Knowledge about stars

Stars are composed of hot gas and are spherical or spherical-like objects that can emit their own light. Because stars are too far away from us, it is difficult to detect changes in their positions in the sky without the help of special tools and methods. Therefore, ancient people considered them to be fixed stars. The main star of our solar system, the sun, is a star.

1.1 Stellar Evolution

Stellar Structure Stars are gas planets. On a clear, moonless night and in an area without light pollution, the average person can see more than 6,000 stars with the naked eye. With the help of a telescope, you can see hundreds of thousands or even millions of them. It is estimated that there are about 150-200 billion stars in the Milky Way. Two important characteristics of stars are temperature and absolute magnitude. About 100 years ago, Einar Hertzsprung of Denmark and Henry Norris Russell of the United States each drew graphs to find out whether there was a relationship between temperature and brightness. A relationship diagram is called a Hertz-Rubber diagram, or H-R diagram. In the H-R diagram, most stars form a diagonal region called the main sequence in astronomy. In the main sequence, as the absolute magnitude of a star increases, the star evolves and its surface temperature also increases. More than 90 stars belong to the main sequence, and the sun is also one of these main sequences. Giant stars and supergiants are higher and farther to the right of the HR diagram. Although the surface temperature of white dwarfs is high, they are not very bright, so they are only in the middle and lower part of the diagram.

1.2 Stellar Evolution

The continuous changes of a star during its lifetime (the period of luminescence and heat generation). The life span varies according to the size of the star. The evolution of a single star cannot be completely observed because the processes may be too slow to be detected. So astronomers use observations of many stars at different stages of their lives and use computer models to simulate their evolution. The astronomer Hertzsprung and the philosopher Russell first proposed the relationship between star classification and color and luminosity. Stars - Herbert-Rubber diagram system, established the stellar evolution relationship called "Herbert-Rubber diagram", which 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 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.

1.3 Star formation

When the universe develops to a certain period, the universe is filled with uniform neutral atomic gas clouds. Large-volume gas clouds become unstable due to their own gravity and collapse. 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 toward 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, thereby establishing a new mechanical equilibrium configuration, called a star blank. The mechanical balance of the star base is caused by the internal pressure gradient competing with self-gravity, while the existence of the pressure gradient depends on the unevenness of the internal temperature (that is, the temperature at the center of the star base is higher than the temperature at the periphery), so in thermal science On the other hand, 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.

The latest observation discovered the star S1020549. Below we will 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-atom ideal gas, μ is the molar mass, and 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 little by little to infinity. The work done by moving the ball completely away from the field is equal to -Eg. When the mass of the ball is m and the radius is r, the work done by the field force during removing dm from the surface is: 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). The Soul Nebula will form a new planet. Thermal motion makes the gas distribution even, and gravity concentrates the gas. Now both of them work together. When Egt; 0, thermal motion dominates, and the gas cloud is stable, and small disturbances will not affect the balance of the gas cloud; when Elt; 0, gravity dominates, and small density disturbances occur. For uniform deviation, the gravity increases where the density is high, which intensifies the deviation and destroys the equilibrium. 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 density of the original gas cloud is small and the critical mass is very large, so few stars are produced individually, and most of them are produced together into star clusters. Spherical star clusters can contain 10^5→10^7 stars, which can be considered to be produced at the same time. We know: the mass of the sun: MΘ=2×10^33, the radius R=7×10^10, we put into (2) to get the total gravitational energy L released by the sun shrinking to its current state. =4×10^33erg.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 5×10^9 years, so , the star embryo stage can only be a short transitional stage before the sun reaches a stable state like today. This raises a new question, how does the star embryo's gravitational contraction stop? What is the source of solar radiation?

1.4 Stellar Stability Period

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 is less than r Smaller and faster, part of the shrinking gas cloud reaches the critical point 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 star continues to shrink after adsorbing the surrounding gas clouds. The surface temperature remains unchanged and the core temperature continues to increase, causing various nuclear reactions of temperature, density and gas composition to generate heat energy and causing the temperature to rise extremely high. The gas pressure resists gravity and stabilizes the protostar. To become stars, the evolution of stars begins with main sequence stars.

