As you may remember from your school natural history course, stars are objects that have the ability to emit their own light. Unlike them, others celestial bodies, such as planets, satellites, asteroids and comets are visible in the sky due to reflected light; they do not have their own glow. The only exceptions are meteorites that fall into the Earth's atmosphere and fall due to the force of its gravity. They burn partially or completely during the fall due to friction with air particles, and glow due to this.
But why do stars glow? This interest Ask, to which astronomers are ready to give a comprehensive answer.
History of the study of stars and their glow
For a long period of time, astronomers could not come to a consensus regarding the nature of starlight. This question has given rise to numerous disputes over many centuries. These disputes were not only of a scientific nature - at the dawn of civilization, people built numerous myths, legends and religious conjectures explaining the presence of stars in the sky and their glow. In the same way, legends and everyday explanations were created for other astronomical phenomena observed in the sky - comets, eclipses, the movements of luminaries.
Interesting fact: Some civilizations believed that the stars in the sky were the souls of the dead, others believed that these were the heads of nails with which the sky was nailed down. The sun was always considered separately; for thousands of years it was not classified as a star, it was too different in its appearance observed from the Earth's surface.
With the development of astronomy, the fallacy of such conclusions was revealed, and the stars began to be studied anew - like the Sun. Subsequently, it was possible to clarify that the Sun is also a star. Modern scientists classify the closest star to us as a red dwarf. However, the nature of the glow of the Sun and other stars gave rise to a lot of controversy until very recently.
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Theories explaining the glow of stars
In the 19th century, many scientific minds believed that a combustion process occurs on stars - exactly the same as in any earthly stove. But this theory was completely unjustified. It is difficult to imagine how much fuel a star must have in order for it to provide heat for millions of years. Therefore, this version does not deserve consideration. Chemists believed that exothermic reactions occur on stars, which provide a powerful release of large volumes of heat.
But physicists will not agree with this explanation, for the same reason as with the combustion process. The supply of reactants must be enormous to maintain the stars' luminosity and their ability to provide heat.
After Mendeleev's discoveries, the situation changed again, as the era of studying radiation and radioactive elements began. At that time, the heat and light generated by the stars and the Sun were unconditionally attributed to radioactive decay reactions; this version became generally accepted for decades. Subsequently, it was modified many times.
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Modern opinion of scientists about the causes of stellar glow
Modern scientists are completely convinced that nuclear fusion occurring in the cores of stars is capable of releasing the amount of energy that each star emits every second. This process is capable of providing glow and heat generation in huge volumes over billions of years.
Therefore, the theory is considered generally accepted. Energy from the interior passes into the gas shells of the star, from where it is radiated outward. There is an opinion in astronomer circles that it takes tens, hundreds of thousands of years to move energy from the depths of a star to its surface - this is by no means an instantaneous process. Therefore, a star can continue to shine for a long time even after synthesis in its depths ceases due to a lack of initial chemical elements.
Light from any star does not reach the surface of the Earth instantly either. Even from the Sun, the closest star to our planet, it takes about 8 minutes. The next closest star to our planet is Proxima Centauri. It takes more than four years for light to reach Earth.
In ancient times, people thought that the stars were the souls of people, living ones, or nails that held up the sky. They came up with many explanations for why the stars glow at night, and the Sun was for a long time considered an object completely different from the stars.
The problem of thermal reactions occurring in stars in general and on the Sun, the closest star to us, in particular, has long worried scientists in many areas of science. Physicists, chemists, and astronomers tried to figure out what leads to the release of thermal energy, accompanied by powerful radiation.
Chemists believed that exothermic chemical reactions occur in stars, resulting in the release of large amounts of heat. Physicists did not agree that in these space objects reactions occur between substances, since no reactions could provide so much light for billions of years.
When did Mendeleev start his famous table? new era in the study of chemical reactions - radioactive elements were found and soon it was the reactions of radioactive decay that were the main cause of radiation from stars.
The debate stopped for a while, as almost all scientists recognized this theory as the most suitable.
Modern theory about stellar radiation
In 1903, the already established idea of why stars shine and emit heat was overturned by the Swedish scientist Svante Arrhenius, who developed the theory of electrolytic dissociation. According to his theory, the source of energy in stars is hydrogen atoms, which combine with each other and form heavier helium nuclei. These processes are caused by strong gas pressure, high density and temperature (about fifteen million degrees Celsius) and occur in the inner regions of the star. This hypothesis began to be studied by other scientists, who came to the conclusion that such a fusion reaction is enough to release the colossal amount of energy that stars produce. It is also likely that hydrogen fusion would allow stars to shine for several billion years.
