After the Sun enters adulthood, it will remain stable for 10 billion years
Now the Sun is already in adulthood (lasted for 5 billion years)
5 billion After 10 years, the sun will enter its old age and expand rapidly, forming a red giant star that is 1,000 times as bright as it is today. This process will last for 1 billion years.
Ten years later, the sun will collapse and its size will shrink sharply, forming a red giant star that is only the size of the Earth. A white dwarf of large size but extremely dense
After a period of time, it turns into a brown dwarf and eventually dies
The evolution process of stars
1. The formation of stars
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When the universe develops to a certain period, the universe is filled with uniform neutral atomic gas clouds. Large-volume gas clouds become unstable and collapse due to their own gravity. In this way the star enters the formation stage. At the beginning of the collapse, the internal pressure of the gas cloud is very small, and the material accelerates to fall towards the center under the action of self-gravity. When the linear dimension of matter shrinks by several orders of magnitude, the situation becomes different. On the one hand, the density of the gas increases dramatically. On the other hand, due to the partial conversion of the lost gravitational potential energy into heat energy, the temperature of the gas also increases significantly. With a large increase, the pressure of a gas is proportional to the product of its density and temperature. Therefore, during the collapse process, the pressure increases faster. In this way, a pressure field sufficient to compete with the self-gravity is quickly formed inside the gas. This pressure The field finally stops the gravitational collapse, thus establishing a new mechanical equilibrium configuration, which is called star failure.
The mechanical balance of the star base is caused by the internal pressure gradient competing with self-gravity, and the existence of the pressure gradient depends on the unevenness of the internal temperature (that is, the temperature in the center of the star base is higher than that of the periphery temperature), so thermally, this is an unbalanced system, and heat will gradually flow out from the center. This natural tendency toward thermal equilibrium has a weakening effect on mechanics. Therefore, the star base must slowly shrink, and its gravitational potential energy decreases to increase the temperature, thereby restoring the mechanical balance; at the same time, the gravitational potential energy decreases to provide the energy required for star base radiation. This is the main physical mechanism of star blank evolution.
Below we use the classical gravity theory to roughly discuss this process. Consider a spherical gas cloud system with density ρ, temperature T and radius r. The thermal motion energy of the gas is:
ET= RT= T
(1) Treat the gas as a single Atomic ideal gas, μ is the molar mass, R is the gas universal constant
In order to obtain the gravitational energy Eg of the gas cloud ball, imagine that the mass of the warp ball is moved to infinity little by little, and all the balls are removed The work done by the field force is equal to -Eg. When the mass of the ball is m and the radius is r, the field force does work when removing dm from the surface:
dW=- =-G( )1/3m2/3dm
( 2) So: -Eg=- ( )1/3m2/3dm= G( M5/3
So: Eg=- (2),
The total energy of the gas cloud: E=ET+EG (3)
Thermal motion makes the gas distribute evenly, and gravity makes the gas concentrate. When E>0, the thermal motion dominates and the gas cloud is stable. , small perturbations will not affect the balance of gas clouds; when E<0, gravity dominates, small density perturbations produce deviations from uniformity, and gravity increases where density is high, intensifying deviations and destroying the balance, and the gas begins to collapse. . The critical radius for shrinkage is obtained from E≤0:
(4) The corresponding critical mass of the gas cloud is:
(5) The original gas cloud density is small and the critical mass is very small. Large. So few stars are produced alone, and most are produced together into star clusters. Spherical star clusters can contain 105→107 stars, which can be considered to be produced at the same time.
We know: The mass of the sun. : MΘ=2×1033, radius R=7×1010, we bring it into (2) and we can get the gravitational energy released by the sun shrinking to its current state
The total luminosity of the sun L=4×1033erg .s-1 If this radiation luminosity is maintained by gravity as the energy source, then the duration is:
Many proofs show that the sun has stably maintained its current state for 5×109 years. Therefore, The starburst phase could only be a brief transitional phase before the Sun reached a stable state like it is today.