Hubble observed two violently burning super stars. Most of the stars are H and He. When the temperature reaches above 104K, particles are formed. The average thermal kinetic energy reaches more than 1eV, and hydrogen atoms are fully ionized through thermal collision (the ionization energy of hydrogen is 13.6eV). 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 produced by the reaction may be the main source of 3He in the early stages of the star. Due to convection, this 3He reaching the surface of the star may still remain until now. Light nuclei such as Li, Be, and B 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) and (p, α) The reaction quickly became 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 (both 1H and 4He) p-p3 or assuming that the weight ratio of 1H and 4He is equal. As the temperature increases, the reaction gradually transitions from p-p1 to p-p3. When Tgt; 1.5×107K, the process of burning H in the star can transition to 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 the total reaction rate depends on the slowest The reaction branch ratio of (p, α) and (p, γ) of 14N (p, γ) 15O and 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 26.7 MeV energy released, most of it is consumed to heat and illuminate the star, becoming the main source of the star.

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 shrinkage 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 in a power form piecewise: L∝Mν. Where υ is not a constant, its value is approximately 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), which is the main sequence star. The lifespan of a sequence star decreases according to a power law as the mass increases. If the existing age of the entire star cluster is T, then a cut-off mass MT can be calculated from the relationship between T and M. 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 a constant combustion stage of 25M. Ignition temperature (K) Core temperature (g. cm-3) Duration (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. Si: 3.5×109 1×108 3×10-3. The total life of the combustion stage is 7.5×106.

The star evolution model lists the ignition temperatures and combustion durations of various elements. 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. Table 1 25M star evolution model of this 25M star. The total lifespan of the burning stage of the model star is 7.5×106 years, and more than 90% of the time is in 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.

1.5 Stars in their later years

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 third stage of star evolution is The first stage is always the combustion stage of hydrogen, which 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 region, it goes out. At this time, the core region is mainly helium, which is a product of combustion. The material in the outer region is mainly unburned hydrogen. After the core flameout When a star loses the energy of radiation, its gravitational contraction 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. There is a hydrogen shell between them. After the hydrogen shell is ignited, the core area is in a high temperature state and there is still no nuclear energy, so 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 will increase the temperature, and The increase in temperature in turn increases the rate of nuclear reactions, so once ignited, it will soon burn so violently that it explodes. This method of ignition is called a "helium flash", so a sudden increase in the luminosity of the star will be seen. It reached a very high level, and then dropped to a very low level.

On the other hand, when gravity contracts, its density does not reach the level of 103g. cm-3. At this time, the pressure of the gas is proportional to the temperature. The increase in ignition temperature causes the pressure to increase, and the nuclear combustion zone There will be expansion, and the expansion causes the temperature to decrease, so combustion can proceed stably, so the impact of these two ignition conditions on the evolution process is different. How do stars evolve after a "helium flash"? The release of a large amount of energy in the flash is likely to blow away all the hydrogen in the star's outer layers, leaving behind the helium core. The helium core area reduces its density due to expansion, and helium may burn normally in it in the future. The product of helium burning is carbon. After the helium flameout, the star will have a helium shell in the carbon core area. Because the remaining mass is too small, the gravitational contraction cannot reach the ignition temperature of carbon, so it ends the evolution of helium burning, and Towards thermal death.

Since gravitational collapse is related to mass, stars with different masses evolve differently. Mlt; 0.08M star: Hydrogen cannot ignite, and it will die directly without a helium burning stage. 0.08lt;Mlt;0.35M star: Hydrogen can ignite. After the hydrogen is extinguished, the hydrogen core area will not reach the ignition temperature, thus ending the nuclear combustion stage. 0.35lt; Mlt; 2.25M star: Its main characteristic is that helium will ignite and appear "helium flash". 2.25lt;Mlt;4M star: Helium can burn normally after the hydrogen is extinguished, but after the flameout, the carbon will not reach the ignition temperature.

The reactions here are: in the early stage of the He reaction, when the temperature reaches the 108K level, the 13C and 17O produced by the CNO cycle can react with 4He to form 16O and 20Ne. The He reaction continues for a long time. Finally, 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 are not important as energy sources, but The emitted neutrons can further undergo neutron nuclear reactions. 4lt;Mlt;8→10M stars, this is a range where the situation is unclear. Maybe the carbon cannot ignite, maybe there is a "carbon flash", or maybe it can burn normally, because this is the last core temperature that is already higher, and some are Sensitive factors such as neutrino energy losses muddy the picture. After the He reaction is completed, when the core temperature reaches 109K, the combustion reaction of C, O, and Ne begins to occur. This is mainly the C-C reaction, O-O reaction, and the γ and α reactions of 20Ne: 8→10Mlt; 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.