The earth is a member of the solar system. The solar system family consists of the sun, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Pluto and 500,000 asteroids, satellites and comets. The sun is the mother of the solar system. Before the formation of the solar system, it was a nebula composed of hot gas. When the gas cools and contracts, it makes the nebula rotate. Due to the action of gravity, the gas and wind grass contract, the rotation speed is accelerated, and the nebula becomes a flat disk. As we know, the washing machines used to wash clothes in modern families all have dehydrators. When wet clothes are put in, the dehydrator rotates quickly, and the water in the clothes will be "thrown out", and the wet clothes will become dry clothes. The force that throws water out is the force that leaves the center when the water drops move in a circular motion, which is called centrifugal force. Similarly, when the rotating nebula rotates while contracting, the centrifugal force of the surrounding matter exceeds the gravity of the center, and a ring is separated. In this way, one ring after another came into being. Finally, the central part becomes the sun, and the surrounding rings become planets, one of which is the earth, which was produced four or five billion years ago.
This is a scientific hypothesis, which was put forward by the German philosopher Kant and the French mathematician Laplace in the18th century. People call it Kant-Laplace nebula theory. 1944, the German physicist Weizak developed this theory.
The research on the origin of the solar system and the formation of the earth continues and is constantly improving. However, the earth is our mother, nurturing our growth. We humans should know it and understand it. Even if one day, humans move to other planets, we will always miss it.
The temple of the world was built on the nuclear ruins.
Exploring Nature 2003/ 1 2003-03-06 Fu Chengqi
Can you believe it? The temple of the world is actually built on the "ruins" of nuclear reaction. Whether it is mountains and rivers, forests and oceans, stars and galaxies, or the sun and the earth, they are all products of nuclear reactions. Fundamentally speaking, even you, me and all other life are products of nuclear reaction. Of course, you don't need to worry-because most of these "ruins" are not radioactive. On the contrary, it is they who create our life and the whole splendid material world. Why do you say that? It all started with the birth of stars.
From clouds to stars
White clouds are floating in the blue sky. In fact, there are "clouds" floating around the stars, which are much larger than white clouds. They are composed of various atoms, molecules and dust, and are called "interstellar clouds". Among them, interstellar molecules tend to assemble into a group to form a "molecular cloud." Molecular clouds are usually large, hundreds of light-years in diameter, but very thin, much thinner than the vacuum in the laboratory. But because of their huge size, they still contain a lot of substances, and the total mass can reach hundreds of thousands to tens of millions of times the mass of the sun. Molecular clouds are very cold, and the temperature is usually only 10~20K ~ 20k (Kelvin, the measurement unit of absolute temperature scale), which is 273. 15K different from centigrade scale, that is, zero degree Celsius (freezing point) is equal to 273. 15K, or absolute zero is equal to-273.6438+00. Astronomical temperature refers to absolute temperature. Therefore, molecular clouds are large and cold celestial bodies that cannot be seen in the optical band. They can only be seen in infrared or radio bands. However, they are hotbeds of brilliant stars.
The interior of the molecular cloud is very uneven, some places are dense and some places are thin. Where the density is high, the matter gathers more and the gravity is greater. Under the action of gravity, interstellar molecules fall to the dense central part of the molecular cloud, and the molecular cloud begins to shrink. Because the molecular cloud is very cold and thin, and the pressure generated by the thermal movement in the cloud is very small, the falling of interstellar molecules is like a free fall, with almost no resistance.
Maybe you have seen a meteor: under the gravity of the earth, space debris passing through the earth falls into the earth's atmosphere and collides with atmospheric molecules, and part of its kinetic energy is converted into heat energy and emits light. A similar situation happened during the falling of interstellar molecules. Gravitational potential energy is converted into kinetic energy, which falls faster and faster, and collides with other molecules and atoms, so that its kinetic energy is converted into heat energy. At first, the temperature of molecular clouds did not change, because they were so thin that almost all the heat was radiated into interstellar space. However, due to the contraction of various parts of the molecular cloud, the original huge molecular cloud began to disintegrate and became a smaller molecular cloud. This fragmentation process will continue until the molecular cloud breaks down to the size of the protostar cloud, and then it will not break down again. At this time, because the density of protostellar cloud is already very high, the heat dissipation is blocked, so the temperature in the cloud begins to rise.