This raises a new question: How does the gravitational contraction of the star blank stop? What does solar radiation use as energy source after that?
2.2 In the main sequence star stage, the density increases during the contraction process. We know that ρ∝r-3, from equation (4), rc∝r3/2, so rc decreases faster than r. , a part of the shrinking gas cloud reaches criticality under new conditions, and small disturbances can cause new local collapse. If this continues, under certain conditions, the large gas cloud shrinks into a condensate and becomes a protostar. The protostar continues to shrink after adsorbing the surrounding gas clouds. The surface temperature remains unchanged and the core temperature continues to increase, causing changes in temperature, density and gas composition. Various nuclear reactions. The generation of heat energy causes the temperature to rise extremely high, and the gas pressure resists gravity to stabilize the protostar and become a star. The evolution of stars begins with main sequence stars.
Most of the components of stars are H and He. When the temperature reaches above 104K, that is, the average thermal kinetic energy of the particles reaches above 1eV, hydrogen atoms are fully ionized through thermal collision (the ionization energy of hydrogen is 13.6 eV), after the temperature further increases, the collision of hydrogen nuclei in the plasma gas may cause a nuclear reaction. For high-temperature gases of pure hydrogen, the most effective nuclear reaction series is the so-called P-P chain:
The main one is the 2D(p,γ)3He reaction. The D content is only about 10-4 of hydrogen, and it burns out quickly. If there is more D than 3He at the beginning, the 3H generated by the reaction may be the main source of 3He in the early stages of the star. Due to convection, this 3He that reaches the star surface may still remain until now.
Li, Be, B and other light nuclei have a very low binding energy like D, and their content is only about 2×10-9K of H. When the core temperature exceeds 3×106K, they start to burn, causing (p, α ) reacts with (p,α) and quickly becomes 3He and 4He. When the core temperature reaches 107K and the density reaches about 105kg/m3, the generated hydrogen is converted into He in the 41H→4He process. This is mainly the p-p and CNO cycles. Containing 1H and 4He at the same time causes a p-p chain reaction, which consists of the following three branches:
p-p1 (only 1H) p-p2 (containing 1H and 4He at the same time) p-p3
< p> Or assume that the weight ratios of 1H and 4He are equal. As the temperature increases, the reaction gradually transitions from p-p1 to p-p3.When T>1.5×107K, the process of burning H in stars can transition to being dominated by the CNO cycle.
When heavy elements C and N are mixed in stars, they can act as catalysts to change 1H into 4He. This is the CNO cycle. The CNO cycle has two branches:
Or total The reaction rate depends on the slowest 14N(p,γ)15O, and the (p,α) and (p,γ) reaction branch ratio of 15N is approximately 2500:1.
This ratio is almost independent of temperature, so one in 2500 CNO cycles is CNO-2.
During the p-p chain and CNO cycle, the net effect is that H is burned to produce He:
Of the 26.7MeV energy released, most of it is consumed to heat and illuminate the star, becoming The main source of stars.
We mentioned earlier that the evolution of stars begins with the main sequence, so what is the main sequence? When H is steadily burned into He, the star becomes a main sequence star. It was found that 80 to 90 percent of stars are main sequence stars. Their most common feature is that hydrogen is burning in the core region. Their luminosities, radii, and surface temperatures are all different. It was later proved that: The quantitative difference between main sequence stars is mainly their mass, followed by their age and chemical composition. This process of the sun takes about tens of millions of years.
The minimum observed mass of a main sequence star is approximately 0.1M⊙. Model calculations show that when the mass is less than 0.08M⊙, the star's contraction will not reach the ignition temperature of hydrogen, and thus a main sequence star cannot be formed. This shows that there is a lower mass limit for main sequence stars. The maximum observed mass of a main sequence star is on the order of tens of solar masses. Theoretically, stars with too much mass emit strong radiation and have violent internal energy processes, so their structures are more unstable. But theoretically there is no absolute upper limit to quality.
