Two important features of a star are temperature and absolute magnitude. About 100 years ago, Einar hertzsprung of Denmark and henry norris russell of the United States drew a chart to find out whether there is a relationship between temperature and brightness, which is called Herro chart or H-R chart. In the H-R diagram, most stars form a diagonal region, which is called the main star sequence in astronomy. In the main sequence, when the absolute magnitude of a star increases,
Evolution of stars
Its surface temperature will also rise. More than 90% of the stars belong to the main sequence, and the sun is also one of these main sequences. Superstar and Supergiant star are on the right of the H-R diagram. Although the surface temperature of the white dwarf is high, its brightness is not great, so it is only in the middle and lower part of the figure.
The evolution of stars is the constant change of stars in their lifetime (luminous and heating period). Life span varies with the size of a star. The evolution of a single star cannot be completely observed, because these processes may be too slow to be detected. Therefore, astronomers observe many stars in different life stages and use computer models to simulate the evolution of stars.
Astronomer hertzsprung and philosopher Russell first put forward the relationship between star classification and color and luminosity.
Star hero
System, the star evolution relationship named "Herzog-Roto" is established, and the secret of star evolution is revealed. In "Herro-Roto", from the high-temperature strong light area at the upper left to the low-temperature weak light area at the lower right, it is a narrow star-intensive area, and our sun is also in it; This sequence is called the main sequence, and more than 90% stars are concentrated in the main sequence. Above the main sequence area are the giant star and Supergiant star area; On the lower left is the white dwarf region.
Astronomers can measure the mass, age, metal content and many other properties of stars by observing their spectra, luminosity and motion in space. The total mass of a star is the main factor that determines the evolution and ultimate fate of a star. Other characteristics, including diameter, rotation, movement and temperature, can be measured in evolutionary history. A diagram describing the relationship between the temperature and luminosity of many stars, that is, Herro diagram (HR diagram), can measure the age and evolution stage of stars.
The distribution of stars in galaxies is not uniform. Most stars will be influenced by gravity to form multiple stars, such as binary stars, triplets, and even clusters of tens of thousands to millions of stars. When the orbits of two binary stars are very close, their gravity may have a great influence on their evolution. [4] For example, a white dwarf gets accretion disk gas from its companion star and becomes a new star.
form
When the universe develops to a certain period, the universe is full of uniform neutral atomic gas clouds, and the massive gas clouds are due to itself.
Strange stars (13 photos)
Gravity instability leads to collapse. In this way, the star has entered 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 its own gravity. When the linearity of matter is reduced by several orders of magnitude, the situation is different. On the one hand, the density of gas increases sharply. On the other hand, because the lost gravitational potential energy is partially converted into heat energy, the temperature of the gas is also greatly increased. The pressure of a 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 that is 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 its own gravity, 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 trend of heat balance plays a weakening role in mechanics. Therefore, the star blank must shrink slowly, and the decrease of its gravitational potential energy will increase the temperature, thus restoring the mechanical balance; At the same time, it also provides the energy needed for star blank radiation by reducing gravitational potential energy. This is the main physical mechanism of stellar blank evolution.
The latest observation found the star S 1020549.
Let's discuss this process roughly with the classical theory of gravity. Considering the spherical gas cloud system with density ρ, temperature t and radius r, the thermal motion energy of gas:
ET= RT= T
(1) gas is regarded as a monatomic ideal gas, μ is the molar mass, and r is the universal constant of gas.
In order to get the gravitational energy Eg of the gas cloud ball, imagine that the mass of the curved ball moves to infinity bit by bit, and the work of 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 in the process of removing dm from the surface:
dW=- =-G( ) 1/3m2/3dm
(2) So: -Eg=- () 1/3m2/3dm= G( M5/3
Therefore: Eg=- (2), [3]
Total energy of gas cloud: E=ET+EG (3)
The soul nebula will form a new planet
Thermal motion makes the gas evenly distributed, and gravity makes the gas concentrated. The two work together. At E>0 point, the thermal motion is dominant, the gas cloud is stable, and the small disturbance will not affect the gas cloud balance; When 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 forming a cluster together. Globular clusters can contain 10 5 → 10 7 stars, which can be considered as simultaneous.
We know that the mass of the sun: mθ = 2× 10 33, and the radius r = 7× 10 10. 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× 10 33 erg. S- 1 If this luminosity is maintained by gravity as energy, then the duration is:
Many proofs show that the sun has been stable for 5× 10 9 years in today's state, so the star blank stage can only be a short transitional stage before the sun forms a stable state like today. 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?
locking phase
During the contraction of the main sequence star, the density increases. We know that ρ∝r-3 is given by formula (4), and rc ∝ R3/2, so rc drops faster than R, and some contracted gas clouds reach the critical value under new conditions. A small disturbance may cause a 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 clouds, it continues to contract, the surface temperature remains unchanged, and the central temperature keeps rising, causing various nuclear reactions of temperature, density and gas composition. The generated heat energy 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.