In some stars, helium synthesis has ended, but they continue to shine as long as they have enough energy.
The energy released in the interior of stars is transferred to the outer regions of the gas, to the surface of the star, from where it begins to be emitted in the form of light. Scientists believe that light rays travel from the cores of stars to the surface for many tens or even hundreds of thousands of years. After this, the radiation reaches the Earth, which also requires large quantity time. Thus, the radiation of the Sun reaches our planet in eight minutes, the light of the second closest star, Proxima Centrauri, reaches us in more than four years, and the light of many stars that can be seen with the naked eye in the sky has traveled several thousand or even millions of years.
Stars do not reflect light, as planets and their satellites do, but emit it. And evenly and constantly. And the blinking visible on Earth is possibly caused by the presence of various microparticles in space, which, when entering the light beam, interrupt it.
The brightest star, from the point of view of earthlings
From school we know that the Sun is a star. From our planet, this is, and by the standards of the Universe, it is slightly less than average both in size and brightness. A huge number of stars are larger than the Sun, but there are much fewer of them.
Star gradation
Ancient Greek astronomers began dividing celestial bodies by size. By the concept of “magnitude”, both then and now, they mean the brightness of the star’s glow, and not its physical size.
Stars also differ in the length of their radiation. Based on the wave spectrum, and it is indeed diverse, astronomers can tell about the chemical composition of the body, temperature and even distance.
Scientists argue
The debate on the question “why do the stars shine” has lasted for decades. There is still no consensus. It is difficult even for nuclear physicists to believe that the reactions occurring in a stellar body can release such a huge amount of energy without stopping.
The problem of what passes through the stars has occupied scientists for a very long time. Astronomers, physicists, and chemists have attempted to figure out what triggers the eruption of thermal energy, which is accompanied by bright radiation.
Chemists believe that the light from a distant star is the result of an exothermic reaction. It ends with the release of a significant amount of heat. Physicists say that chemical reactions cannot take place in the body of a star. For none of them is capable of going non-stop for billions of years.
The answer to the question “why do stars shine” became a little closer after Mendeleev’s discovery of the table of elements. Now chemical reactions have begun to be viewed in a completely new way. As a result of the experiments, new radioactive elements were obtained, and the theory of radioactive decay becomes the number one version in the endless debate about the glow of stars.
Modern hypothesis
The light of a distant star did not allow Svante Arrhenius, a Swedish scientist, to “sleep”. At the beginning of the last century, he turned the idea of radiation of heat by stars, developing the concept. It consisted of the following. The main source of energy in the body of a star is hydrogen atoms, which are constantly involved in chemical reactions with each other, form helium, which is much heavier than its predecessor. Transformation processes occur due to gas pressure of high density and a temperature that is wild for our understanding (15,000,000°C).
The hypothesis was liked by many scientists. The conclusion was clear: the stars in the night sky glow because a fusion reaction occurs inside and the energy released during this process is more than enough. It also became clear that the combination of hydrogen can proceed non-stop for many billions of years in a row.
So why do stars shine? The energy that is released in the core is transferred to the outer gas shell and radiation visible to us occurs. Today, scientists are almost sure that the “road” of the beam from the core to the shell takes more than a hundred thousand years. The beam from the star also takes quite a long time to reach the Earth. If radiation from the Sun reaches the Earth in eight minutes, the brighter stars - Proxima Centauri - in almost five years, then the light of the rest can travel for tens and hundreds of years.
One more “why”
Why stars emit light is now clear. Why is it flickering? The glow coming from the star is actually even. This is due to gravity, which pulls the gas expelled by the star back. The flickering of a star is a kind of error. The human eye sees a star through several layers of air, which is in constant motion. A star ray passing through these layers appears to flicker.
Since the atmosphere is constantly moving, hot and cold air flows, passing under each other, form turbulence. This causes the light beam to bend. also changes. The reason is the uneven concentration of the beam reaching us. The star pattern itself is shifting. This phenomenon is caused by gusts of wind passing through the atmosphere, for example.
Multicolored stars
In cloudless weather, the night sky pleases the eye with its bright colors. Arcturus also has a rich orange color, but Antares and Betelgeuse are soft red. Sirius and Vega are milky white, with a blue tint - Regulus and Spica. The famous giants - Alpha Centauri and Capella - are juicy yellow.
Why do stars shine differently? The color of a star depends on its internal temperature. The “coldest” ones are red. On their surface there is only 4,000°C. with surface heating up to 30,000°C - are considered the hottest.
Cosmonauts say that in reality the stars shine evenly and brightly, and they only wink at earthlings...
