Now the sun is in adulthood (it has lasted for 5 billion years)
After 5 billion years, the sun enters old age, and its volume expands rapidly, forming a red giant whose brightness is 1 times that of the present one. This process lasts for 1 billion years
After 1 years, the sun collapses and its volume shrinks sharply. The formation of a white dwarf
, which is only the size of the earth but has a very high density, will become a brown dwarf after a period of time, and eventually perish
The evolution process of stars
1. The formation of stars
When the universe develops to a certain period, the universe is full of uniform neutral atomic gas clouds, and the massive gas clouds are unstable due to their own gravity and collapse. In this way, the star enters the formation stage. At the initial stage of collapse, the pressure inside the gas cloud is very small, and the matter accelerates to fall toward the center under the action of self-gravity. When the linearity of matter has shrunk by several orders of magnitude, the situation is different. On the one hand, the density of gas has increased dramatically. On the other hand, due to the partial conversion of lost gravitational potential energy into heat energy, the temperature of gas has also increased greatly. The pressure of gas is directly proportional to the product of its density and temperature, so the pressure increases faster in the process of collapse. In this way, a pressure field enough to compete with self-gravity is quickly formed inside the gas.
the mechanical balance of the star blank is caused by the internal pressure gradient and self-gravitation, but the existence of the pressure gradient depends on the inhomogeneity of the internal temperature (that is, the temperature in the center of the star blank is higher than that in the periphery), so it is an unbalanced system in terms of heat, and the heat will gradually flow out from the center. This natural tendency to balance in heat plays a weakening role in mechanics. Therefore, the star blank must shrink slowly, and its gravitational potential energy is reduced to raise the temperature, so as to restore the mechanical balance; At the same time, it also provides the energy needed for the radiation of the star blank by reducing the gravitational potential energy. This is the main physical mechanism of star blank evolution.
let's discuss this process roughly by using the classical theory of gravity. Considering the spherical gas cloud system with density ρ, temperature t and radius r, the energy of gas thermal motion is:
ET= RT= T
(1) The gas is rEgarded as a monatomic ideal gas, μ is the molar mass, and r is the universal constant of gas
In order to obtain the gravitational energy eg of gas cloud ball, imagine that the mass of the ball moves to infinity a little bit, and the work of field force is equal to. 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 = Now the two work together. When E> At o'clock, the thermal movement is dominant, the gas cloud is stable, and the small disturbance will not affect the gas cloud balance; When e <: , gravity is dominant, small density disturbance produces deviation from uniformity, and gravity increases at high density, which strengthens deviation and destroys balance, and gas begins to collapse. The critical radius of contraction is obtained from E≤:
(4) The critical mass of the corresponding gas cloud is:
(5) The original gas cloud has a small density and a large critical mass. So few stars are produced alone, and most of them are produced by a group of stars together to form a cluster. Spherical clusters can contain 15→17 stars, which can be considered as simultaneous production.
as we know, the mass of the sun: mθ = 2× 133, and the radius R=7×11. We bring in (2) the gravitational energy released by the sun's contraction to today's state
The total luminosity of the sun L=4×133erg.s-1 If this luminosity is maintained by gravity as the energy source, then the duration is: < p This raises a new question, how does the gravitational contraction of the star blank stop? After that, what is the energy source of solar radiation?
2.2 The density of the main sequence star phase increases during the contraction process. We know that ρ∝r-3 is given by formula (4) and rc ∝ R3/2, so RC decreases faster than R, and a part of the contracted gas cloud reaches the critical value under new conditions. Small disturbance can cause new local collapse. In this way, under certain conditions, the large gas cloud shrinks into a condensate and becomes a protostar. After the protostar absorbs the surrounding gas cloud, it continues to shrink, the surface temperature remains unchanged, and the central temperature keeps rising, causing various nuclear reactions of 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 into a star, and the evolution of the star begins with the main sequence star.