When doing statistical analysis on a certain star cluster, people found that there is an upper limit for main sequence stars. What does this mean? We know that the luminosity of main sequence stars is a function of mass. This function can be expressed piecewise by a power formula:
L∝Mν
Where υ is not a constant, its value Probably between 3.5 and 4.5. A large M reflects that there is more mass available for burning in the main sequence star, while a large L reflects the fast burning. Therefore, the lifespan of the main sequence star can be approximately marked by the trademarks of M and L:
T∝M- (ν-1)
That is, the lifetime of the main sequence star decreases according to the power law as the mass increases. If the age of the entire star cluster is T, it can be calculated from the relationship between T and M A cutoff mass MT. Main-sequence stars with masses greater than MT have ended the H-burning stage in their cores and are not main-sequence stars. This is why it is observed that star clusters composed of a large number of stars of the same age have an upper limit.
Now we will discuss the reason why most of the observed stars are main sequence stars. Table 1 is based on the constant combustion stage ignition temperature (K) of 25M⊙ and the core temperature (g.cm-3). Time (yr)
H 4×107 4 7×106
He 2×108 6×102 5×105
C 7×108 6×105 5×102
Ne 1.5×109 4×106 1
O 2×109 1×107 5×10-2
Si 3.5×109 1× 108 3×10-3
The total lifetime of the combustion stage is 7.5×106
The star evolution model lists the ignition temperatures of various elements and the duration of combustion. It can be seen from the table that the nucleus with a large atomic number has a higher ignition temperature. The largest nucleus is not only difficult to ignite, but also burns more violently after ignition, so the combustion lasts for a shorter time. This 25M⊙ Table 1 25M⊙ star evolution model, the total lifespan of the burning stage of the model star is 7.5×106 years, and more than 90% of the time is the hydrogen burning stage, that is, the main sequence stage. Statistically speaking, this suggests that the odds of finding a star on the main sequence are higher. This is the basic reason why most of the observed stars are main sequence stars.
2.3 Post-main sequence evolution. 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 star evolution is always the burning of hydrogen. stage, that is, the main sequence stage. During the main sequence stage, the star maintains a stable pressure distribution and surface temperature distribution inside the star, so throughout the long stage, its luminosity and surface temperature only change slightly. Next we discuss how the star will further evolve after the hydrogen in the star core is burned out.
After the star burns out the hydrogen in the core area, it goes out. At this time, the core area is mainly hydrogen, which is the product of combustion. The material in the outer area is mainly unburned hydrogen. After the core area burns out, the star Without the energy of radiation, it will have gravitational contraction, which is a key factor. The end of a nuclear burning phase indicates that the temperature everywhere in the star has dropped below the temperature required to cause ignition there. The gravitational contraction will increase the temperature everywhere in the star. This is actually the search for the next nuclear ignition. At the required temperature, gravitational contraction will cause an overall increase in temperature everywhere in the star. The gravitational contraction after the main sequence will first ignite not the helium in the core (its ignition temperature is too high), but the core and periphery. Between the hydrogen shell, after the hydrogen shell is ignited, the core area is in a high temperature state, but there is still no nuclear energy, and it will continue to shrink. At this time, due to the gravitational potential energy released in the core region and the nuclear energy released by the burning hydrogen, the non-burning hydrogen layer in the periphery must expand violently, that is, the medium radiation becomes more transparent. The expansion of the hydrogen layer reduces the surface temperature of the star, so this is a process in which the luminosity increases, the radius increases, and the surface cools. This process is the transition of the star from the main sequence to the red giant. When the process proceeds to a certain extent, the hydrogen region The temperature in the center will reach the temperature of hydrogen ignition, and then it will transition to a new stage - the helium combustion stage.
