Despite the simple formulation of the question “Why does the Sun shine?” the answer to it requires some base of physical knowledge and to present it in one sentence is a difficult task. We will try to solve it towards the end of the article, which we will begin with historical background.
Story
One of the first who tried to explain the nature of the Sun from a scientific point of view was the ancient Greek astronomer and mathematician Anaxagoras, according to whom the Sun is a hot metal ball. For this, the philosopher was imprisoned. Before the instrumental study of the Sun began in the 17th century, there was still a lot of speculation about the nature of sunlight, including forests that were constantly burning on the surface.
Since the 17th century, scientists have discovered such a phenomenon as sunspots, and it becomes possible to calculate the period of rotation of the Sun. It becomes clear that our star is a kind of physical body with a complex structure. In the 19th century, spectroscopy appeared, with the help of which it was possible to decompose a sunbeam into its component colors. Thus, thanks to the absorption lines, Fraunhofer manages to detect a new chemical element that is part of the star - helium.
In the mid-19th century, scientists were already trying to describe the glow of the Sun using more complex scientific hypotheses. So Robert Mayer suggested that the star is heating up due to bombardment by meteorites. Somewhat later, in 1853, a more plausible idea of the so-called “Kelvin-Helmholtz mechanism” arose, according to which the Sun heated up due to gravitational compression. However, in this case, the age of the star would be much less than it actually is, which contradicts some geological studies.
Why does the sun shine
The British physicist Ernest Rutherford was the first to come up with the correct answer to this question, who suggested that radioactive decay occurs in the Sun and it is this that is the source of the star’s energy. Later, in 1920, the English astrophysicist Arthur Eddington developed Rutherford's idea, arguing that a thermonuclear fusion reaction could occur in the Sun's core under the influence of the internal pressure of the Sun's own mass. After 10 years, the main fusion reactions generating the observed amount of energy were calculated.
Briefly, the thermonuclear reaction that causes the Sun to shine can be described as the fusion of protons (hydrogen nuclei) into a helium-4 nucleus. Since the helium-4 nucleus has less mass than the hydrogen nucleus, the energy difference (free energy) is emitted in the form of photons - particles that are electromagnetic radiation.
Thermonuclear reaction
Proton-proton thermonuclear fusion reactions occurring inside stars with a solar mass or less can be divided into three chains: ppI, ppII, ppIII. Of these, ppl accounts for more than 84% of the Sun's energy. The proton-proton reaction consists of three cycles, where the first is the interaction of two protons (two hydrogen nuclei). With enough energy to overcome the Coulomb barrier, the two protons merge to form a deuteron. Since the deuteron nucleus, consisting of two protons, has less mass than two individual protons, free energy is generated, due to which a positron and an electron neutrino are created, which are emitted from the region where the reaction took place.
Next, due to the interaction of a deuteron and another proton, helium-3 is formed with the release of energy in the form of electromagnetic radiation. Further stages of the reaction can be clearly seen in the diagram below.
Reactions taking place inside the sun
In addition to the proton-proton thermonuclear fusion reaction, a small contribution to the energy released by the Sun is made by a reaction of the proton-electron-proton type (0.23%).
Thus, summarizing the above, the Sun emits electromagnetic waves of various frequencies, including in the region of visible light, which are formed by particles born as a result of released energy during the proton-proton (proton-electron-proton) thermonuclear fusion reaction.
The fourth state of matter.
Part six. Why does the sun shine
Why does the sun shine? The same exact answer to this question is known today. The sun shines because in its depths, as a result of the thermonuclear reaction of converting 4 protons (nuclei of hydrogen atoms) into one helium nucleus, free energy remains (since the mass of the helium nucleus is less than the mass of four protons), which is emitted in the form of photons. Photons in the visible range are the sunlight we see.
Now let’s speculate and imagine the path that scientists have taken. And at the same time, let’s think about what will happen when hydrogen completely burns out in the Sun? Will it definitely go out? We advise you to read the article to the end - a very interesting assumption is made there.