Because gravity is still greater than internal pressure, protostellar clouds will continue to shrink. With the increasing density, it is more and more difficult to dissipate heat, and the temperature in the cloud is rising faster and faster. Finally, the temperature in the center of the cloud reaches such a value that the repulsion and attraction generated by the thermal movement of internal molecules reach a balance. At this time, the cloud center no longer shrinks, forming a cloud core. However, the pressure and gravity outside the cloud core are not balanced, and the material keeps falling, which makes the pressure on the surface of the cloud core increase. In this way, the balance that the cloud core has reached will be broken, shrink again, and the temperature will continue to rise. When the temperature of the cloud core reaches 2000K, the hydrogen molecule dissociates, that is, one hydrogen molecule decomposes into two hydrogen atoms. The dissociation of molecules absorbs a lot of heat, which makes the pressure inside the cloud core plummet, so the cloud core collapses into a smaller and denser core, which we call a "protostar".
In the protostar stage, energy comes from gravitational energy. As the outer material continues to fall, the surface pressure increases, the protostar continues to shrink, and the central temperature continues to rise. When the temperature reached 7 million K, the nuclear reaction of hydrogen fusion to helium suddenly ignited and a new star was born. Since then, the star has started a stable and long career. Hydrogen-helium fusion energy can replace gravitational energy as the source of star energy. The stars at this stage are called "main sequence stars".
The time from the contraction of molecular clouds to the birth of stars is about 654.38+0 million years to tens of millions of years. The greater the mass, the shorter the star formation time. This is because mass means more matter, greater gravity, greater primitive acceleration, faster free fall of matter and shorter falling time, and molecular clouds will soon collapse into protostars. Because this stage is very short compared with the star's main sequence stage, we call the star that has just reached the main sequence "zero-age main sequence star" as the star's initial age. It's a bit like counting the baby's age from birth, and we omit the time when the baby is pregnant in the mother's body.
The "nuclear melting pot" of the universe
If we put the stars in the sky on a picture, the abscissa of the picture is the temperature or color of the stars, and the ordinate is their brightness or luminosity, we will find that most of the stars are in a strip from upper left to lower right, which is called "main sequence", and the stars in the main sequence are called "main sequence stars", and this picture is called "Herotto diagram". It is named after the first word of the names of two astronomers. hertzsprung and Russell first used this picture at the beginning of last century.
From the upper left corner to the lower right corner of the main sequence, the temperature of the star is from high to low, the mass is from large to small, the luminosity is from bright to dark, and the color is from blue to white to red. The sun is in the lower part of the main sequence, yellow and orange. The stars in the main sequence rely on the energy released by the hydrogen-helium fusion reaction to maintain their own luminescence. Because this reaction is very slow, the star's life is mainly spent at this stage.
Hydrogen-helium fusion reaction has high production efficiency, and each reaction will release nearly 27 mev of energy, which is much higher than other nuclear reactions. For example, when coal is burned, the process of generating carbon dioxide by one carbon atom and two oxygen atoms produces only one-sixth of the heat energy of the former. The sun emits nearly 40 billion joules of energy every second. If all the energy of the sun comes from hydrogen-helium fusion, it needs to consume 600 million tons of hydrogen nuclei every second. The sun has 200 billion tons of matter. If it is all hydrogen, it can maintain the state of the sun today for 654.38+000 billion years. In fact, when the hydrogen in the core of the main sequence star is exhausted, the star will leave the main sequence and become a red giant, and the mass of the core of the red giant only accounts for more than 10% of the total mass of the star. In addition, stars are not all composed of hydrogen, so the life span of the sun is much shorter, only about 65.438+0 billion to 65.438+0.2 billion years.
If the temperature inside the star is higher than, for example, 1 6 million k, the main energy source will be the cyclic reaction of carbon, helium and oxygen-four hydrogens fuse into1helium nucleus, releasing about 25 mev of energy. Stars whose mass is more than twice that of the sun mainly rely on this reaction to provide energy.
The energy sources of the sun and stars have aroused the curiosity of many scientists. What are burning things that can make them glow for a long time? The heat released per gram of coal combustion is 6.5 calories, and that of oil is 10 calories. If the sun relies on burning coal or oil to provide energy, it is impossible for the sun to run out of fuel in just four to seven years. Before the discovery of nuclear energy, scientists also calculated the energy produced by gravitational contraction.