Before helium ignition occurs in the center of the star, gravity shrinks so that its density reaches the order of 103g.cm-3. At this time, the pressure of the gas is very weakly dependent on the temperature, so the energy released by the nuclear reaction The temperature will rise, and the temperature rise will in turn increase the nuclear reaction rate. Once ignited, it will burn so violently that it will explode. This method of ignition is called "flash?", so in the phenomenon You will see that the star's luminosity suddenly rises to a very high level, and then drops to a very low level.
On the other hand, when gravity contracts, its density does not reach the level of 103g.cm-3. The pressure of the gas is proportional to the temperature. When the ignition temperature increases, the pressure increases, and the nuclear combustion zone expands. The expansion causes the temperature to decrease, so combustion can proceed stably. Therefore, the impact of these two ignition conditions on the evolution process It's different.
How does a star evolve after a "helium flash"? The flash releases a large amount of energy, which is likely to blow away the hydrogen in the outer layer of the star, leaving behind the helium core. The density of the helium core area is reduced due to expansion, and helium may burn normally in it. The product of helium burning is carbon. After the helium burns out, the star will have a helium shell in the carbon core area, due to the remaining mass. The gravitational contraction is too small to reach the ignition temperature of carbon, so it ends the evolution of helium burning and goes to thermal death.
Since gravitational collapse is related to mass, stars with different masses are evolving. There is a difference.
M<0.08M⊙ star: hydrogen cannot ignite, it will die directly without helium burning stage.
0.08 0.35 2.25 In the early stage of the He reaction, when the temperature reaches the 108K level, 13C and 17O produced by the CNO cycle can react with 4He in new (α, n) reactions to form 16O and 20Ne. After the He reaction proceeds After a long time, 20Ne(p,γ) 21Na(β+,ν) 21Na in 21Na and 22Ne formed by 14N absorbing two 4He can undergo (α,n) reaction to form 24Mg and 25Mg, etc. These reactions serve as energy sources. It's not important, but the neutrons emitted can further cause neutron nuclear reactions. 4 He After the reaction is completed, when the core temperature reaches 109K, C, O, Ne combustion reactions begin to occur, which are mainly C-C reactions, O-O reactions, and γ, α reactions of 20Ne: 8→10M⊙< M stars: hydrogen, helium, carbon, oxygen, neon, and silicon can burn normally step by step. Finally, a core area is formed in the center that cannot release energy. Outside the core area are various shells of hydrogen elements that can burn but are not burned out. At the end of the nuclear burning stage, the entire star presents a layered (Fe, Si, Mg, Ne, O, C, He, H) structure from the inside to the outside. 2.4 The end of stars Now we already know that for stars with a mass less than 8→10M⊙, it will end its life because it cannot reach the next stage and ignition temperature. Nuclear burning stage; for a more massive star, it will end its nuclear burning stage after the core region runs out of fuel. After that, what is the final fate of the star? Once the nuclear burning stops , the star must undergo gravitational contraction, because the pressure inside the star to maintain mechanical equilibrium is related to its temperature. Therefore, if the star is in a "final" equilibrium configuration, it must be in a "cold" equilibrium configuration, that is, its pressure has nothing to do with its temperature. Main-sequence star core H After exhaustion, the exit from the main sequence begins its final journey. The outcome depends mainly on quality. For stars with very small masses, the self-gravity inside the object is not important due to their small mass. The balance inside the solid is achieved by the net Coulomb attraction between positive and negative ions and the pressure between electrons. When the mass of the star becomes larger and the self-gravity cannot be ignored, then the self-gravity increases the internal density and pressure. The increase in pressure causes the material to undergo pressure ionization, which gradually becomes a solid electrolyte. The confinement breaks down and the transition is to plasma gas. Increase the mass, that is, increase the density. At this time, the pressure has nothing to do with the temperature, thus reaching a "cold" equilibrium configuration. The kinetic energy of the electrons in the plasma is large enough to cause beta decay inside the material: Here p is the proton in the