Let us assume that the Sun burns the most calorific of all types of fuel - the purest carbon, which burns entirely, without any ash. Let's do a simple calculation. It is known how much heat this “fire” sends to the Earth. The sun is a globe, so it emits heat evenly in all directions. Knowing the sizes of the Earth and the Sun, it is not difficult to calculate that in order to maintain the flow of heat from the Sun, about 12 billion tons of coal must burn in it every second! The figure is huge on an earthly scale, but for the Sun, which is more than three hundred thousand times heavier than the Earth, this amount of coal is small. And yet all this coal on the Sun would have to burn out in just six thousand years. But the data of many sciences - geology, biology, etc. - irrefutably indicate that the bright Sun has been heating and illuminating our planet for at least several billion years.
The idea that the sun burns with coal had to be rejected. But maybe there are chemical reactions that release even more heat than when burning coal? Let's assume they exist. But even these reactions could extend the life of the Sun by a thousand, two thousand years, even doubled, but no more.
But if the Sun is not able to provide itself with fuel for any long time, then perhaps outer space does this from the outside? It has been suggested that meteorites are constantly falling onto the Sun. We have already said that when approaching the Earth, meteorites, due to braking in the Earth’s atmosphere, often completely burn out, heating the air along the way. Why not assume that there is no atmosphere around the Sun, that the braking of meteorites occurs directly in the solar matter, and it heats up to a high temperature?
Let's turn again to the calculations. How many meteorites must fall on the Sun to ensure its long-term burning? The calculation gives an absolutely incredible figure: even if the weight of all the meteorites that fell on the Sun was equal to the weight of the Sun itself, it would still shine for only about a million years.
But maybe once upon a time such a huge number of meteorites fell on the Sun, heated it to a huge temperature, and now the Sun is slowly cooling? Nothing like this! There is a lot of evidence that the Sun shone and warmed a billion, a million, and a thousand years ago, just as it does today. So, the second assumption also fails.
The amazing constancy of solar activity also buried the third, most tempting assumption about the cause of the “burning” of the Sun. It boiled down to the following. According to the law of universal gravitation, all bodies move closer to each other. The Earth is attracted by the Sun and moves around it. The stone is attracted by the Earth and falls on it if it is released from the hands.
Let's imagine that the Sun is some kind of huge vessel with gas. The molecules of this gas, subject to the action of mutual attraction, despite the collisions that throw them away from each other, must gradually attract each other and come closer. The sun as a whole would then shrink, the gas pressure in it would increase, and this would lead to an increase in temperature and the release of heat.
If we assume that over 100 years the diameter of the Sun decreases by only a few kilometers, then this phenomenon could completely explain the emission of radiation from the Sun. However, such a slow reduction cannot be detected using astronomical instruments.
But there is a “device” that works for a much longer time. This device is the Earth itself. During its existence, the Sun would have to shrink tens of times: from a size many times greater than the extent of the entire solar system to its present size. Such compression would certainly affect . However, the history of the Earth knows nothing like this. She knows of major geological disasters in which the highest mountains perished, new oceans and entire continents were born, but all this can be fully explained by the activity of the Earth itself, and not the Sun.
So, all three mentioned hypotheses about the reasons for the “burning” of the Sun turned out to be untenable. Science, which managed to explain many of the most complex phenomena on Earth, gave up for a very long time before the mystery of the activity of the Sun. Now it has become clear that the solution to this riddle must be sought not in the depths of space, but in the depths of the Sun.
And here the science of the super-large - astronomy - came to the aid of the science of the super-small - the physics of the atomic nucleus.
- a fairly ordinary star for the Milky Way - not the brightest, not the largest and is only 4.5 billion years old. Currently, the Sun is the only star we know of whose light and heat support life on the only habitable planet we know of. Fortunately for us, the Sun was still shining at the time when the first people appeared several hundred thousand years ago. But where can the Sun have so much fuel? Why hasn’t it gone out yet, like a candle or a fire? And when will our star finally burn out?
Why does the sun shine?
