"We know that the mass of the Sun: M⊙=2×10, radius R=7×107, and we can bring in (2) the gravitational energy released by the Sun shrinking to its present state.

The total luminosity of the Sun is L=4×10e

g·s-1 If this radiant luminosity is maintained by gravity as an energy source, then the duration is: 11×108 years

Many proofs suggest that the Sun has remained stable in its present state for 5×108 years, so the stellar phase can only be a brief transition before the Sun forms a stable state like today. This raises the new question, how did the gravitational contraction of the stellar billet stop, and what was the energy source of solar radiation after that?"

The teacher's voice seemed tireless, and it had been a week of lessons in a row.

At this moment, even Yun Meng and Bai Feng, who had always been very patient, were a little impatient, but they accompanied Hua Feng to know more things that they had never thought about and thought about before.

The main sequence stage increases in density during contraction, and we know ρ∝

-3, by Eq. (4),

c∝

3/2, so

C ratio

The reduction is faster, and a part of the shrinking air cloud reaches a critical point under the new conditions, and a small disturbance can cause a new local collapse. Under certain conditions, the large gas cloud shrinks into a condensate and becomes a protostar, and the protostar continues to shrink after adsorbing the surrounding gas cloud, the surface temperature remains unchanged, and the core temperature continues to increase, causing various nuclear reactions in temperature, density and gas composition. The heat energy generated makes the temperature rise extremely high, and the gas pressure resists gravity to stabilize the protostar and become a star, and the evolution of the star begins with the main sequence star.

When the temperature reaches more than 104K, that is, the average thermal kinetic energy of the particles reaches more than 1 eV, the hydrogen atom is fully ionized through thermal collision (the ionization energy of hydrogen is 13.6 eV), and after the temperature is further increased, the collision of hydrogen nuclei and protons in the plasma gas may cause nuclear reactions. For high-temperature gases of pure hydrogen, the most effective nuclear reaction series is the so-called P-P chain:

Among them, the main reaction is 2D(p,γ)3He. D (deuterium, an isotope of hydrogen, consisting of one proton and one neutron) has only about 10-4 of hydrogen, which burns out quickly (the principle is similar to that of modern ** weapons). If D is more than 3He (helium 3, the isotope of helium, consisting of 2 protons and 1 neutron) at the beginning, then the resulting 3H (tritium, an isotope of hydrogen, composed of 1 proton and 2 neutrons, which decays into helium3) may be the main source of the early 3He of the star, and it is possible that this 3He is still preserved due to the convection that reaches the surface of the star.

Of the 26.7 MeV energy released, most of it is consumed to heat and emit light to the star, becoming the main source of the star.

We mentioned earlier that the evolution of stars starts from the main sequence, so what is the main sequence? When H burns steadily to He, the star becomes the main sequence. It was found that 80 to 90 percent of the stars are main-sequence stars, and their common feature is that hydrogen is burning in the core region, and their luminosity, radius, and surface temperature are different.

The minimum observed mass of the main-sequence star is approximately 0.1M⊙. The model calculation shows that when the mass is less than 0.08M⊙, the contraction of the star will not reach the ignition temperature of hydrogen, so that the main sequence star cannot be formed, which indicates that it has a lower mass limit for the main sequence star. The maximum mass observed for main-sequence stars is about a few dozen solar masses. Theoretically, a star with too much mass has strong radiation, and the energy processes inside it are intense, so the more unstable the structure. But theoretically, there is no absolute upper limit to quality.

When a certain star cluster is statistically analyzed, it is found that the main-sequence star has an upper limit, what does this mean? We know that the luminosity of the main-sequence star is a function of mass, which can be segmented and expressed by power:

L∝Mν

where v is not a constant, it has a value between 3.5 and 4.5. M large reflects the mass available for combustion in the main sequence star, while L large reflects fast combustion, so the lifetime of the main sequence star can be similarly marked by the trademarks M and L:

T∝M-（ν-1)

That is, the lifetime of the main sequence star decreases according to the power law with the increase of mass, and if the existing age of the whole cluster is T, then a cut-off mass MT can be obtained from the relationship between T and M. Main-sequence stars with masses greater than MT have ended the H-burning phase of the core instead of the main-sequence star, which is why it is observed that there is an upper limit on the cluster composed of a large number of stars of the same age.

Table 1 lists the ignition temperature of each element and the duration of combustion based on a 25M⊙ stellar evolution model.

