Gas cloudIs onuniverseAt a certain time, the universe is full of uniform neutral atoms or molecular gas clouds, and large gas clouds collapse due to instability of their own gravity.It is believed that stars form in dense molecular gas clouds.The star then enters the formation stage.At the beginning of collapse, the pressure inside the gas cloud is very small,materialAccelerate to fall to the center under the action of self gravity.When the linearity of matter shrinks by several orders of magnitude, the situation is different. On the one hand, the density of gas has increased dramatically; on the other hand, because the lost gravitational potential energy is partially converted into heat energy, the temperature of gas has also increased greatly. The pressure of gas is proportional to the product of its density and temperature, so during the collapse process, the pressure increases faster,In the gas, a pressure field that is sufficient to counteract the self gravity is quickly formed. This pressure field finally stops the gravitational collapse, thus establishing a new mechanical equilibrium configuration, called the star blank.
The mechanical balance of the star billet is caused by the balance between the internal pressure gradient and the self gravity, while the existence of the pressure gradient depends on the non-uniformity of the internal temperature (that is, the temperature in the center of the star billet is higher than the temperature in the periphery)heatThis is an unbalanced system, and the heat will flow out from the center gradually.This natural tendency towards equilibrium in heat plays a weakening role in mechanics.Therefore, the star billet must slowly shrink, and its gravitational potential energy will decrease to increase the temperature, so as to restore the mechanical balance;At the same time, the reduction of gravitational potential energy is also used to provide the energy required for the radiation of the star billet.This is the main physical mechanism of the evolution of star billets.
Now we will discuss this process roughly using the classical gravity theory.Consider the spherical gas cloud system with density ρ, temperature T and radius r, and the energy of gas thermal movement:
ET= RT= T
(1) Consider the gas as a monatomic ideal gas, μ isMolar mass, R is the universal constant of gas
In order to obtain the gravitational energy Eg of the gas cloud sphere, imagine that the mass of the sphere moves to infinity little by little, and the work done by moving the sphere away from the field is equal to - Eg.When the ball mass is m and the radius is r, the work done by the field force in the process of removing dm from the surface:
dW=- =-G( )1/3m2/3dm
(2) So: - Eg=- () 1/3m2/3dm=G (M5/3)
So: Eg=- (2),
Total energy of gas cloud: E=ET+EG (3)
Heat movement makes the gas distribute evenly, and gravity makes the gas concentrate.Both work together.When E>0, the thermal movement is dominant, the gas cloud is stable, and small disturbances will not affect the gas cloud balance;When E ＜ 0, gravity is dominant, small density disturbance will produce deviation from uniformity, gravity at large density will increase, which will strengthen the deviation and destroy the balance, and the gas will begin to collapse.The critical radius of shrinkage is obtained from E ≤ 0:
(4) The critical mass of the corresponding gas cloud is:
(5) The density of the original gas cloud is small and the critical mass is large.So there are fewfixed starThey are produced separately, and most of them are clusters of stars.Spherical clusters can contain 105 → 107 stars, which can be considered to be generated simultaneously.
We know that the mass of the sun: M θ=2 × 1033, radius R=7 × 1010
main sequence
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The total luminosity of the sun L=4 × 1033erg.s-1 If this luminosity is maintained by gravity, then the duration is:
Many proofs show that the sun has been stable in today's state for 5 × 109 years. Therefore, the stellar bloom 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 starlet stop?After that, what is the energy source of solar radiation?
In the main sequence star stage, the density increases during the contraction process. We know that ρ ∝ r-3, according to formula (4), rc ∝ r3/2, so rc decreases faster than r, part of the contracted gas cloud reaches the critical value under new conditions, and small disturbances can cause new local collapse.Under certain conditions, the massive gas cloud will shrink into a condensate and become a protostar. After the protostar absorbs the surrounding gas cloud, it will continue to shrink, with the surface temperature unchanged and the center temperature rising, causing various nuclear reactions of temperature, density and gas composition.The generation of heat energy makes the temperature rise extremely high. The gas pressure resists gravity to stabilize the protostar into a star. The evolution of the star starts from the main sequence star.
