The accumulation energy of iron core is the largest, so it is impossible for stars to continue burning when they fuse into iron. At this time, there is no longer the tension of nuclear fusion reaction to counter the huge gravity of the star, and the iron core in the center of the massive star begins to accelerate the collapse. After the violent collapse, the material in the outer layer of the star will also fall inward, and a supernova explosion will occur at this time. The outer shell of a star will be ejected into the universe in a supernova explosion, and the central core will exist in the form of a neutron star or a black hole.
In the extreme high temperature and high pressure state of supernova explosion, elements heavier than iron, such as heavy metal elements such as gold, will be synthesized and thrown into space. These supernova projectiles may become the raw materials of a new generation of star systems, forming a new star system, just like our present sun, earth and life.
You may not believe that our common heavy metals such as gold, silver and copper all come from supernova explosions!
Iron, as well as elements before iron, such as carbon, oxygen, calcium and other elements, all come from the nuclear fusion of stars. In this process, elements with low atomic number will combine with elements with high atomic number and generate energy at the same time. For example, hydrogen can combine into helium and generate energy at the same time.
But this process cannot last forever. Once the ordinal number of elements exceeds that of iron, nuclear fusion cannot produce energy. On the contrary, it will absorb energy. If a reaction absorbs energy, it is unsustainable as long as there is no external energy input.
So the elements behind iron are not produced by nuclear fusion, but by supernova explosion.
The main energy of supernova explosions usually comes from gravity. When the star's own nuclear reaction can't support its own mass, it will start to collapse. At the same time, the gravitational potential energy is transformed into huge heat energy, triggering a supernova explosion.
In this process, endothermic nuclear fusion can occur, producing heavy metals such as copper and gold that we are familiar with today.
Because the nuclear fusion reaction of iron will consume the energy of the star, it will cause imbalance inside the star, and then it will be impossible to continue nuclear fusion to synthesize heavier elements. As far as it is known, there are two sources of ultraferrous elements.
The first is the supernova explosion of massive stars. When the nuclear fusion of iron causes a star to explode, it will produce a considerable number of free neutrons. Through slow neutron and fast neutron processes, iron atoms can capture free neutrons, and then continuously synthesize various super-iron elements naturally existing in the universe, from the 27th element cobalt to the 94th element plutonium. The heavy elements synthesized by nuclear fusion and the super-iron elements synthesized by iron atoms capturing neutrons will be released into space with the super-new explosion, becoming the raw materials of the new planetary system, providing an important foundation for the emergence of life. The heavy elements that make up life on earth all come from a supernova before the formation of the solar system.
The second is the merger of two neutron stars. According to the first gravitational wave event of neutron stars discovered last year, the debris generated by the collision of neutron stars will also evolve into heavy elements, such as gold and platinum.
The reason why stars converge to become iron is that protons and neutrons in iron nuclei and the binding energy between protons and neutrons is the largest among all nuclei, that is to say, every proton or neutron added to a nucleus smaller than iron nuclei will release energy, and every proton or neutron after fusion into iron nuclei needs to absorb energy. So if you want to produce elements heavier than iron, that is, nuclei heavier than iron nuclei, you need a lot of energy from outside.
Previously, it was thought that all these heavy elements came from red giant stars and supernova explosions. In fact, through nuclear physics calculation, it is found that the neutrons captured by the nuclei of small-mass stars in the red giant stage are the source of most carbon and nitrogen and a small amount of heavier nuclei (the green part in the figure), while the neutrons captured by the nuclei of large-mass stars in the supernova explosion stage are the source of most light elements (the yellow part in the figure), and the rest come from the explosion of white dwarfs (the silver-gray part in the figure).
However, the calculation of nuclear physics also found that the above process will not produce the nuclei of those heavier radioactive elements, and only the neutron star and this rare high-energy event in the universe can produce these nuclei (purple part in the picture). Because there is no direct observation result of neutron star merger, it was only a hypothesis until last year, but the gravitational wave event GW 1708 17 observed by LIGO last summer directly proved the existence of neutron star merger event, which drew a satisfactory full stop for this problem.
Stellar nuclear fusion does end with iron.
Heavy elements in the universe, such as gold and silver, are all produced when supernovae explode.