As can be seen from the table, the nucleus with the largest atomic number has a higher ignition temperature, and the largest nucleus is not only difficult to ignite, but also burns more violently after ignition, so the combustion duration is shorter. The 25M⊙ model star has a total lifetime of 7.5×109 years during the combustion phase, of which more than 90 percent is the hydrogen combustion phase, the main sequence phase. Statistically speaking, this indicates that there is a better chance of finding a star in the main sequence phase. This is the basic reason why most of the observed stars are main-sequence stars.

Since the main component of star formation is hydrogen, and the ignition temperature of hydrogen is lower than that of other elements, the first stage of stellar evolution is always the combustion phase of hydrogen, that is, the main sequence stage. During the main sequence phase, the star maintains a steady balance of pressure distribution and surface temperature distribution inside the star, so that its luminosity and surface temperature change only slightly throughout the long phase. Let's discuss how the star will evolve further after the hydrogen in the core of the star is burned.

After the star burns out the hydrogen in the core of the star, it goes out, at this time the core area is mainly helium, which is the product of combustion, and the material in the outer area is mainly unburned hydrogen.

The end of a nuclear combustion phase indicates that the temperature in all parts of the star has been lower than the temperature required to cause ignition there, and the gravitational contraction will increase the temperature in various parts of the star, which is actually the temperature required to find the next nuclear ignition, and the gravitational contraction will increase the temperature of all parts of the star, and the gravitational contraction after the main sequence first ignites not the helium in the core area (its ignition temperature is too high), but the hydrogen shell between the core and the periphery, after the hydrogen shell is ignited, the core area is in a high temperature state, and there is still no nuclear energy, it will continue to shrink。

At this time, due to the gravitational potential energy released in the core area and the nuclear energy released by the hydrogen in combustion, the hydrogen layer that does not burn through the periphery must expand violently, that is, the medium radiation becomes more transparent, and the excess heat energy is discharged to maintain the thermal balance. The expansion of the hydrogen layer lowers the surface temperature of the star, so this is a process of increasing luminosity, increasing radius, and cooling the surface, this process is the transition of the star from the main sequence to the red giant, and the process progresses to a certain extent, the temperature in the center of the hydrogen region will reach the temperature of helium ignition, and then it transitions to a new stage - helium combustion stage.

Before helium ignition occurs in the center of the star, the gravitational contraction to make its density reach the order of 103g·cm-3, at this time the pressure of the gas is very weak dependent on temperature, then the energy released by the nuclear reaction will increase the temperature, and the temperature increase in turn will increase the rate of the nuclear reaction, so once ignited, it will soon burn very violently, so that it explodes, the ignition in this way is called "helium flash", so the phenomenon will see that the luminosity of the star suddenly rises to a very large level, and then decreases very low.

On the other hand, when the gravitational force shrinks, its density does not reach the order of 103g·cm-3, and the pressure of the gas is proportional to the temperature, and the increase of the ignition temperature leads to an increase in pressure, and the expansion causes the temperature to decrease, so the combustion can be carried out stably, so the influence of these two ignition situations on the evolution process is different.

How do stars evolve after the "helium flash"? The flash releases a lot of energy, which is likely to blow away all the hydrogen in the outer layer of the star, leaving the helium core.

The density of the helium core region is reduced due to expansion, and it is possible for helium to burn normally in it in the future. The product of helium combustion is carbon, after helium extinguishment, the star will have a helium shell in the carbon core region, because the remaining mass is too small to reach the ignition temperature of carbon, so it ends the evolution of helium combustion and goes to thermal death.

Since gravitational collapse is mass-dependent, stars with different masses evolve differently.

M<0.08M⊙ star: hydrogen cannot be ignited, it will not have a helium combustion phase and will die directly.

0.08

A star of 0.35

2.25

In the early stage of the nuclear reaction, when the temperature reaches the order of 1010K, the 13C, 17O energy and 4He produced by the CNO cycle are new (α,

) reaction to form 16O and 20Ne, and after a long time of nuclear reaction, Na and N in Ne(p,γ)Na(β+,ν) Na absorb the Ne energy formed by two 4He α,

These reactions are not important as an energy source, but the neutrons emitted can further neutron nuclear reactions.

They had already listened to the teacher three times, and although they didn't know what the intentions of the Dawning Academy were, Hua Feng still didn't dare to slack off.