Hubble observed two burning giants.
Stars are mostly composed of H and He. When the temperature is above 104K, that is, the average thermal kinetic energy of particles is above 1ev, hydrogen atoms are completely 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 gases containing pure hydrogen, the most effective nuclear reaction series is the so-called P-P chain:
The main reaction is 2D(p, γ)3He reaction. The content of D (deuterium, an isotope of hydrogen, consisting of a proton and a neutron) is only about 10-4% of that of hydrogen, and it will soon burn out (its principle is similar to that of modern hydrogen bomb weapons). If d is greater than 3He (helium 3, an isotope of helium, consisting of two protons and 1 neutron) at the beginning, 3H (an isotope of tritium and hydrogen, consisting of 1 proton and two neutrons, which will decay into helium 3) produced 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 through convection may still be retained.
The binding energy of Li, Be, B and other light nuclei is as low as that of D, and the content is only about 2× 10-9K of H. When the central temperature exceeds 3× 106K, they start to burn, causing (p, α) and (p, α) reactions, and soon become 3He and 4He. When the center temperature reaches 107K and the density reaches about 105kg/m3, the generated hydrogen is transformed into the process of 4 1H→4He of he. This is mainly the p-p and CNO cycles. At the same time, what contains 1H and 4He is a p-p chain reaction, which consists of the following three branches:
P-p 1 (only 1H) p-p2 (1H and 4he at the same time) P-P3.
Or assume that the weight ratio of 1H and 4He is equal. With the increase of temperature, the reaction gradually transited from p-p 1 to p-p3.
However, when the temperature is 1.5× 107K, the process of star burning H can transition to CNO cycle.
When 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, which has two branches:
Or the total reaction rate depends on the ratio of (p, α) and (p, γ) reaction branches of the slowest 14N(p, γ) 15O and 15N, which is about 2500: 1.
This ratio is almost independent of temperature, so one of the 2500 CNO cycles is CNO-2.
In the process of p-p chain and CNO cycle, the net effect is that H burns to produce he:
Of the 26.7MeV energy released, most of it is consumed to heat and emit light to the star, which becomes 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 burns stably into He, the star becomes the main sequence star. It has been found that 80% to 90% of the stars are main sequence stars, and 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 difference in the number of main sequence stars was mainly quality, followed by age and chemical composition. The cycle of the sun is about 10 million years.
The observed minimum mass of the main sequence star is about 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 the main sequence star will not be formed, 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 its internal energy process is very intense, so its structure is more unstable. But there is no absolute upper limit of quality in theory.
In the statistical analysis of a cluster, it is found that there is an upper limit for the main sequence stars. 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 segments by power:
l∧mν
Where υ is not a constant and its value is about 3.5 to 4.5. M reflects that the main sequence star has more mass to burn, and L reflects the rapid burning, 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 age of the whole cluster is t, a cutoff mass MT can be obtained from the relationship between t and m, and the main sequence star with a mass greater than MT has ended the core H burning period, not the main sequence star, which is why a large number of stars of the same age are observed to have an upper limit.
We will discuss why most observed stars are main sequence stars. Table 1 ignition temperature (k), center temperature (g cm-3) and duration (yr) based on 25M constant combustion stage.
H 4× 107 4 7× 106
He 2×1086×1025×105
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 life of combustion stage is 7.5× 106.
The star evolution model lists the ignition temperature and combustion duration of various elements. As can be seen from the table, the ignition temperature of nuclei with large atomic number is higher, and nuclei with large Z are not only difficult to ignite, but also burn more violently after ignition, so the combustion duration is shorter. In this 25M⊙ table 1 25M⊙ star evolution model, the total life of the model star in the combustion stage is 7.5× 106, and more than 90% 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 stars in the main sequence stage. This is the basic reason why most observed stars are main sequence stars.
old age
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 inside the star are stable, so its luminosity and surface temperature change only slightly during the whole long period. Let's discuss how the stars will evolve further when the hydrogen in the core region burns.