Some relatively large stars, in the later stage of evolution, the heat is not enough to maintain the gravity of the stars, so they will collapse inward, and the material structure will burst during the collapse. In this process, due to the huge gravitational potential energy converted into heat energy, coupled with high temperature, a supernova explosion occurred. The moment of this explosion is the formation of heavy metal elements.
This is the current mainstream view.
So, what is the residue after the stellar supernova explosion? The answer is neutron star. Neutron stars are no longer normal matter, and all these atoms are crushed by gravity. Therefore, you can think that heavy metal elements such as gold and silver are fugitives in the process of neutron star production. These fugitives retained the atomic structure, but became heavy metal atoms.
Of course, it is not excluded that there are other material mechanisms that can produce heavy metal elements. Especially in the early universe, the temperature is very high, and in this melting pot, the nuclei of heavy metal atoms may also be produced-of course, this situation is difficult to happen, but there is also a small probability.
Hydrogen, helium, lithium, beryllium, boron, carbon, nitrogen, oxygen, neon, sodium, magnesium, aluminum, silicon and phosphorus ... For most people, the chemical "periodic table of elements" is certainly no stranger. However, apart from hydrogen and helium, how other heavy elements in the universe are formed is still an unsolved mystery.
At present, the scientific community generally believes that some heavy elements are produced by the nuclear fusion reaction of hydrogen and helium in stars. When a star explodes into a supernova, it will form other heavy elements. However, a new theoretical model recently published in Physical Review Letters shows that miniature black holes may destroy neutron stars from the inside, and may also produce heavy elements, including precious gold. In addition, there are some other speculations about the source of heavy elements.
Heavy elements were born in a supernova explosion.
After the nuclear fusion of the core of a massive star produces iron and its previous heavy elements, the star will collapse violently to form a supernova explosion. Iron in stars will react with free neutrons, electrons and protons at high temperature and high pressure to produce all heavy elements before uranium.
At present, the mainstream view in the scientific community believes that in the period after the Big Bang, space is full of the most common light elements such as hydrogen and helium, and some heavy elements in the universe come from nuclear fusion inside stars.
Scientists point out that under extremely high temperature and pressure, electrons outside the nucleus can get rid of the shackles of the nucleus, so that the two nuclei attract each other and collide together, and nuclear polymerization will occur, resulting in a new nucleus with heavier mass. This is called nuclear fusion. The heavy elements before iron are all produced by nuclear fusion in the inner core of stars.
All the energy in the early days of a star comes from the accumulation of hydrogen into helium. Fusion is the energy source for stars to resist their own gravitational collapse. When the hydrogen on the massive star burns out, it will collapse under its own gravity, which will greatly increase the core temperature and pressure, and then reach the condition of helium fusion to produce carbon and oxygen. When helium is gradually consumed, the star begins to collapse again, the temperature and pressure rise further, and carbon and oxygen fuse to produce silicon. Then, in the same way, silicon melts to produce iron. Because the energy produced by iron fusion is not worth the loss, the chain of fusion to iron stops. At this time, the outermost layer to the innermost layer of the star is hydrogen, helium, carbon, silicon and iron in turn.
But the evolution of stars has not completely stopped at this point. Because the high temperature of the star is not enough to "cook" the elements behind iron, such as copper, nickel, zinc and uranium. In order to promote the birth of these heavy elements, we need a bigger melting pot, that is, supernova explosion.
Scientists pointed out that after a massive star produces an iron core, the core will collapse violently due to the cessation of fusion reaction, forming a supernova explosion. Iron will react with free neutrons, free electrons, protons and other nuclei at extremely high temperature and pressure to produce all heavy elements before uranium 92, which will spread into space with the explosion of supernovae.
When two neutron stars collide, some matter will be thrown into space. These substances are rich in neutrons, and many neutrons will shoot at the "seed nucleus", thus forming elements with increasing atomic weight.
Although most scientists believe that about half of the stable heavy elements in nature, from iron to uranium, are produced by supernova explosions at the end of life. However, some scientists have given different possibilities. They pointed out that the origin of these heavy elements may be a more violent and rare mechanism-the collision between ultra-high density neutron stars.
Neutron stars are the remnants of star decay and supernova explosion, and their density is extremely high. A neutron star with a diameter of hundreds of kilometers can have the same mass as the sun, or even more. On earth, if you hold a spoonful of neutron star material, the weight of this spoonful of material will reach 5 billion tons.