Take the pendulum as an example, the pendulum keeps swinging, which is the process of mutual transformation between the gravitational potential energy and kinetic energy of the pendulum. When the pendulum is vertical, the kinetic energy of the pendulum is the largest and the potential energy is zero; When placed in the highest position, the potential energy is the largest and the kinetic energy is zero. Due to the friction between the pendulum and the air, the pendulum will eventually stop, and all the potential energy will be converted into heat energy and distributed to the surrounding air.
For the sun, if gravity is to make the sun shine all the time, then it will shrink from its present size to a point and only provide enough energy for the sun for 2000 years. In 1930s, British astronomer Eddington proposed that the energy source of stars can be explained according to Einstein's mass-energy relationship. 1938, American astronomer Betty pointed out that hydrogen-helium fusion reaction and carbon-nitrogen cycle nuclear reaction provided the energy of stars, thus solving the energy problem of stars, and Betty won the 1967 Nobel Prize in physics.
The time a star stays in the main sequence stage accounts for 80% of its life. The greater the mass of the star, the shorter the residence time, the smaller the mass and the longer the residence time. There is a simple reason. If a star is made up of all hydrogen, the time it can maintain hydrogen combustion is equal to the mass of the star divided by the energy it emits per second, which is the time for the main sequence star to exist. The mass of the most massive star is about 100 times that of the sun, but its luminosity is 100 times that of the sun. Obviously, it can keep hydrogen burning for only one tenth of the time of the sun, that is, only a few million years. For example, a massive star is like a profligate rich man, while a small star is a thrifty poor man. Although the former is rich, it is profligate and its wealth is rapidly exhausted. The latter can live long.
The "ashes" of nuclear reaction
With the passage of time, after hydrogen-helium fusion, the hydrogen fuel in the core of the star gradually decreases and the helium element gradually increases. But there is still abundant hydrogen in the outer shell of the star core. Therefore, once the core hydrogen fuel is exhausted, the hydrogen-helium fusion reaction will transfer to the shell wrapped outside the helium core. At this time, there is a helium core in the center of the star, and the surrounding shell is undergoing hydrogen-helium fusion. The nuclear combustion of the shell makes the whole outer layer of the star warm up and expand, and the expansion will cool down. From the appearance, the star began to become bigger and redder, and the star began to enter the old age stage. It only takes hundreds of millions of years for a main sequence star to become a red giant with a diameter of dozens or even hundreds of times. When the sun becomes a red giant, the earth may also be swallowed up by the sun. At that time, as a molten remnant core, the earth may still revolve around the sun-through the thin solar atmosphere with a high temperature of several thousand degrees Celsius, and finally rotate to the center of the sun after a long time. In fact, once the final stage of the main sequence is reached, the nuclear reaction has gradually shifted to the shell, and the stars begin to turn red and become bigger. Therefore, in the next 2 billion to 3 billion years, the sun will enter this stage, and by then, the earth will have become very unfit for human habitation.
The evolution and final outcome of the star after the main sequence are closely related to its mass. For small and medium-mass stars (less than two solar masses), because the hydrogen-helium fusion reaction stopped and the heat could not be replenished, the central helium nucleus began to gravitationally contract. However, the helium "ash" from hydrogen combustion in the shell layer keeps falling into the helium core, which increases the attraction of the nuclear region and makes the helium core bear more and more pressure. In the nuclear region of a star, the extremely high temperature ionizes electrons, leaving the nucleus and becoming free electrons. Now, under great pressure, free electrons fill possible gaps between nuclei.
When the temperature of helium core reaches several hundred million K, it will ignite the reaction of helium polymerization into carbon-oxygen nucleus. Two helium nuclei collide to produce beryllium, beryllium and helium nuclei collide to produce carbon, and carbon and helium nuclei produce oxygen. The whole process will release about 14 MeV of energy. When helium burns out, it will leave a carbon-oxygen core.
Helium combustion is different from hydrogen combustion. In the stage of hydrogen combustion, the core of a star is in a gaseous state and can expand greatly after being heated. Expansion plays a role in controlling the speed of nuclear reaction-if the core temperature drops slightly, the speed of nuclear reaction will slow down, less heat will be released, and the core will shrink slightly, which is a bit like a controllable nuclear reaction. However, in the helium combustion stage, the helium nucleus of a star, like a solid, expands little when heated, so the helium nucleus reaction is uncontrollable, but like a pulse.