stars are mostly composed of H and He. When the temperature reaches above 14K, that is, the average thermal kinetic energy of particles reaches above 1eV, hydrogen atoms are fully ionized by thermal collision (the ionization energy of hydrogen is 13.6eV). After the temperature rises further, the collision between hydrogen nuclei in plasma gas may cause nuclear reaction. For high-temperature gas with pure hydrogen, the most effective nuclear reaction series is the so-called P-P chain: < P > Among them, 2D(p,γ)3He reaction is the main one. The content of D is only about 1-4 of that of hydrogen, and it will burn out soon. If the content of D is more than 3He at the beginning, the 3H generated by the reaction may be the main source of 3He in the early stage of the star, and this 3He that reaches the surface of the star due to convection may still remain until now.
the binding energy of light nuclei such as Li, Be and B is as low as that of D, and the content is only about 2×1-9K of H. When the central temperature exceeds 3×16K, they start to burn, causing (p,α) and (p,α) reactions, and soon become 3He and 4He. When the center temperature reaches 17K and the density reaches about 15kg/m3, the generated hydrogen is transformed into He from 41h to 4He. This is mainly the p-p and CNO cycles. P-p chain reaction occurs when 1H and 4He are contained at the same time, which consists of the following three branches:
p-p1 (1H only 1H) p-p2 (1H and 4He at the same time) P-P3
or assuming that the weight ratio of 1h and 4he is equal. With the increase of temperature, the reaction gradually transited from p-p1 to p-p3, < P >, while when T >; When the temperature is 1.5×17K, the process of burning H in the star can transition to CNO cycle.
when the stars are mixed with heavy elements c and n, they can be used as catalysts to change 1H into 4He, which is the CNO cycle. The CNO cycle has two branches:
or the total reaction rate depends on the slowest 14N(p,γ)15O and 15N (p,α) and (p,γ) reaction branches with a ratio of about 25: 1.
this ratio is almost independent of temperature, so one of the 25 CNO cycles is CNO-2.
In the process of p-p chain and CNO cycle, the net effect is that H burns to generate He:
Most of the 26.7MeV energy released is used to heat and emit light to the stars, which becomes the main source of the stars.
As we mentioned earlier, the evolution of stars begins with the main sequence, so what is the main sequence? When H burns stably into He, the star becomes the main sequence star. It has been found that 8% to 9% of the stars are main sequence stars. Their common feature is that hydrogen is burning in the core area, and their luminosity, radius and surface temperature are different. Later, it was proved that the quantitative difference of main sequence stars is mainly the quality, followed by their age and chemical composition. The course of the sun is about ten million years.
the minimum mass of the observed main sequence star is about .1M⊙. The model calculation shows that when the mass is less than .8M⊙, the contraction of the star will not reach the ignition temperature of hydrogen, so it will not form a main sequence star, which shows that it has a lower mass limit for the main sequence star. The observed maximum mass of the main sequence star is about dozens of solar masses. Theoretically, a star with too much mass radiates strongly, and the internal energy process is very intense, so the structure is more unstable. But there is no absolute upper limit of mass in theory.
when a cluster is statistically analyzed, people find that there is an upper limit for the main sequence star. What does this mean? As we know, the luminosity of the main sequence star is a function of mass, and this function can be expressed in a piecewise power form:
L∝Mν
where ν is not a constant, and its value is about 3.5 to 4.5. M indicates that there is more mass available for burning in the main sequence star, while L indicates that it burns fast, so the life of the main sequence star can be approximately marked by the trademarks of M and L:
T∝M-(ν-1)
that is, the life of the main sequence star decreases according to the power law with the increase of mass. If the existing age of the whole cluster is t, then a cutoff mass can be obtained from the relationship between T and M. The main sequence star whose mass is greater than MT has ended the core H-burning phase instead of the main sequence star, which is the reason why it is observed that the cluster composed of a large number of stars of the same age has an upper limit.
now let's discuss why most of the observed stars are main sequence stars. Table 1 According to the ignition temperature (k), center temperature (g cm-3) and duration (yr) of a constant combustion stage of 25M⊙.
h 4× 17 47× 16
he 2× 18 6× 12 5× 15
c 7× 18 6× 15 5× 12
ne 1.5× 19 4× 16 1
o 2× 19 1× 19. As can be seen from the table, the nuclei with large atomic number and higher ignition temperature, and the nuclei with large Z are not only difficult to ignite, but also burn more violently after ignition, so the combustion duration is shorter. The evolution model of this 25M⊙ table 1 25M⊙ star shows that the total life of the model star in the combustion stage is 7.5×16 years, and more than 9% of the time is the hydrogen combustion stage, that is, the main star sequence stage. Statistically speaking, this shows that it is more likely to find a star in the main sequence stage. This is the basic reason why most of the observed stars are main sequence stars.