Although most neutron stars are separate, two neutron stars will form a binary system. They can rotate around each other for a billion years, but in the process they will gradually get closer to each other until one day, the two neutron stars finally fall into a devastating collision.
Edo Berg, a scientist at Harvard Smithsonian Center for Astrophysics in the United States, said that at this time, most of the matter of the two neutron stars will further collapse to form a black hole, while the other part will be thrown into space. These substances are rich in neutrons, which will form elements with increasing atomic weight. Daniel Carson, an astrophysicist at the University of California, Berkeley, explained that you need a lot of neutrons and shoot them at those "seed nuclei" to synthesize heavy elements such as gold, lead or platinum. Like mud on the fender of a car.
Scientists came to this conclusion because of gamma ray bursts. This gamma ray burst is about 3.9 billion light years away from the earth. Although it lasts less than 0.2 seconds, its infrared afterglow can last for several days. After comparing the observation results with the theoretical model, scientists concluded that this is a radioactive glow produced by the formation of a large number of heavy metal elements, which were produced in a neutron star collision event.
Carson made a rough estimate of the collision and thought that gold equivalent to 20 times the mass of the earth was produced in this incident. This gold is enough to fill 100 trillion oil drums. Moreover, the amount of platinum produced in this collision event is even seven times that of gold.
In addition, scientists also found seven stars containing many heavy elements among the nine brightest stars in a dwarf galaxy, Netherseat II, which is more than that found in any dwarf galaxy. Scientists say that the heavy elements in these stars are 100 times closer than those observed in other similar galaxies. The discovery of so many elements in a dwarf galaxy proves that there must be something rarer than supernova explosions, such as neutron star collisions, because most supernova explosions produce heavy elements that are far lighter than those on the lattice.
Black holes destroy neutron stars and become the source of heavy elements.
Primordial black hole swallowed the neutron star from the inside, which made the neutron star shrink rapidly and transform itself, and eventually some parts were thrown out of the body. These separated parts rich in neutrons may be the source of heavy elements.
Some researchers speculate that the heavy elements in the universe (such as gold, silver, platinum and uranium) may have been formed by black holes when the early universe was born.
In BIGBANG, its unusual intensity will squeeze some substances very tightly, forming a "primitive black hole". This kind of black hole is not formed by the collapse of stars. Theoretically, primitive black holes are smaller than ordinary black holes, even so small that they are invisible to the naked eye.
In this latest study, researchers believe that primordial black hole will collide with neutron stars, which are almost entirely composed of neutrons and have a very high density. primordial black hole will sink into the central region of neutron stars and devour them from the inside. Alexander Kusenko, a theoretical physicist at UCLA, believes that when this happens, the black hole will devour the neutron star from the inside, and this process may last about 1 10,000 years. After that, the neutron star will rotate faster and faster with its own contraction, which will eventually lead to some small parts being thrown out of the body. These separated parts rich in neutrons are likely to be the sources of heavy elements.
However, Kushenko also said that the possibility of neutron stars capturing black holes is very low, which is consistent with the observation that only a few galaxies are rich in heavy elements. The theory that black holes formed in the early universe collide with neutron stars to produce heavy elements also explains the scarcity of neutron stars in the central region of the Milky Way. It is understood that later this year, Kushenko and his colleagues will cooperate with scientists from Princeton University to conduct a computer simulation of the process of heavy elements produced by the interaction between neutron stars and black holes, and hope to compare the simulation results with the observation results of heavy elements in neighboring galaxies to determine whether the gold, platinum and uranium existing on the earth originated from black holes in the early universe?
A: Atoms heavier than iron can be produced in other ways, such as supernovae.
Among the average nuclear masses of atoms, the average nuclear mass of iron is the lowest.
It means that iron -56 is the most stable atom;
(1) atoms smaller than iron can fuse and release huge energy at the same time;
(2) atoms larger than iron can undergo fission, which will also release huge energy;
(3) But when iron atoms fuse to produce heavier atoms, they will absorb a lot of energy;
The theory of star formation and evolution points out that the polymerization of iron atoms needs a high temperature of more than 6 billion degrees, while the highest temperature inside the star is only a few hundred million degrees, so the temperature inside the star is not enough for the polymerization of iron atoms, and the nuclear fusion inside the star takes iron as the end point.