For stars with large mass, such as stars with less than 6 ~ 8 solar masses, the evolution after the main sequence is different from that of small and medium-mass stars. A star with a large mass can form a helium nucleus with a large mass because it has enough matter, and its temperature can also rise higher. Therefore, it can also ignite a series of nuclear reactions. For example, when the temperature reaches 800 million K, carbon is ignited and can be polymerized into oxygen, neon, sodium and magnesium. Stars whose mass is more than 8 times that of the sun can also ignite nuclear reactions of heavier elements, such as neon when the temperature reaches10.50 billion K, oxygen when it reaches 2 billion K, and silicon when it reaches 3 billion K, until iron is polymerized and formed. Since then, there is no new energy source, and when it is polymerized into elements heavier than iron, it absorbs heat instead of releasing it.
In the stage of helium combustion, the luminosity of stars is often bright and dark, and many types of variable stars are at this stage. The red giant stage is short, generally only 20% of the main sequence stage.
Stars like the sun can burn helium for 3 billion years, while stars with five times the mass of the sun can only last for more than 65.438+million years.
After the red giant stage, various "nuclear ashes" will be left in the stars: carbon, oxygen, neon, magnesium, silicon, argon, calcium, titanium and iron ... These have become indispensable elements in today's material world and all life.
The final destination of the stars
Once the helium core or carbon-oxygen core is burned, the star enters the last stage of life. The nuclear reaction in the core region stops, helium combustion moves to the outer layer of the core, while hydrogen burns in the outer shell, maintaining the last light of the star. The core will shrink again, while the shell of the core will expand when heated. At this time, the massive star will become a big and bright Supergiant star.
The main feature of the last stage of a star is that it throws a lot of matter outward. For small and medium-mass stars, the ejected matter and strong radiation pressure will cause the matter to flow out at a high speed, which is the so-called "superstar wind". The speed of the giant star wind can reach more than 65,438+0,000 kilometers per second, which will blow away the shell of the star core, disperse the residual molecular cloud material around it, expose the star core, and the star will become a "white dwarf". Sometimes, a ring nebula is left around the star, which is called a "planetary nebula". White dwarfs are very small. Stars as big as the sun eventually produce white dwarfs with a diameter of only 10000 km, which is smaller than the earth. However, the density of white dwarfs is extremely high. A spoonful of substance can weigh tens of thousands of tons, and its density is 6.5438+0.0000 times that of water. The mass of the white dwarf will not exceed 1.44 times of the mass of the sun, otherwise the star nucleus will shrink and eventually form a "neutron star". After the formation of white dwarfs, they will rely on the afterheat to glow, gradually darken, become "brown dwarfs", and finally become invisible "black dwarf", just like cinders that burn out combustible materials, and their colors will change from bright dark red to grayish black.
Stars often appear in pairs. We call them "binary stars". When one of the binary stars becomes a white dwarf, there will be a continuous flow of matter from the companion star to the white dwarf, which will trigger a thermonuclear reaction and appear in the form of "supernova explosion". The result of the explosion may be that there is only one white dwarf left, or the whole white dwarf is blown up, leaving nothing behind. This thermonuclear reaction and supernova explosion are important sources of iron group elements (iron, nickel, cobalt, etc.). ) and medium quality elements (calcium, silicon, sulfur, magnesium, etc.). ).
For more massive stars, the final product is an iron core. Once the iron core is formed, the nuclear fusion reaction stops and the iron core begins to shrink due to gravity, which makes the density and temperature rise continuously. When the temperature reaches 5 billion K, the photon energy will destroy all kinds of heavy nuclei and turn them into protons and neutrons. Finally, protons capture electrons to produce neutrons. These processes absorb a lot of heat, which makes the pressure of the iron core suddenly drop, and can no longer resist the strong gravity, thus causing rapid collapse, and the diameter is reduced to about 10 km.
The whole process is short. An iron core with a density of 10000 tons per cubic centimeter can collapse in only 1 millisecond, so that all the substances in it can be compressed into a smaller core with a diameter of 10 km, with a density of several hundred million tons per cubic centimeter. The collapsed core can no longer be compressed, but the outer layer material is still falling towards the core at supersonic speed. When the matter falls to the surface of the iron core, its speed suddenly drops to zero, so according to the law of conservation of energy, it will bounce back like a ball, thus causing a supernova explosion, throwing the shell into space, leaving neutron stars or black holes, and often leaving nebula remains around. The remaining remnant nuclei depend on the original mass of the star and the mass of the remnant nuclei, and the remnant nuclei larger than 3 times the mass of the sun will form black holes.
