The spectral line of a star actually comes from the electronic energy level transition in the atmosphere on the surface of the star.

Astronomical research is very similar to archaeology. They all restore the truth of past historical events. The starlight we usually see actually contains a lot of information worthy of our discussion.

Comte is a French philosopher. He once put forward an assertion that "man can never understand the chemical composition of stars". He put forward this assertion in order to come up with a proposition to see what man can't do.

However, before his voice fell, astronomers began to observe starlight with spectrometers. Now astronomy knows more about the chemical composition of stars and nebulae than we know about the drugs in the cupboard. "This is a passage in Mathematics and Imagination, which vividly reveals such a feature that astronomers use starlight to explore the internal structure of stars. Today, if we compare the internal structure of the sun and the earth, you will find that the understanding of the sun is much richer and deeper than that of the earth. From the core of the sun to the surface of the sun, the changes of the basic physical parameters of the sun obtained by people's physical models are very accurate, and you may wonder why it is so far away.

Answer: "Starlight".

Newton made a great contribution in 1666. He used a prism to break down sunlight into polychromatic light. In fact, we don't need special instruments or equipment to discuss starlight or sunlight. Walking in nature, we can see the rich physical content contained in sunlight. In the Yuan Dynasty, Baipu had such a saying that "green mountains and green waters are all white". Various colors represent the fragrance of nature, but what people don't think is that they actually reflect the colors contained in sunlight, because in fact, the colors we see don't come from the radiation of the object itself, but reflect the sunlight to express its colors, so in this sense, all the colors we see come from sunlight.

If we want to measure how high the temperature of the sun is, it seems an impossible task for people on earth, because the sun is too far away from us and its temperature is too high, and we may never be able to do this, but in fact it is not. By receiving the radiation from the sun, we can get the whole continuous radiation spectrum of the sun, which reflects the wavelength of the sun, and the wavelength is in microns. 1 micron is one millionth of 1 meter, which is about equivalent to 1/60 of a human hair. In fact, the sun's radiation exists in all bands, except our common visible light, which has radiation in infrared and ultraviolet bands, and the shape of this radiation provides a temperature information.

Thought experiment:

Imagine an object called a "black body". Black body has such a characteristic that it only receives radiation and does not reflect. Therefore, if such a special device is made and a hole is made in a small ball, the light coming in from outside the hole can only be reflected inside, and there is almost no chance to escape from the ball, which can be considered as a "black body".

When it receives radiation, its temperature will rise and it will produce radiation itself. There is a very close relationship between its intensity and its temperature. Generally speaking, the higher the temperature, the shorter the radiation spectrum will extend to, so this provides a method to measure the temperature of celestial bodies. In this way, the continuous spectrum of blackbody with different temperatures is compared with the continuous spectrum of celestial bodies, so that the temperature of celestial bodies can be measured. For example, our sun can use this method.

The famous Orion has two very bright stars, one is Betelgeuse and the other is Betelgeuse. If you look closely, they are different colors in the telescope. The color of Betelgeuse tends to be red, while that of Betelgeuse tends to be white. The different colors actually reflect that the strongest bands corresponding to their continuous spectra are different, so the colors are different.

The above is the continuous spectrum, but you will find that there are many details besides the continuous spectrum, the most obvious of which is the "absorption line", which is the depression in the continuous spectrum. Absorption lines can provide richer and more detailed information about stars. In order to understand the source of absorption lines, it is necessary to go into the microscopic world and see their production process. Therefore, the study of the properties of such a huge celestial body as a star actually begins with the smallest particles.

The concept of "atom" was put forward by ancient Greek philosophers, especially democritus. They think that atoms are the smallest inseparable unit in our world, but today we know that atoms can still be divided into smaller particles, such as protons and neutrons. Atoms are extremely tiny. After magnifying atoms, we can see that their core is the nucleus, and the size of the nucleus is 654.38+ million times different from that of a prokaryote. Therefore, without the electrons around the nucleus, our world is almost a vacuum, so only electrons and the nucleus can form atoms. Their relationship is very special. Electrons revolve around the nucleus, but the orbit of this rotation is not random. It follows the quantization law, and the size of its orbit is specific. If the atom is divided into a tall building, then the nucleus is only a particle about 1 mm in size.