The composition of stars is mostly H and He. When the temperature reaches above 104K, that is, the average thermodynamic energy of particles reaches above 1eV, the hydrogen atoms are fully ionized through thermal collision (the ionization energy of hydrogen is 13.6eV). After the temperature further rises, the collision of hydrogen nuclei with hydrogen nuclei in the plasma gas may cause nuclear reaction.For high temperature gas of pure hydrogen, the most effective nuclear reaction series is the so-called P-P chain:
The main reaction is 2D (p, γ) 3He reaction.D content onlyhydrogenAbout 10-4, 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. Because of the 3He that convection reaches the surface of the star, it may still remain.
Light nuclei such as Li, Be, and B have very low binding energy as D, and the content is only about 2 × 10-9K of H, when the center 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 converted into the 41H → 4He process of He.This is mainly p-p and CNO cycle.With 1H and 4He at the same time, the p-p chain reaction occurs, which has the following three branches:
P-p1 (only 1H) p-p2 (both 1H and 4He) p-p3
Or assume that the weight ratio of 1H and 4He is equal.With the increase of temperature, the reaction gradually transits from p-p1 to p-p3,
When T>1.5 × 107K, the process of burning H in the star can transition to CNO cycle.
When there are heavy elements C and N mixed in the star, they can be used as catalyst to make 1H become 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 the ratio of (p, α) and (p, γ) reaction branches of 15N is about 2500:1.
This ratio is almost independent of temperature, so it is CNO-2 once in 2500 CNO cycles.
During the p-p chain and CNO cycle, the net effect is that H burns to generate He:
Most of the 26.7MeV energy released is consumed for star heating and luminescence, becoming the main source of stars.
In the main sequence star stage, 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 star.It was found that 80% to 90% of stars are main sequence stars. Their common feature is that there is hydrogen burning in the core area, and their luminosity, radius and surface temperature are different. It was later proved that the quantitative difference of main sequence stars is mainly due to their different masses, 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 0.1M ⊙.The model calculation shows that when the mass is less than 0.08M ⊙,Astral bodyThe contraction of will not reach the ignition temperature of hydrogen, so that the main sequence star cannot be formed, which means that there is a lower mass limit for the main sequence star.The maximum mass of the observed main sequence star is about dozens of solar masses.Theoretically, stars with too much mass radiate very 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 making statistical analysis of a star cluster, 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, which can be expressed piecewise in power form:
L∝Mν
Where nu is not a constant, its value is about 3.5 to 4.5.Large M indicates that there is more mass available for combustion in the main sequence star, while large L indicates fast combustion, so the life of the main sequence star can be approximately marked by the trademarks M and L:
T∝M-(ν-1)
That is, the life of the main sequence star decreases exponentially with the increase of its mass. If the existing age of the entire cluster is T, then a cut-off mass MT can be obtained from the relationship between T and M.The main sequence star with mass greater than MT has ended the H burning phase of the core instead of the main sequence star, which is why it is observed that the star cluster composed of a large number of stars of the same age has an upper limit.
We will discuss the reason why most of the observed stars are main sequence stars. Table 1 is based on the ignition temperature (K) in the constant combustion stage of a 25M ⊙ center 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 1
O 2×109 1×107 5×10-2
Si 3.5×109 1×108 3×10-3
Total life in combustion stage 7.5 × 106
Star evolution model, listing the ignition temperature of various elements and the duration of combustion.It can be seen from the table that the nuclei with large atomic numbers and higher ignition temperatures are not only difficult to ignite, but also burn more intensely after ignition, so the duration of combustion is shorter.Table 1 25M ⊙ of this 25M ⊙Stellar evolutionThe total life of the combustion phase of the model star is 7.5 × 106 years, and more than 90% of the time is the hydrogen combustion phase, that is, the main star sequence phase.Statistically speaking, this indicates 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.