Of course, a very violent explosion can also destroy the residual nuclear, leaving nothing behind. Elements heavier than iron, such as platinum, gold and uranium, were formed by capturing neutrons in the final stage of supernova explosion. Because neutrons are neutral and will not be affected by the force between charges, they are easy to approach the nucleus and form heavier elements.
Supernovae were recorded in Oracle Bone Inscriptions during the Shang Dynasty in China. Since the Han Dynasty, the "guest star" recorded in ancient books sometimes refers to supernovae. Especially from the Northern Song Dynasty to the first year of Yong (1054), a guest star was recorded by the company at that time, and the details were later recorded in the history book Yao. This guest star is now the famous Crab Nebula, named after its own shape.
In the 1920s, it was discovered that this nebula was expanding outward. It was estimated that these cloud materials flew out from a center about 900 years ago, which was considered as a supernova explosion in the last century. Through research, it is confirmed that it is a guest star relic of 1054 recorded in the Song Dynasty. This supernova explosion has high research value, and it is one of the youngest supernova remnants in the Milky Way. 1968, a pulsar was found in the center of the crab nebula, which proved that neutron stars were indeed produced by supernova explosions.
In 2000, astronomers discovered three stars with abnormal lead content in the Milky Way, each of which contained lead equivalent to the mass of the moon (700 billion tons). Lead is heavier than iron, so it is impossible for them to form in the red giant stage of stars. But it is impossible to form so much lead in the brief process of supernova explosion, which shows that there is another slow and gentle process of heavy elements capturing neutrons. This process may occur when the star is near the end of its life and ignites the internal helium fuel.
When helium is ignited, it will produce isotope carbon 13(6 protons and 7 neutrons). When carbon 13 is hit by helium 4(2 neutrons and 2 protons), oxygen 16(8 neutrons and 8 protons) will be produced. In this process, one neutron is actually missing. It is this neutron that is captured by heavy elements, which makes the formation of heavier metal elements possible. The three lead stars discovered in 2000 are all binary stars, and their companion stars are all white dwarfs. In fact, before becoming a white dwarf, the companion star of the leading star must eject a lot of matter. At this time, the iron family heavy elements were also thrown into space and entered the atmosphere of the lead star. They capture neutrons and produce lead, which gradually accumulates to the existing content. The battery of the car you are riding may contain lead produced in this way.
In 2002, astronomers discovered two celestial bodies that were denser than neutron stars but not dense enough to form black holes through X-ray telescopes. They think that these celestial bodies may be composed of quarks, so they are called "quarks". A spoonful of kwakexing substance can weigh 65.438 billion tons. All heavy elements are scattered into interstellar space with the super storm wind and supernova explosion, and mixed into interstellar gas and dust like seeds, becoming the raw materials of the next generation of stars.
Rebirth in "nuclear ashes"
After millions of years, interstellar gas dust mixed with heavy elements will slowly regroup into huge clouds, and then repeat the previous process. The shock wave produced by the supernova explosion will also directly trigger the contraction of the surrounding interstellar clouds and begin to form a new generation of stars. In this way, generation after generation, each generation of stars will leave their own "nuclear ashes"-heavy elements to the next generation, creating the proportion of various elements in the universe today.
The universe began to form atoms about several hundred thousand years after the Big Bang, and galaxies and stars began to form after 65.438 billion years. In the early days of the universe, the main components were hydrogen and helium, only a very small amount of deuterium and lithium, and no heavier elements. The age of the sun is 5 billion years. By mass, it contains 73% hydrogen, 25% helium and 2% metal elements. Obviously, the sun is not the first generation star, because it contains quite a lot of metal. The metal content of the Milky Way and other nearby galaxies reaches 1% ~ 3%, indicating that most stars in the galaxy are not the first generation. Today, in the Milky Way and other galaxies, the process of star formation continues, but it is far less active than in the early universe.
After the formation of stars, the residual dust gas mixed with "nuclear ash" around some stars slowly forms an astrolabe in the process of rotating around the stars, and finally planets like the Earth and Mars, small celestial bodies like comets and asteroids, and everything on the planet are born. Therefore, whether it is rocks or soil, you who are reading this article and me who wrote this article-we are all products of the "nuclear melting pot" of stars.