out as almost entirely hydrogen and helium when it forms, the gas that returns to the interstellar medium will be slightly enriched with metals, that is, the metallicity will go up. This enriched gas that has been returned to the interstellar medium will go on to form a second generation of stars, which will still be almost entirely hydrogen and helium, but now with a tiny fraction more of metals. The process will repeat, and each time it does the gas will become more enriched with metals. In order to have enough metals that rocky planets such as the earth can form the gas must go through at least 20 star formation and enrichment cycles. To date, the highest metallicity ever observed in a star is about three times the metallicity of the sun.

Because of something called the nucleon binding energy only elements up to iron can be produced in the conventional way inside of stars. Anything heavier than iron needs to be produced in another way because it is en endothermic reaction and requires huge amounts of energy. Some heavy elements are produced in supernovas but there is a subtle problem with that. While there certainly is enough energy in a supernova explosion to produce the heavier elements, most of the mass that is blown off in a supernova is hydrogen. It would take an immense amount of energy and a string of complex, and highly improbably reactions to convert that much hydrogen into elements as heavy as gold and lead.

In nuclear physics there are two processes which can produce heavier elements, named the r-process and the s-process (unimaginatively the r and s stand for rapid and slow respectively). The s-process takes less energy and a much lower neutron flux, and can happen over long time scales. In the s-process heavy elements are built up slowly one neutron at a time, and allows for neutrons to decay into protons.

The r-process requires huge amounts of energy, and a truly astronomical neutron flux. A parent element is bombarded with a huge number of neutrons to make an extremely unstable isotope. The only thing keeping it from decaying into smaller elements is the rate at which neutrons are bombarding the nucleus. While a supernova has enough energy for the r-process, there is a distinct lack of neutrons to achieve the neutron flux necessary for the r-process to take place. It does happen, just not at a high enough rate to explain the amount of gold, lead, uranium, and other really heavy elements we observe in the universe. So while normal stellar processes can explain the carbon, oxygen, and nitrogen we see, and novas and supernovas can explain the amount of aluminum, iron, nickle, and zinc we see, neither of those can explain the amount of gold, silver, lead, and uranium we see.

This is where merging neutron stars come in. In the collision there certainly is enough energy for the nucleosynthesis to take place, and because there are two massive sources of neutrons being ripped apart, the problem of meeting the minimum neutron flux is solved. But up until now we had no hard confirmation of neutron star mergers, much less finding evidence of r-process production of heavy elements. It has been suspected for years, but only with the LIGO detection and the followup observations of the nova remnant has this been confirmed. With the detection of r-process reactions in the remnant of the merger we can now conclude that almost all of the gold, uranium and other very heavy elements come from neutron star mergers.

Below is an updated periodic table of elements that shows where each element comes from. Some come from more than one source, but you can see just how many elements were detected in the neutron star merger. The purple shows elements from neutron star mergers. It is much more than gold. This is why the detected merger was so important. It showed us where many of the heavy elements came from like gold, silver, lead, platinum, iodine, bismuth, tin, uranium, and many more.

From Wikipedia. You can see a larger version here.

For my own research this does not change what I am doing. While most very heavy elements come from neutron star mergers, most metals come from normal stellar process that are already accounted for in my models. The detection of the merger does not change the overall metallicity, but it does slightly change the relative ratios of the different metals. But this change is not significant enough to impact what I do. The overall metallicity is extremely important, but the heavy elements are still too rare to make any difference. This could be more relevant to those who work on rocky planet formation, and also nucleosynthesis in interstellar space. But my work does not get down to that level of refinement. I work on fairly large objects where individual stars, and even supernovas are below the level of my resolution. So while it is exciting, it does not affect my work directly, but at some point someone may provide a slight modification to some of the models that I use that may change a few of the minor outputs.

Cheating at Connect the Dots: Failing to see the whole picture of Mormonism

Recently I was reading an article by a historian of Mormon history and it got me thinking about playing connect the dots. You see, playing connect the dots is easy... if you ignore most the dots. Consider the image below. It contains a number of different color dots. The question is, can you see the star in the dots?

How about now? Can you see the star? All I have done is emphasize a few important dots, and now all I have to do is connect the dots.

With the dots now connected the star is easy to see. Sure the dots aren't perfectly aligned with the star but that's only because I didn't take the time fully flesh out the star. With a little more work the misalignment could be fixed and the minor discrepancies could be smoothed over. Only those who are extremely nitpicky will complain about the misalignment, and that only distracts from my point that there is a red star in there.