This question was raised by scientists already in the 19th century. At that time, scientists knew only two ways in which the Sun could generate energy: either it created heat and light as a result of gravitational compression - it was pulled towards the center and radiated energy (in the form of heat that we feel on the surface), so over time it would become decrease. Or the Sun literally burned like coal in a furnace - as a result of a chemical reaction familiar to us all, which occurs when we light a fire. Taking as a basis the fact that any of the listed hypotheses could support an explanation of the functioning of the Sun, scientists of those years accurately calculated how long our star could exist if the corresponding process took place on it. But not a single result coincided with the figure that researchers knew about the age - 4.5 billion years. If the Sun had contracted or burned, it would have run out of fuel long before we arrived on the scene of evolution. It became obvious that something else was happening on the Sun.
Einstein's equation
A few decades later, armed with Enschnein's famous equation E = mc2, which predicted that any mass must have an equivalent amount of energy, British astronomers in the 1920s proposed that the Sun was actually converting its mass into energy. However, instead of a furnace that turns wood and coal into ash and blackened carbon (emitting light and heat), the center of the Sun is more like a giant nuclear power plant.
Solar fusion fuel
The sun contains a huge number of hydrogen atoms. Typically, a neutral hydrogen atom contains a positively charged proton and a negatively charged electron that orbits around it. When this atom meets another hydrogen atom, their respective outer electrons magnetically repel each other. This prevents one of the protons from colliding with each other. But the Sun's core is very hot and under such pressure that the atoms move with great kinetic energy, which allows them to overcome the force that binds their structure, and electrons begin to separate from their protons. This means that protons normally found inside the nucleus of a hydrogen atom can touch each other and combine into the nuclei of other elements in a process called thermonuclear fusion. This reaction occurs with the release of a colossal amount of energy.
Just like inside a nuclear reactor, the atoms inside the Sun's core crash into each other every second. What often happens in these collisions is that four hydrogen protons fuse together to create one helium atom. As a result of this fusion, part of the mass of these four microscopic protons is “lost”, since the helium atom weighs less than the total of four protons. But because the Universe retains matter, it can't just disappear forever; that mass turns into an incredible amount of energy - every second the Sun emits 3.9 x 10 to the power of 26 watts of power. (This is such a huge amount of energy that, frankly, no analogy can be made with earthly processes. Perhaps this number can be estimated as follows: this number of watts is much more than all the electricity that the entire world will be using at the current rate of more than several hundred thousand centuries).
How long will the sun burn?
The efficiency of the fusion reaction is the main reason the Sun constantly emits heat - the energy released by converting just one kilogram of hydrogen into helium is equivalent to that released by burning 20,000 tons of coal. Because the Sun is quite massive and relatively young, it is believed that it has only used about half of its fuel, hydrogen.
Eventually, the Sun's core will convert all of its hydrogen into helium and the star will die. But don't worry. This won't happen for about 5 billion years.
Lesson type: combined
Target
the formation of ideas about stars, that the Sun is the closest star to Earth, to show the diversity of stars; acquaintance with the natural satellite of the Earth - the Moon, and its features.
Planned results
Subject
Will learn: compare the apparent and real sizes of stars; observe the picture of the starry sky, find the constellation Leo.
Will get the opportunity learn to work with the atlas - determinant; observe the picture of the starry sky; build arguments on a given topic.
Metasubject
Regulatory: understand the learning objective of the lesson, strive to complete it and evaluate your achievements.
Cognitive: work in pairs: model the shape and comparative parameters of some stars, carry out mutual verification.
Communicative: listen to your interlocutor; formulate your own opinion and position, ask questions.
Personal results
Determine the motives for educational activities; accept and master the social role of the student; understand a holistic picture of the world; motivation for educational activities (educational and cognitive).
Basic concepts and definitions
Star, Sun, star size, sunrise, sunset, day, night, constellation.
Preparing to learn new material
Let's find out what the Sun is. Let's learn how to make models of stars. We will learn to find the constellation Leo in the sky.
Remember what you can see in the sky day and night. What do you know about the Sun and stars?