nucleus. Such a reaction will gradually change the atomic nucleus in the negative ion body into a neutron-rich nucleus when the density reaches 108 g.cm-3. There will be too many neutrons in the nucleus. This results in a loose nuclear structure. When the density exceeds 4×1011g.cm-3, neutrons begin to separate from the nucleus and become free neutrons. The self-gravity balances the pressure between neutrons. If when the mass increases, the pressure between the neutron gas can no longer resist the self-gravity of the material, and a black hole is formed. However, due to the post-evolution stages of most stars, the mass is less than its initial mass, such as stellar wind, "helium flash", supernova explosion, etc., They will cause the star to lose a large percentage of its mass. Therefore, the fate of the star cannot be judged by its initial mass. It actually depends on the process of evolution. Then we can draw this conclusion. Stars below 8→10M⊙ eventually throw away part or most of their mass and become a white dwarf. Stars above 8→10M⊙ will eventually become neutron stars or black holes through the gravitational collapse of the star core. 3. Ending The currently observed stellar mass range is 0.1→60M⊙. Objects with a mass less than 0.08M⊙ cannot reach the ignition temperature. Therefore, if it does not emit light, it cannot become a star. The center temperature of celestial bodies with a mass greater than 60M⊙ is too high and unstable, and it has not yet been discovered. Through discussion, we can generally understand the evolution process of stars, which mainly go through: gas cloud → collapse stage → main sequence star stage → post-main sequence stage → final stage. This is of great significance to our further understanding of stellar evolution. Looking at the night sky from the earth, the universe is a world of stars. The distribution of stars in the universe is uneven. From the day they were born, they have gathered in groups, reflected each other, and formed binary stars, star clusters, and galaxies... Stars are burning planets. Generally speaking, stars are relatively large in size and mass. It is only because they are so far away from the earth that the starlight appears so weak. Ancient astronomers believed that the position of a star in the starry sky was fixed, so they named it "Xingxing", which means "eternal star". But we know today that they are constantly moving at high speed. For example, the sun moves the entire solar system around the center of the Milky Way. But other stars are so far away from us that we can hardly detect changes in their positions. Stars have strong or weak abilities to emit light. In astronomy, it is represented by "luminosity". The so-called "luminosity" refers to the power radiated in the form of light from the surface of the star. Star surface temperatures also vary from high to low. Generally speaking, the lower the temperature of a star's surface, the redder its light is; the higher the temperature, the bluer its light. The higher the surface temperature, the larger the surface area, and the greater the luminosity. From the color and luminosity of stars, scientists can extract a lot of useful information. Historically, the astronomer Hertzsprung and the philosopher Russell first proposed the relationship between star classification and color and luminosity, established the stellar evolution relationship known as the "Hertz-Router diagram", and revealed The secrets of stellar evolution. In the "H-Ro diagram", from the high temperature and strong luminosity area on the upper left to the low temperature and weak luminosity area on the lower right, there is a narrow star-dense area, including our sun; this sequence is called the main sequence , more than 90% of stars are concentrated in the main sequence. Above the main sequence region are the giant and supergiant regions; at the lower left is the white dwarf region. Stars are born from interstellar dust in space (scientists vividly call them "nebulae" or "interstellar clouds"). The "youth" of a star is the longest golden stage in its life - the main sequence stage, which accounts for 90% of its entire life. During this time, the star glows and heats with almost constant luminosity, illuminating the surrounding space. After this, the star will become turbulent and turn into a red giant; then, the red giant will complete its entire mission in an explosion, ejecting most of its material back into space, The debris left behind may be a white dwarf, a neutron star, or even a black hole... In this way, the star comes from the nebula and returns to the nebula, completing its glorious life. The gorgeous stars will always be the most beautiful sight in the night sky.