So why am I talking about a manufactured star and connecting the dots? As historians approach history, by definition, they do not have all the facts, nor were they impartial observers of the totality of events. It is the job of a historian to take as much information as possible and attempt to build a coherent image of the past, or at least a single person or event, based on as much information as possible. Historian effectively play an extremely complicated game of connect the dots.

Their work is very similar to mine, I cannot go directly and observe stars and galaxies close up, nor can I see how they behave over millions and billions of years, so I am reduced to playing a very complicated game of connect the dots.

But in these games there are certain rules. It is easy to connect the dots of history into just about any natural progression of events after the fact. But in doing so we have to be careful not to ignore those things that undermine the point we are trying to make.

For example, I could use the Declaration of Independence, Shays' Rebellion, the Whiskey Rebellion, the Civil War, and the modern Tea Party to make the case that the founding principle of the United States is rebellion and distrust of government. I could probably write a fairly convincing argument, backed up with quotes from Thomas Jefferson, centered on the idea that to be American is to rebel.

But in order to make this argument I would have to ignore many other important founding principles of American society, such as representative democracy, constitutional government, separation of powers, personal liberties, and English common law. A similar argument regarding many other aspects of American society showing a coherent progression of history culminating in the latest manifestation of that defining characteristic. In fact most modern political positions attempt to do just that by tying current debates to what are assumed to be foundational principles.

Whenever we do this we run the risk of taking a few minor points of history and trying to superimpose a particular overall shape or interpretation to it. This is even more tempting when a few of the points are not just minor, but seemingly dominate the historical map (just like in the second image above). If I were to make the case that rebellion runs deep in American society I could make that argument quite persuasively using well known events such as the Revolutionary War, but to say that the United States is founded on rebellion ignores the fact that the United States was not organized from rebellion, but under a constitution formed out of democratic compromise. And it ignores the fact that the vast majority of American history did not involve rebellion, but democratic debate and the application of constitutional law.

So when I recently read an article by Grant Shreve entitled How Mormons Have Made Religion Out of Doubt, I thought about how he was connecting the dots. In his article Shreve makes the case that "The Church of Jesus Christ of Latter-day Saints was founded on the idea of an evolving scriptural canon." While this is certainly true because it is a large and important point in LDS thought, he connects this central idea to other contemporary and historical events in such a way that he is effectively playing connect the dots by blatantly ignoring other large and just as important parts of LDS thought.

Shreve uses the story of James Colin Brewster to try to show that "If the church seems to have lost its way, a new revelation may be just around the bend." and that "The legacy of James Colin Brewster and numerous other Mormon dissidents lives on in the Remnant, a diffuse international movement of disaffected Mormons." This Remnant organization that Shreve sees as a natural product of LDS though, "answers the doubts many Mormons harbor by offering more revelations and additional scriptures."

Thus Shreve takes the idea of an open canon and makes the case that these historical and contemporary dissident movements are simply a natural extension of LDS thought. But in making this argument Shreve ignores other important points just as integral to LDS thought. Points such as priesthood authority and keys, and church hierarchy. He does briefly acknowledge these things, but they are not mentioned as foundational and fundamental to LDS thought.

While Shreve uses the concept of the open canon to make his point he completely ignores the actual content of that new canon. Just as someone might argue that the United States was founded on rebellion, they would have to ignore the fact that the United States is actually founded on constitutional democracy. They would have to ignore the bulk of US history and law.

The concepts of priesthood authority and church hierarchy are acknowledged by Shreve, but he views these not as foundational but later additions where Joseph Smith sought to "consolidated revelatory power in himself and a select cadre of elites." Furthermore, the structure of the church is only described in negative terms such as "elaborate", "complex" and made of "byzantine bureaucracies" which "disrupted the mystical and communal experiences of Mormonism’s salad days."

Thus the point that Shreve is trying to make is that the impulse to have new mystical revelations is the natural state of Mormonism, with new prophets popping up "if the church seems to have lost its way" to provide new books of revelation. Thus, based on this reasoning, the Remnant movement is truly following the principle started by Joseph Smith, and the elaborate, complex and byzantine bureaucracies of the modern LDS church only serve to stifle the true expression of Mormonism.

Unfortunately this game of connecting the dots blatantly ignores much of the rest of the content of Mormonism. The "consolidation of revelatory power" was not a late development imposed by Joseph Smith, but rather was an early and foundational principle. If we ignore this fact, we fail to see the whole picture of Mormonism.