It is said that "the sun is like fire", whether it is ancient or modern, almost all civilizations in historical periods know that the sun is very hot, but how hot is it?
The ancients could not give the answer, and it was not until the end of the 19th century that physicists measured that the surface temperature of the sun was around 5500 degrees Celsius; as for the 15 million degrees Celsius core of the sun, it was measured in the second half of the 20th century.
Before introducing the temperature of the sun, it is necessary to understand how the sun "burns", after all, this is directly related to the core temperature and surface temperature.
Due to the limitations of the times, in the early years of science, scientists put forward many hypotheses to explain how the sun burned, some of which seem very strange to us now, such as the most famous one, the "coal ball burning theory".
At that time, a group of scientists believed that the sun was actually a super invincible coal ball, and that the light and heat on the earth were provided by this big coal ball, but this hypothesis was quickly rejected.
The main loophole is that even if the size of the sun is the same size as the coal ball, its burning time is only two thousand years, but according to the solar system formation hypothesis at that time, the earth and the sun were formed at the same time, and the estimated life of the earth is more than ten million years, so the sun can never be a coal ball.
The solar system formation hypothesis
After that, a lot of various hypotheses have been proposed, which will not be repeated here.
Looking directly at the "nuclear fusion hypothesis" proposed by British scientist Eddington in the 1920s.
Arthur. Eddington
Because the scientific community had already discovered the existence of nuclear transmutation, and Einstein's theory of relativity had also emerged, considering that with the "help" of the mass energy equation, nuclear fusion in the sun's interior should be able to run for billions of years, which was not a big problem.
And we now know that the real reason the sun "burns" is nuclear fusion. Knowing the principle of energy generation, how to measure the temperature?
How to measure temperature? Considering daily life, presumably everyone's first reaction is a thermometer, for example, when we are sick with a fever, we will use a mercury thermometer to measure the body temperature under the arm socket.
However, this method is impossible to use on the sun, not to mention that it does not have a carcass, and even if there is, then we are not so easy to send, after all, the average distance between the sun and the earth is 150 million kilometers, and at that time, not to mention the rocket spaceflight (even today, it is a difficult project).
Some friends want to refute it, what an age! Also mercury thermometer, temperature gun has not seen, do not need to touch the skin to know the temperature.
Yes, physicists at the time actually used a giant "temperature gun" to give the temperature of the sun, but the temperature gun here is not the kind of handheld temperature measuring device we commonly see, but the earth itself, yes, you heard it right.
Before you can understand this, you also need to know a very famous law of physics called the Stefan-Boltzmann Law (proposed in 1879 and 1884), which is the name of the law for the synthesis of two names.
In simple terms, it gives the relationship between energy and temperature (the energy emitted by an object is proportional to the quaternion of its own temperature). Thus knowing the temperature of the sun becomes how to measure how long the energy emitted by the sun at any moment is.
So how do you measure it? Theoretically, it is not difficult, to take a simple example, because we all live on the earth, so the basis of measurement is built on the earth, so the distance between the earth's "temperature gun" and the sun is 150 million kilometers, and the "thermoelectric conversion element" of this temperature gun can be a basin of water, which can make the sun shoot it directly for a while, record the temperature before and after, get the temperature difference, and then calculate how much heat the basin absorbs per unit time through known physical conditions such as mass and specific heat capacity.
Finally, multiply the spherical area with a radius of 150 million kilometers to get the total energy of the sun's radiation per second. The total energy is then evenly distributed to the surface of the Sun, and finally substituted for the Steffan-Boltzmann law, the surface temperature of the Sun can be obtained, which is about 5800 Kelvin, that is, about 5500 degrees Celsius.
Compared with the measurement of the surface temperature of the sun, the core temperature is more difficult, because the core temperature is buried in the central region of the sun, and the electromagnetic wave depth emitted inside is not visible (because the structure of the sun is not simple, the core area has to go through the radiation region and convection region after the production capacity, and then to the photosphere, the photon needs to go through countless collision absorption and release processes during this journey, and the whole time takes tens of thousands of years or even hundreds of thousands of years).
So it is impossible to take it for granted that we can also use the Stefan-Boltzmann law, for example, I already know the total energy emitted by the sun into space every second, and now just divide it equally on the surface of the core region, can't we calculate the temperature? In fact, the temperature is only in the early 10,000s, and it is still early to the threshold of nuclear fusion.
So when Eddington proposed the fusion hypothesis, he also made a mistake in the temperature of the sun's core, and this is not a general difficulty, if you just use the ideal gas and hydrostatic equilibrium model, and add some hypothetical proportions of internal elements, the calculated temperature is unstable, about 20 million to 40 million degrees Celsius.
The reason for this is that nuclear physics research at that time was still in its infancy, because the sun's interior belonged to hydrogen fusion, so the strong and weak interactions at the microscopic scale and quantum mechanical effects needed to be considered, and as for the details of these processes, it was not suitable for detail, so it was only mentioned an important link - "quantum tunneling effect".
Because this effect is very important and very interesting, we know that when two protons are close to each other, due to the same sex repulsion, they will encounter a strong electromagnetic repulsion force, and nuclear fusion is to overcome this repulsion force, so it needs a high temperature and high pressure state.
However, due to the existence of tunneling effect, the difficulty of this process has decreased a lot, for example, according to classical theory, for example, the distance between two protons is less than 1, so that the strong force is greater than the electromagnetic force, and then fusion, and considering quantum tunneling, then this distance can be appropriately relaxed, such as 1.1 can be.
(The example just now is just a popular way, not rigorous, but the quantum tunneling effect does exist, for example, the following GIF is made of moving molecules with a scanning tunnel electron microscope, and the principle of this microscope is the quantum tunneling effect)
So in the end, people calculated that the core temperature of the sun was corrected to about 15 million degrees Celsius.
But this is not the end of the matter, 15 million degrees Celsius is only calculated after all, we should have experiments to measure and verify ah.
And this matter fell to two experiments, one is neutrino capture, and the other is heliostoseological observation.
I have to mention again that the reason why one is capture and the other is observation is because the sun is too far away from the earth, the surface temperature is too high, and there is no object on the earth that can go deep into the sun without melting, so there are only two means left, one is a common observation, and the other is to capture the neutrinos produced by the sun's core.
The following is a very simple explanation of the principle of these two:
The first is neutrinos, because the rate at which neutrinos are produced inside the Sun is very sensitive to core temperature, so we can determine what the core temperature is based on the fluxes of neutrinos captured in different energies on Earth.
The second is the means of heliostoseism, the so-called heliostoseism, popularly but not rigorously speaking, is to observe the vibrations on the sun, so it is called helioquake (analogous to the vibration on the earth, called earthquake). And we know that through earthquakes can study the internal structure of the earth, then the same is true for solar earthquakes, the temperature of the sun's interior, the proportion of elements, rotation speed, structure, etc., can be studied.
In the end, it was found that the results of the two measurements were consistent, and the temperature of the sun's core was 15 million degrees Celsius.