In 2013, an amazing event occurred in astronomy. Scientists saw the light of a star that exploded... 12,000,000,000 years ago, in Dark Ages Universe - this is the name in astronomy for the time period lasting one billion years that passed after Big Bang.
When the star died, our Earth did not yet exist. And only now earthlings saw its light - wandering throughout the Universe for billions of years, farewell.
Why do stars glow?
Stars glow because of their nature. Each star is a massive ball of gas held together by gravity and internal pressure. Intense thermonuclear fusion reactions take place inside the ball, the temperature is millions of kelvins.
This structure ensures the monstrous radiance of a cosmic body, capable of traveling not only trillions of kilometers (the closest star from the Sun, Proxima Centauri, is 39 trillion kilometers), but also billions of years.
The most bright stars observed from Earth - Sirius, Canopus, Toliman, Arcturus, Vega, Capella, Rigel, Altair, Aldebaran, others. 
Their visible color directly depends on the brightness of stars: blue stars are superior in radiation intensity, followed by blue-white, white, yellow, yellow-orange and orange-red.
Why are stars not visible during the day?
The reason for this is the closest star to us, the Sun, in whose system the Earth is included. Although the Sun is not the brightest or largest star, the distance between it and our planet is so small in terms of cosmic scales that sunlight literally floods the Earth, making all other faint glow invisible.
In order to personally verify the above, you can conduct a simple experiment. Make holes in the cardboard box and mark the inside with a light source (table lamp or flashlight). In a dark room, the holes will glow like small stars. And now “turn on the Sun” - the overhead room light - the “cardboard stars” will disappear. 
This is a simplified mechanism that fully explains the fact that we cannot see starlight during the day.
Are stars visible during the day from the bottom of mines and deep wells?
During the day, the stars, although not visible, are still in the sky - they, unlike the planets, are static and are always at the same point.
There is a legend that daytime stars can be seen from the bottom of deep wells, mines, and even high and wide enough (to fit a person) chimneys. It has been considered true for a record number of years - from Aristotle, an ancient Greek philosopher who lived in the 4th century BC. e., before John Herschel, an English astronomer and physicist of the 19th century.
It would seem: what’s easier - get down into the well and check! But for some reason the legend lived on, although it turned out to be absolutely false. The stars are not visible from the depths of the mine. Simply because there are no objective conditions for this.
Perhaps the reason for the appearance of such a strange and tenacious statement is the experience proposed by Leonardo da Vinci. To see the actual image of the stars as seen from Earth, he made small holes (the size of a pupil or smaller) in a piece of paper and placed them on his eyes. What did he see? Tiny dots of light - no jitter or "rays".
It turns out that the radiance of stars is a merit of the structure of our eye, in which the lens bends light, having a fibrous structure. If we look at the stars through a small aperture, we pass such a thin ray of light into the lens that it passes through the center, almost without bending. And the stars appear in their true form - as tiny dots.
The question of why stars glow is a childish question, but, nevertheless, it baffles a good half of adults who have either forgotten school course physics and astronomy, not that we skipped a lot in childhood.
Explanation of star glow
Stars are essentially balls of gas, therefore, during their existence and the chemical processes occurring in them, they emit light. Unlike the moon, which simply reflects the light of the sun, stars, like our sun, glow themselves. If we talk about our sun, it is medium in size, as well as a star in age. As a rule, those stars that visually appear larger in the sky are closer, those that are barely visible are further away. There are still millions of those that are not visible to the naked eye at all. People became acquainted with them when the first telescope was invented.
A star, although it is not alive, has its own life cycle, which is why at different stages it has a different glow. When her life path comes to an end, it gradually turns into a red dwarf. In this case, its light is, accordingly, reddish, pulses are possible, the light seems to flicker, like the glow of an incandescent lamp during sudden changes in voltage in the network. Certain parts of it either become crusty or explode again with renewed vigor, visually forming such blinking lights.
Another reason for the difference in cross-sections of stars lies in their spectrality. It's like the length and frequency of the light rays they emit. It depends on the chemical composition star, as well as its size.
All stars are also different in size. But what is meant here is not how they look to us when looking at the sky in the evening or at night, but their actual sizes, which are calculated with varying degrees of accuracy by astronomers.
It must be said that the stars glow not only at night, but also during the day. It’s just that the sun illuminates the atmosphere during the daytime, we see it consisting of many layers of clouds. At night, the sun illuminates the other side of the earth and where it is dark, the atmosphere becomes transparent. This is how we see what surrounds our planet - the stars, its satellite, the Moon, sometimes even meteorites, comets, even another planet solar system- Venus. It appears to be a large star, but its glow, like that of the Moon, is due to the fact that it reflects sunlight. Venus is visible mainly in the early evening or at dawn.
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