Post main sequence stage
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Since star formation is the main component of the evolution after the main sequence, and the ignition temperature of hydrogen is lower than that of 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 star maintains a stable pressure distribution and surface temperature distribution inside, so its luminosity and surface temperature have only small changes in the whole long stage.Now we will discuss how the stars will further evolve when the hydrogen in the core region is burned out?
After the star burns out the hydrogen in the core, it will extinguish. At this time, the core area is mainly hydrogen, which is the product of combustion. The material in the peripheral area is mainly unburned hydrogen. After the core extinguishs, the star loses the radiant energy, so it needs gravitational contraction to be a key factor.The end of a nuclear combustion stage indicates that the temperature in all parts of the star has been lower than the temperature needed to ignite there. The gravitational contraction will increase the temperature in all parts of the star, which is actually the temperature needed to find the next nuclear ignition. The gravitational contraction will increase the temperature in all parts of the star,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 ignition of the hydrogen shell, 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 by the core area and the nuclear energy released by the burning hydrogen, both need to expand violently through the outer non burning hydrogen layer, that is, make the medium radiation more transparent.The expansion of the hydrogen layer reduces 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 star sequence to the red giant star. When the process goes to a certain extent, the temperature in the center of the hydrogen region will reach the temperature of hydrogen ignition, and then it will transition to a new stage -- helium combustion stage.
Before the helium ignition in the center of the star occurs, the gravity shrinks to make its density reach the order of 103g.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 raise the temperature, which in turn will increase the nuclear reaction rate, so once the ignition is started, it will soon burn very violently, so that it will explode,The ignition in this way is called flash, so the brightness of the star will suddenly rise to a large value and then drop to a very low value.
On the other hand, when gravity shrinks, its density does not reach the order of 103g. cm - 3. At this time, the pressure of the gas is proportional to the temperature. When the ignition temperature rises, the pressure rises, and the nuclear combustion zone will expand, and the expansion will reduce the temperature, so combustion can proceed stably. Therefore, the impact of these two ignition conditions on the evolution process is different.
How does the star evolve after the helium flash?The flash makes a large amount of energy release, which is likely to blow away the hydrogen in the outer layer of the star, leaving the core area of helium.The density of helium core area is reduced due to expansion, and helium may burn normally in it later.The product of helium combustion is carbon. After helium flameout, the star will have a helium shell in the carbon core area. Because the remaining mass is too small, the gravitational contraction can no longer reach the ignition temperature of carbon, so he ended the evolution of burning with helium and went to thermal death.
Since gravitational collapse is related to mass, stars with different masses have different evolutions.
Stars with M ＜ 0.08M ⊙: hydrogen cannot ignite, and it will die without helium combustion stage.
Star with 0.08 ＜ M ＜ 0.35M ⊙: hydrogen ignition, after hydrogen flameout, the ignition temperature of hydrogen core area will not be reached, thus ending the nuclear combustion phase.
Star with 0.35 ＜ M ＜ 2.25M⊙: its main feature is that helium will ignite and "helium flash" will appear.
2.25 ＜ M ＜ 4M ⊙ stars: hydrogen can burn normally after flameout, but carbon will not reach ignition temperature after flameout.The reactions here are:
At the initial stage of He reaction, when the temperature reaches the order of 108K, a new (α, n) reaction occurs between 13C, 17O energy generated by CNO cycle and 4He to form 16O and 20Ne. After a long time of He reaction, 21Na in 20Ne (p, γ) 21Na (β+, ν) 21Na and 22Ne formed by 14N absorbing two 4He can occur (α,n) The reaction forms 24Mg and 25Mg, etc. These reactions are not important as energy sources, but the neutrons emitted can further occurNeutron nuclear reaction。
4 ＜ M ＜ 8 → 10M ⊙ stars, this is a range of unclear conditions. Maybe the carbon can not be ignited, or there may be "carbon flash", or it can burn normally, because this is the final center temperature has been higher, and some more sensitive factors, such as the energy loss of neutrinos, have blurred the situation.