Learning new material
Why does the sun shine during the day?Astars at night?
Did you know that the Sun is the closest star to Earth? All stars are huge flaming balls. Many of them are much larger than the Sun. Think about why they appear to us as small luminous dots
Make models of the stars shown in the picture from plasticine. Correctly show their shape, color and comparative sizes. Check each other's work.
Consider the constellation Leo in the picture. Think about why it is called that. Which star shape can you use to find this constellation in the sky? Test yourself with the help of the atlas-identifier.
Make a model of the constellation Leo
In the evening, when it gets dark, look for the constellation Leo in the sky. If necessary, use the atlas-identifier.
Conclusion
The sun doesn't just shine during the day, it creates the day itself. When it rises and illuminates the earth, the day begins. And when it comes, the night begins. It is then that stars become visible, which shine much weaker than the Sun.
1. What new have you learned about the Sun and stars? 2. Why are there no stars visible in the sky during the day? 3. Why do they become visible »» at night? 4. How to find the constellation Leo in the sky?
WhyshinesSun -1
WhyshinesSun-2
WhyshinesSun?
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WhyWewe seestars?
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It is calculated that on average the amount of radiation emanating from each square meter of the solar surface is 62 thousand kilowatts, which is approximately equal to the power of the Volkhov hydroelectric station. The radiation power of the entire Sun is equivalent to the work of 5 billion billion (5·10 18) such power plants!
Let us give one more figure: every square meter of the solar surface emits as much light as could be produced by 5 million 100-watt light bulbs... So, tirelessly, our radiant luminary “works” not for centuries or even millennia, but for billions of years !
What happens on the Sun? Where does it continuously draw a truly colossal amount of energy?
In 1920, the outstanding English astronomer Arthur Eddington (1882-1944) first suggested that thermonuclear fusion could be a source of solar energy. Subsequently, other scientists developed this idea. According to modern ideas, nuclear reactions occur in the depths of the Sun and similar stars, that is, processes during which not chemical compounds are formed, but nuclei of new chemical elements. And in the hot interior of the star, where the temperature can reach 15 million degrees, the nuclei of hydrogen atoms - protons, overcoming the force of mutual repulsion, come closer and, “merging”, form helium nuclei. This process of converting hydrogen into helium consists of a chain of three successive nuclear interactions called proton-proton cycle, as a result of which one helium nucleus is formed from four hydrogen nuclei. But the mass of the helium nucleus is slightly less than the mass of four protons. Thus, when synthesizing 1 g of hydrogen, the “mass defect” is 7 mg. Knowing this and using it discovered by Albert Einstein (1879-1955) law of relationship between mass and energy, it can be calculated that only the “combustion” of 1 g of hydrogen releases 150 billion calories! In a solar thermonuclear “boiler,” 564 million tons of hydrogen should “burn” every second, that is, turn into 560 million tons of helium. And if half of the hydrogen reserves remaining on the Sun were used for thermonuclear fusion, then the Sun would shine and warm the Earth with unabated force for another 30 billion years. This means that the thermonuclear process may be that inexhaustible source of solar energy that has not been established for so long.
Thermonuclear reactions occur only at temperatures above 10 million degrees. Such a high temperature can only prevail in the very “central” region of the Sun with a radius equal to about a quarter of the solar one. The energy in this self-controlled thermonuclear reactor is released in the form of hard gamma rays.
The “leakage” of radiation from the center of the Sun to the surface occurs extremely slowly. In this case, in the process of energy transfer from layer to layer, gamma quanta are crushed. First they turn into X-ray quanta, then into ultraviolet ones... About 10 million years will pass before the gamma ray quanta born in the bowels of the star emerge from it as photons of visible light. Thus, the light emitted by the Sun today was generated at the end of the Tertiary period, that is, long before the appearance of modern man on Earth.
But the optical (visible) radiation of the Sun does not reflect the physical essence of the phenomena occurring in the depths of the star. And if so, then solar thermonuclear fusion is just a hypothesis that needs to be proven.