Shulou(Shulou.com)11/24 Report--

Original title: How did the gold on earth come from? Understand the principle and "dig gold" together!"

LIGO (Laser Interferometer Gravitational Wave Observatory) is a large research instrument that uses laser interferometers to listen for gravitational waves from deep in the universe. The first gravitational wave source in history, GW150914, was detected in 2015.

Since then, the discovery of gravitational waves has mushroomed. By the end of 2018, LIGO had discovered 11 gravitational wave sources.

11 sources of gravitational waves discovered by LIGO by the end of 2018 The above figure shows the basic situation of these 11 sources of gravitational waves.

The blue and purple dots represent black holes, while the yellow and orange dots represent neutron stars. The numbers on the vertical axis mark how many times the mass of the sun these objects are. The arrow in the figure represents an event in which two smaller objects combine to form a larger object and emit gravitational waves. Ten of these gravitational wave events were the merger of two black holes, and only one was the merger of two neutron stars, which was of course the first binary star merger observed in human history.

What we want to popularize is this double neutron star merger event. Because it answers a question that interests most people on Earth: Where does gold come from?

Gold is a simple metal consisting of gold with atomic number 79 (that is, 79 protons in the nucleus). It has a beautiful golden color and hardly reacts with any other elements. Because of its scarcity and chemical stability, gold has historically been used as legal tender and as a symbol of wealth.

Source: pexls But where did all this gold come from? This question has puzzled mankind for thousands of years.

Many strange ideas have been put forward: some say it is concentrated sunlight, others say it is hardened water, and others say it is transformed from cheap metals. Of course, we now know that none of these assumptions are reliable. The earth itself cannot make gold; all gold comes from heaven.

It wasn't until the second half of the twentieth century that two astronomers found the key clue to solving this century's puzzle. They are Hoyle and Fowler, whom we met earlier.

Before introducing Hoyle and Fowler's theory, let's briefly review some of the basics of stellar evolution.

A star's birth is marked by hydrogen fusion ignited in its central region. During the main sequence, nuclear fusion in the center of a star can occur in two ways: for a star with a small mass, such as the Sun, the fusion ends with oxygen; for a star with a mass more than ten times that of the Sun, the fusion ends with iron.

Why does the fusion reaction in massive stars end in iron? The diagram below explains why. The horizontal axis of this graph is the number of nucleons (nucleons contain protons and neutrons) contained in a given nucleus, and the vertical axis is the average binding energy of that nucleus. The average binding energy is the average energy required to bind each nucleus together. As shown in the figure, iron has the largest average binding energy of all the chemical elements.

Iron has the largest average binding energy with the consequence that iron is the most stable of all elements. In other words, all chemical elements lighter than iron fuse into iron, releasing energy; all chemical elements heavier than iron split into iron, releasing energy. In other words, iron absorbs energy whether it fuses or splits into other elements.

Nuclear fusion at the center of the star must then cease after iron has been produced. This was because fusing iron into heavier elements not only failed to release energy, but also absorbed energy. Xing Xing naturally wouldn't do such a loss-making business.

This has a consequence: gold, heavier than iron, cannot be produced by fusion at the center of a star.

Even stellar fusion couldn't make gold, so how did gold come from? Hoyle and Fowler's answer is, from a supernova explosion.

About supernova explosions. One such event is known as "massive star collapse," in which strong gravity forces electrons into the nucleus of a massive star after death to combine with protons to form neutrons; this process of "neutronisation" of the star creates a neutrino explosion that blows all the material out of the star, creating a supernova explosion. Hoyle and Fowler found that this is not the whole story of supernova explosions.

After the star "neutronizes," the star interior becomes a "sea" of neutrons. Because neutrons are uncharged, they easily fall into the iron nuclei at the center of stars. In other words, iron nuclei can increase the number of nuclei by capturing neutrons, and at the same time, some of the captured neutrons decay inside the nucleus and then turn back into protons. This means that the number of protons and neutrons in the nucleus of iron atoms increases, making it a heavier element.

By capturing neutrons in this way, iron can break through the bottleneck that prevents it from condensing into heavier elements, thus producing all heavy metals, including gold and silver. The gold produced by the neutron capture process diffuses outward with the expanding supernova remnant and melts into the interstellar medium. The interstellar medium can form new stars and planets (such as our solar system and its eight planets). In the process, gold in the interstellar medium is brought to Earth.

For a long time, it was thought that supernova explosions were the only way to produce gold. But in the 1990s, using computer simulations, astronomers discovered a serious problem: supernova explosions are actually a very difficult thing to do.