However, at the end of evolution, a massive star may explode into a supernova. At the moment of supernova explosion, billions of degrees of high temperature will be formed inside, which can meet the conditions of iron atom fusion and thus produce heavier elements.
There is a saying that the heavy elements stored in each of us come from a supernova explosion before the formation of the earth.
In addition, in addition to supernova explosion, neutron star merger and other violent astronomical events may also meet the conditions of iron atom fusion.
As far as iron is concerned, it is a patent for massive stars. For example, our sun has not had a chance to touch iron, but it will be done with carbon and oxygen.
So how did the heavy elements such as gold and silver in the universe come from?
Nuclear fusion of massive stars can produce more substances than iron. In the initial stage of primary nuclear synthesis in the Big Bang, hydrogen, helium and lithium (a little) light nuclei were mainly produced. Light nuclei such as beryllium, boron and lithium can be synthesized by fusion reaction caused by cosmic rays.
The elements after boron are synthesized by stars and their stellar events.
For example, low-mass stars, such as the sun, can evolve into white dwarfs, which will eventually be made up of carbon and oxygen. Of course, if the star is massive, the white dwarf will be composed of oxygen, neon and magnesium.
A massive star (usually more than 8 times the mass of the sun) will have a supernova explosion. Supernova explosion is a processing plant for heavy elements, and violent astronomical events in the universe can produce heavy elements such as gold, silver, platinum, mercury and lead.
A supernova explosion will throw a lot of heavy elements. )
In addition, for example, the merger of neutron stars, the collision of black holes, and the collision between neutron stars and black holes will also throw out heavy elements.
Therefore, the production of heavy elements can not be separated from high temperature and high pressure, such as gold, which is very rare in the universe, which is why all countries use gold as a reserve instead of more expensive jewelry.
My humble opinion, welcome comments! This answer was originally written by a game science fiction fan. Thank you for your attention. Let's imagine and travel the universe together! Give someone a rose, the hand has a lingering fragrance!
First of all, we must understand that nuclear fusion has two important premises. First, the temperature and pressure inside the star are high enough, but the pressure should not be too high, otherwise the whole star will collapse quickly, which means that the outward thrust generated by nuclear fusion needs to be balanced with the outward gravity generated by the star itself!
This strict requirement also explains why stars have minimum and maximum mass requirements. If the mass is too small, stars can't be formed because the internal temperature and pressure can't reach, such as Jupiter. Too much mass is not good, because too much gravity will definitely collapse inward!
It is precisely because of the limited mass of stars that nuclear fusion cannot last forever, and usually stops when it reaches iron.
Once there is no nuclear fusion, the balance between nuclear fusion and gravity is broken, gravity begins to dominate, and the whole star begins to collapse sharply inward, resulting in a rapid rise in temperature and pressure, a violent explosion at the critical value, and a supernova is born!
The energy produced by supernova explosion is beyond imagination, and its brightness is extremely high, even exceeding the brightness of the whole galaxy. At the same time, at the moment of explosion, due to the extremely high temperature and pressure, iron elements had to start to gather together, and finally formed our common heavy elements, which erupted into space with the explosion of supernovae!
However, only massive stars will eventually form supernovae, while stars the size of the sun will not form supernovae, and only white dwarfs will eventually form! As a result of supernova explosion, apart from forming heavier elements, the remaining core is neutron stars or black holes!
The argument that nuclear fusion ends with iron is because the fusion process inside the transverse star can only reach iron, and the fundamental reason is that it will absorb energy after fusion with iron, unlike the previous light element fusion. This is why when a star begins to produce iron, it means that the life of this star begins to end.
When a star produces iron, its fusion process will cause energy to be absorbed by the fusion process. In the absence of energy, the star can no longer maintain its fusion process, so that the matter on the star can't resist its own gravity, and then the planet collapses and a supernova explosion occurs.
Most of the substances exceeding iron come from the fusion reaction caused by the high energy generated by the supernova explosion, which also allows these heavy nuclear elements to reach other regions.
So it is incorrect to say that nuclear fusion ends with iron. To be precise, in the normal cycle of a star, internal nuclear fusion ends with iron. At present, the mass of elements that can be synthesized artificially has far exceeded that of iron, and a lot of energy is needed in the process of particle collision.