Electrons' orbits are quantized, so they can be graded from 1, 2,3 to higher energy levels. Generally speaking, the lower the energy level, the more stable it is, so electrons can jump in some different energy levels, but the transition process will be accompanied by the absorption and emission of energy or "light", and electrons must absorb light when jumping from a lower energy level to a higher energy level. On the other hand, it can emit light when it transitions from a high energy level to a low energy level, and the energy of the absorbed and emitted "light" is exactly equal to the difference between its two energy levels, so the absorption line in the star spectrum actually comes from the energy level transition process.

This is related to which energy level the electron is more likely to be in. The energy level of electrons corresponds to the lowest energy state of atoms, but atoms collide with each other and transfer energy to each other, so that electrons may be above a higher energy level. Generally speaking, the more frequent collisions, the higher the energy level of electrons, so there is a physical correlation between the energy level of electrons and the temperature, and this energy level is directly related to the spectral line generated by electrons, which is generated at a higher temperature.

The center of the star is the region of nuclear reaction and the source of energy. A large amount of light is emitted from the core region of the star to the surface of the star. When it passes through the surface atmosphere of a star, some light will be absorbed by atoms in the atmosphere, or more accurately, by electrons in these atoms. After electrons absorb photons, there will be a transition process, so the spectral lines of stars actually come from the energy level transition of electrons in the atmosphere on the surface of stars.

Chart: the formation of absorption lines in the spectrum of stars

Because the structure of each atom is different, the different spectral lines we see actually represent different types of atoms, so in this sense, the spectrum of a star is similar to our fingerprints, everyone's fingerprints are unique, and the spectrum of a star is unique, because the temperatures corresponding to various elements contained in it are not exactly the same, so the temperature of the atmosphere on the surface of the star can be obtained through the difference of spectral lines.

For example:

Comparing the spectra of the sun and Vega, the position and intensity of their absorption lines are not exactly the same. Each absorption line comes from a specific energy level transition, reflecting a specific temperature. So according to this feature, we can determine the temperature of Vega and the temperature of our sun.

Measuring the surface temperature of a star requires identifying a large number of star spectra. This work began at the beginning of last century. At that time, an astronomer named Pickering of Harvard University hired a group of women to help him identify the star spectrum. Among them, two representatives made great contributions:

"Canon" is almost deaf, but she has a very keen judgment on the spectral discrimination of stars. During her lifetime, she made the spectra of about 350,000 stars and obtained their temperatures. On this basis, cannon put forward a method of star classification. In the past, people only classified the stars from the spectrum itself, but Cannon found that by classifying the temperature, more scientific and effective star spectrum types can be provided: "O, B, A, F,"

The chemical abundance of stars, 1 the first person to complete this work, called Payne. When studying the spectra of stars, Payne found that those spectral lines depend not only on the temperature of stars, but also on the content of elements in stars. Therefore, from this basic condition, Payne not only got their temperature, but also got the content of elements they contained, which is called "element abundance". Payne found that the most abundant element in a star is "hydrogen", accounting for about 70%, followed by "helium" and a few elements heavier than helium, which are usually called "metal elements" or "heavy elements".

The above two points are the two most important information obtained from the star spectrum.

The star spectrum can get the mass of the star, which is also closely related to the star spectrum. This is the "Doppler effect" that we often encounter in our daily life. When a police car horns towards us, its frequency will increase, and if the police car deviates from us, its frequency will decrease. This is the so-called "Doppler effect", which actually reflects how the wavelength or frequency of sound waves changes with its movement. A completely similar phenomenon also occurs in stars. If a star moves towards us, its spectrum will shift to the short wave direction, and vice versa.

If a star is in a binary system, they orbit each other, so each star will periodically approach and leave us. The consequence of this movement is that their spectra will periodically shift from red to blue, so the spectra of stars can not only provide their temperature and element abundance, but also reflect the motion state of stars. According to the displacement, the speed of the star can be determined, and then the star can be obtained by using the speed.

In the process of studying stellar atmosphere and stellar spectrum, people gradually realize that a star is actually a hot gas ball, and its temperature can be as low as several thousand degrees and as high as several thousand degrees, so it can only exist in the form of gas in this state.

On the other hand, the existence of various elements in the stellar atmosphere, although different from that of the earth in abundance, is very similar in species, which proves that the elements in the stellar atmosphere or the stars themselves have similar sources to those in the earth.

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