As mentioned earlier, supernova explosions are the result of neutrino explosions produced by the "neutronization" process of stars, which explode the outer matter of stars. But astronomers have found that, in most cases, neutrino explosions are not enough to blast apart the outer layers of a star. There would be no more supernova explosions; the massive star would simply collapse into a black hole and disappear silently from the night sky. This is the "failed supernova." A "failed supernova," on the other hand, cannot produce gold.

Because of the existence of "failed supernovae," supernova explosions are not enough to produce all the gold in the universe. Astronomers estimate that supernova explosions produce only 10 percent of the gold in the universe. So where does the remaining 90% of gold come from?

In fact, as early as the 1970s, American astronomers Latimer and Schram found the key clue to this problem, that is, the merger of two neutron stars. When two neutron stars collide and merge, some of the neutron star's matter is thrown out. The ejected neutron star matter can emit a large number of electromagnetic waves at high temperatures, thereby increasing its brightness. Although far less luminous than a supernova, it can still shine 1000 times brighter than a normal nova (roughly tens of millions of times brighter than the Sun). Therefore, people call it "thousand new stars."

Kilonova Latimer and Schram found that neutron capture can also occur during the merger of neutron stars. The part of neutron star matter that is thrown out contains a lot of iron. In this way, the iron nucleus can continuously capture neutrons, thus creating all heavy elements, including gold. Unlike supernovae, the merger of neutron stars is not blocked by any outer matter, and there is no possibility of failure.

Thus, a binary neutron star merger can produce more gold than a supernova explosion.

For decades, humans have not observed a thousand new stars produced by the merger of two neutron stars, so it is impossible to verify whether this theory is correct. This situation completely changed on August 17, 2017.

On August 17, 2017, there was a carnival in astronomy. At 12:41 UTC that day, Fermi sent a message to Earth saying it had just detected a gamma ray burst from deep space.

Six minutes later LIGO reported that it had detected a gravitational wave burst at about the same time as the gamma ray burst. LIGO also determined the approximate range of the gravitational wave source and sent the message to telescope control centers around the world at 13:21 p.m. Telescopes all over the world pointed in the same direction at the same time, and indeed found a kilonova resulting from a binary neutron star merger in the galaxy NGC 4993, 130 million light-years away from Earth.

The approximate location of the neutron star merger has astronomers all over the world boiling over. This is the first time in human history that a celestial body has been observed emitting both electromagnetic and gravitational waves. This discovery ushered in a new era of multi-messenger astronomy.

Of course, this discovery also confirmed the theory that the merger of two neutron stars would produce gold. Astronomers estimate that this neutron star merger could yield 1.8×10 to the 24th ton of gold, which is roughly equivalent to the mass of 300 Earths.

Who knows where all this gold will end up.

★ Book Introduction ★

Cosmic Odyssey Through the Milky Way Author: Wang Shuang Tsinghua University Press A Journey to the Stellar World of the Milky Way. We will tour 12 carefully selected sites in the Milky Way: Alpha Centauri, Sirius, 51 Pegasus b, Polaris, Alpha Orion, PSR B1919+21, Cygnus X-1, Crab Nebula, Eagle Nebula, Hels-Taylor Binary, Sagittarius A*, and Panorama of the Milky Way. This book presents you with a complete body of knowledge about the Milky Way. With 40 carefully selected topics, I will take you through what role these 12 galactic attractions have played in the history of human civilization; what major scientific events they are related to; how they have changed our view of the universe; and how they relate to our real lives. We can see the wisdom of the human elite, how scientists think, how we should understand the world. A solid scientific foundation, fictional true stories, beautiful language, and humanistic care. A solid scientific foundation, fictional true stories, beautiful language, and humanistic care.★ Author Profile ★

Wang Shuang, associate professor and doctoral supervisor, School of Physics and Astronomy, Sun Yat-sen University. He has devoted himself to cosmological research for many years, and has published 35 SCI papers, with a total citation of more than 1800 times. Popular science writer, author of "Cosmic Odyssey: Strolling the Solar System,""Talking about the Universe to Children" and "Relativity to Children," has won a series of popular science awards including "30 Good Books in China in 2017" and "100 Excellent Publications Recommended by the State Press and Publication Administration to National Youth in 2018." Sina Weibo, a well-known science blogger with nearly 2 million fans, and a guest speaker at Phoenix TV, Shenzhen TV and TEDx standard-level conferences. Part of the source network copyright belongs to the original author

This article comes from Weixin Official Accounts: origin reading (ID: tupyread), author: Wang Shuang, editor: Zhang Runxin

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