After the star burns all the hydrogen in the core region, it will turn off. At this time, the core area is mainly the product of combustion helium, and the material in the peripheral area is mainly unburned hydrogen. After the core is closed, the star loses its radiant energy, so its gravitational contraction is a key factor. The end of a nuclear combustion phase shows that the temperature of all parts of the star is lower than the temperature needed to ignite there. Gravitational contraction will increase the temperature of all parts of the star, which is actually the temperature needed to find the next nuclear ignition. Gravitational contraction will increase the temperature of all parts of the star. The gravitational contraction after the main sequence first ignites helium not in the core region (its ignition temperature is too high), but in the hydrogen shell between the core and the periphery. After the hydrogen shell is ignited, due to the gravitational potential energy released by the core area and the nuclear energy released by burning hydrogen, the unburned hydrogen layer in the periphery will inevitably expand violently, that is, the medium radiation will become more transparent, thus discharging excess heat energy to maintain the thermal balance. The expansion of the hydrogen layer reduces the surface temperature of the star, so it is a process of increasing luminosity, radius and surface cooling. This process is the transformation of the star from the main sequence to the red giant. When this process goes to a certain extent, the temperature in the center of the hydrogen zone will reach the temperature of helium ignition, and then it will transition to a new stage-helium combustion stage.
Before helium ignition occurs in the center of the star, gravity contraction makes its density reach the order of 103g. Cm -3-3. At this time, the dependence of gas pressure on temperature is weak, so the energy released by nuclear reaction will increase the temperature, which will further aggravate the nuclear reaction rate. Once lit, it will soon burn so violently that it will explode. This ignition method is called "helium flash", so it will be phenomenal.
On the other hand, when gravity contracts, its density cannot reach the order of 103g. Cm -3-3. At this time, the pressure of the gas is directly proportional to the temperature. When the ignition temperature rises, the pressure increases and the nuclear combustion zone expands, which in turn lowers the temperature, so the combustion can be carried out stably. Therefore, these two ignition conditions have different effects on the evolution process.
How do stars evolve after 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 only the helium core. The density of helium core region decreases due to expansion, and helium may burn normally in it in the future. The product of helium combustion is carbon. After the helium is extinguished, the star will have a helium shell in the core region of carbon. Because the residual mass is too small to reach the ignition temperature of carbon, the evolution of burning with helium has ended and it is heading for thermal death.
Because gravitational collapse is related to mass, the evolution of stars with different masses is different.
M & lt0.08M⊙ Star: Hydrogen can't be ignited, and it will die directly without helium combustion stage.
0.08 & ltm < 0.35 m ⊙ Star: Hydrogen can be ignited, and after the hydrogen is extinguished, the ignition temperature in the hydrogen nuclear region cannot be reached, thus ending the nuclear combustion stage.
0.35 & ltm < 2.25m ⊙ Star: Its main feature is that helium will ignite and "helium flash" will appear.
2.25 & ltM & lt4M⊙ star: Helium can burn normally after hydrogen is turned off, but carbon cannot reach the ignition temperature after hydrogen is turned off. The reaction here is:
At the initial stage of nuclear reaction, when the temperature reaches the order of 108K, 13C and 17O generated by CNO cycle can react with 4He to generate 16O and 20Ne. After a long period of nuclear reaction, 20Ne(p, γ) 26550. ν) 2 1Na and 14N of 2 1Na absorb two 4He to form 22Ne energy, and (α, n) react to form 24Mg and 25Mg, etc. These reactions are not important as energy sources, but the released neutrons can further produce neutron nuclear reactions.
4<M<8 → 65438+200m ⊙ Stars, which is an unclear range. Maybe the carbon doesn't ignite, maybe there is a "carbon flash", maybe it can burn normally, because the final central temperature is already very high, and some sensitive factors, such as the energy loss of neutrinos, make the situation blurred.
After the nuclear reaction, when the central temperature reaches 109K, the combustion reactions of carbon, oxygen and neon begin, mainly including carbon-carbon reaction, oxygen-oxygen reaction and γ, α reaction of 20Ne.
8→ 10M⊙& lt； The stars of M: hydrogen, helium, carbon, oxygen, neon and silicon, which can burn normally step by step. Finally, a core area that can't release energy is formed in the center, and all kinds of combustible but unburned hydrogen shells are outside the core area. At the end of the nuclear combustion stage, the whole star presents a layered structure (Fe, Si, Mg, Ne, O, C, he, H) from the inside out.
end
We already know that for a star with a mass less than 8→ 10M⊙, its nuclear combustion phase will end because it cannot reach the next stage and ignition temperature. For a more massive star, it will end the nuclear combustion phase after the fuel in the core region runs out. After that, what is the final destination of the star?
Small-mass stars (such as the sun) will initially expand. At this stage, stars are called red giants, and then they will collapse, become white dwarfs, radiate and lose energy, then become black dwarf, and finally disappear.