2.3 evolution after the main sequence. Because the main component of star formation is hydrogen, and the ignition temperature of hydrogen is lower than other elements, the first stage of star evolution is always the combustion stage of hydrogen, that is, the main sequence stage. In the main sequence stage, the pressure distribution and surface temperature distribution in the star are stable, so its luminosity and surface temperature have only a small change throughout the long stage. Let's discuss how the stars will evolve further when the hydrogen in the core region is burned.
after the star burns all the hydrogen in the core region, it turns off. At this time, the core region is mainly hydrogen, which is the product of combustion. The material in the peripheral region is mainly unburned hydrogen. After the core turns off, the star loses the radiant energy, so its gravitational contraction is a key factor. The end of a nuclear combustion stage shows that the temperature in all parts of the star is lower than the temperature needed to cause ignition there. Gravitational contraction will increase the temperature in all parts of the star, which is actually the temperature needed to find the next nuclear ignition. Gravitational contraction will increase the temperature in all parts of the star. Gravitational contraction after the main sequence first ignites helium not 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 At this time, due to the gravitational potential energy released by the core area and the nuclear energy released by the burning hydrogen, both need to pass through the outer unburned hydrogen layer and must expand violently, which makes the medium radiation more transparent. The expansion of the hydrogen layer reduces the surface temperature of the star, so it 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. When the process goes to a certain extent, the temperature in the center of the hydrogen zone will reach the temperature of hydrogen ignition, and then it will transition to a new stage-helium combustion stage.
Before helium ignition occurs in the center of the star, gravity contracts to make its density reach the order of 13 g cm-3. At this time, the pressure of the gas is weakly dependent on the temperature, so the energy released by the nuclear reaction will increase the temperature, which in turn will accelerate the nuclear reaction rate. Once ignited, it will soon burn so violently that it will explode. This way of ignition is called "flash"? quot; Therefore, in the phenomenon, you will see that the luminosity of the star suddenly rises to a great level and then drops very low.
on the other hand, when gravity contracts, its density can't reach the order of 13 g cm-3, at this time, the pressure of the gas is directly proportional to the temperature. When the ignition temperature increases, the pressure increases, and the nuclear combustion zone expands, while the expansion leads to the temperature decrease, so the combustion can be carried out stably, so the two ignition conditions have different effects on the evolution process.
how do stars evolve after a helium flash? Flash releases a lot of energy, which is likely to blow away all the hydrogen in the outer layer of the star, leaving the core of helium. The density of helium core area is reduced due to expansion, and helium may burn normally in it in the future. The product of helium combustion is carbon. After helium flameout, the star will have a helium shell in the core area of carbon. Because the remaining mass is too small to reach the ignition temperature of carbon, it ends the evolution of burning with helium and goes to thermal death.
Because gravitational collapse is related to mass, the evolution of stars with different masses is different.
M< .8M⊙ Star: Hydrogen can't ignite, it will go directly to death without helium combustion stage.
.8< M< .35M⊙ star: hydrogen can ignite, and after hydrogen is extinguished, the hydrogen core region will not reach the ignition temperature, thus ending the nuclear combustion stage.
.35< M< 2.25M⊙ star: Its main feature is that helium will ignite and "helium flash" will appear.
2.25< M< 4M⊙ Star: Hydrogen can burn normally after the hydrogen is turned off, but after the hydrogen is turned off, the carbon will not reach the ignition temperature. The reactions here are as follows:
At the initial stage of He reaction, when the temperature reaches the order of 18K, 13C, 17O generated by CNO cycle can react with 4He to form 16O and 2Ne, and after a long time of He reaction, 2Ne(p,γ) 21Na(β+,ν) 21Na.
4< M< 8→1M⊙ Stars, this is an unclear range, maybe carbon can't ignite, maybe there is a "carbon flash", maybe