Massive stars with solar density ≥7 (8 → 10m ⊙)
Once the nuclear combustion stops, the star must undergo gravitational contraction, because the pressure to maintain mechanical balance inside the star is related to its temperature. So, if the star is in a? Quot The final equilibrium configuration must be a "cold" equilibrium configuration, that is, its pressure has nothing to do with its temperature.
After the core H of the main sequence star is exhausted, leaving the main sequence is the beginning of its final course. The outcome depends mainly on the quality. For a star with small mass, the self-gravity inside the object is not important because of its small mass. The internal balance of solids is achieved by the net Coulomb attraction between positive and negative ions and the pressure between electrons.
When the mass of the star is large, until the self-gravity can't be ignored, the internal density and pressure are increased by the self-gravity, and the increase of pressure is the pressure ionization of the substance, which gradually leads to the disintegration of the electric confinement of the solid and turns into plasma gas. Increase the mass, that is, increase the density. At this time, the pressure has nothing to do with temperature, thus achieving a "cold" equilibrium configuration. The kinetic energy of electrons in plasma is large enough to cause β decay in matter;
Here, p is the proton in the nucleus. When the density reaches108 g cm-3, this reaction will gradually make the nucleus in the negative ion body become neutron-rich, and there will be too many neutrons in the nucleus, which will lead to the loose structure of the nucleus. When the density exceeds 4×101g. Cm-3, neutrons will begin to separate from the nucleus. If the mass becomes larger, the pressure between neutron gases can't resist the self-gravity of matter, and a black hole will be formed. However, because the mass of most stars is smaller than their initial mass in the later stage of evolution, such as stellar wind, "helium flash" and supernova explosion, a large proportion of the mass of stars will be lost. Therefore, the final quality of a star can not be judged by its initial quality, but actually depends on the evolution process. Then we can come to the conclusion that. Stars below 8→ 10M⊙ will eventually throw off some or most of their mass and become white dwarfs. Stars above 8→ 10M⊙ will eventually collapse into neutron stars or black holes through the gravitational force of the star core, that is, stars with a collapsed core mass of 1.44 to 3.2 times the mass of the sun will eventually become neutron stars, and stars with a collapsed core mass of 3.2 times the mass of the sun will eventually become black holes.
The observed star mass range is generally 0. 1→60M⊙. A celestial body with a mass less than 0.08M⊙ cannot reach the ignition temperature. Therefore, if you don't shine, you can't be a star. The center temperature of celestial bodies with mass greater than 60M⊙ is too high and unstable, so far only less than 20 celestial bodies have been found.
Variable stars, etc.
structure
According to the actual observation and spectral analysis, we can understand the basic structure of the stellar atmosphere. It is generally believed that in some stars, the outermost layer has a high-temperature low-density corona similar to the corona. It is usually related to the star wind. Some stars have discovered the chromosphere that produces some emission lines in the corona, and the inner atmosphere absorbs the continuous radiation of gas with higher temperature to form absorption lines. People sometimes refer to this atmosphere as inversion layer, and the high-temperature layer that emits continuous spectrum is called photosphere. In fact, the formation process of stellar light radiation shows that this layer of photosphere is quite thick, and each layer has emission and absorption. Photosphere layer and inversion layer cannot be completely separated. In the photosphere of the sun star, there is a troposphere with an average radius of about one tenth or more. Inside the upper main sequence star and the lower main sequence star, the position of troposphere is very different. The energy transfer is mainly radiation from photosphere and convection in troposphere.
For the photosphere and troposphere, we often use the model based on the actual measured physical characteristics and chemical composition for more detailed research. Based on the basic assumptions of hydrostatic equilibrium and thermodynamic equilibrium, we can establish some relationships to solve the pressure, temperature, density, opacity, productivity and chemical composition of different regions of the star. In the center of a star, the temperature can be as high as millions or even hundreds of millions of degrees, depending on the basic parameters and evolution stage of the star. There, there are different abilities to respond. It is generally believed that stars are condensed by nebulae, and the stars before the main sequence can't have thermonuclear reaction because of their low temperature, so they can only produce energy by gravitational contraction. After entering the main sequence, the center temperature is as high as 7 million degrees, and the thermonuclear reaction of hydrogen polymerization into helium begins. This process is very long, and it is the longest stage in a star's life. After the completion of hydrogen combustion, the star will shrink inward and expand outward, and evolve into a huge red giant with a low surface temperature, which may cause pulsation. Stars whose internal temperature rises to nearly 1 100 million degrees begin to appear helium-carbon cycle. During these evolutions, the temperature and luminosity of stars change according to certain laws, thus forming a certain trajectory on the Herro diagram. Finally, some stars explode in supernovae, the gas shells fly away, and the cores are compressed into dense stars such as neutron stars and tend to "die" (see the formation and evolution of stars).