After the He reaction, when the center temperature reaches 109K, C, O and Ne combustion reactions begin, mainly including C-C reaction, O-O reaction, and γ and α reaction of 20Ne:
Stars with 8 → 10M ⊙ ＜ M: hydrogen, helium, carbon, oxygen, neon and silicon can burn normally step by step.Finally, a core area that can not release energy is formed in the center. Outside the core area is a shell of various hydrogen elements that can be burned but not burned out.At the end of the nuclear combustion phase, the whole star presents a layered structure (Fe, Si, Mg, Ne, O, C, He, H) from the inside out.
Final stage
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We already know that for stars with mass less than 8 → 10M ⊙, it will end its nuclear combustion stage because it cannot reach the next stage and ignition temperature;For a star with higher mass, it will end its nuclear combustion phase after the core region runs out of fuel. After that, what is the final destination of the star?
Once the nuclear combustion is stopped, the star must undergo gravitational contraction, because the pressure to maintain the mechanical balance inside the star is related to its temperature.Therefore, if the star is in a final equilibrium configuration, it must be a "cold" equilibrium configuration, that is, its pressure is independent of 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 quality.For stars with small mass, the self gravity inside the object is not important because of its small mass. The balance inside the solid is achieved by the net coulomb gravity between positive and negative ions and the pressure between electrons.
When the mass of the star is larger, until the self gravity cannot be ignored, the self gravity increases the density and pressure inside the star. The increase of pressure causes the pressure ionization of matter, which gradually leads to the breakdown of the electrical constraint of solid and the transition to plasma gas.Increase the mass, that is, increase the density. At this time, the pressure is independent of temperature, so as to achieve a "cold" equilibrium configuration. The kinetic energy of electrons in the plasma is large enough to cause beta decay in the material:
Here, p is the proton in the atomic nucleus. When the density of such reaction reaches 108 g.cm-3, it will gradually become a neutron rich nucleus in the negative ionomer. There are too many neutrons in the atomic nucleus, resulting in loose nuclear structure. When the density exceeds 4 × 1011 g.cm-3, neutrons begin to separate from the atomic nucleus and become free neutrons,Self gravity balances the pressure between neutrons.If the mass changes, the pressure between neutron gases can no longer resist the self gravity of matterblack holeHowever, since the mass of most stars in the post evolution stage is less than its initial mass, such as stellar wind, "helium flash", supernova explosion, etc., they will lose a large percentage of the mass of stars. Therefore, the final outcome of stars can not be judged by their initial mass, but actually depends on the evolution process.Then we can draw such a conclusion.Stars below 8 → 10M ⊙ eventually throw off part or most of its mass and become a white dwarf.Stars above 8 → 10M ⊙ will eventually become neutron stars or black holes through gravitational collapse of star cores.
Process summary
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The observed star mass range is 0.1 → 60M ⊙, and the objects with mass less than 0.08M ⊙ cannot reach the ignition temperature.Therefore, it cannot become a star without emitting light.The central temperature of the celestial body with a mass greater than 60M ⊙ is too high and unstable, which has not yet been found.
Through discussion, we can generally understand the evolution process of stars, which mainly goes through: gas cloud → collapse stage → main sequence star stage → main sequence post stage → final stage.This is of great significance for us to further understand the evolution of stars.
Looking at the night sky from the earth, the universe is the world of stars.
The distribution of stars in the universe is uneven.From the day of their birth, they have gathered in groups to form a pair of starscluster, Galaxy
Stars are burning stars.Generally speaking, the size and mass of stars are relatively large.It is only because it is too far away from the earth that the starlight appears so weak.
Ancient astronomers believed that the position of a star in the sky was fixed, so they named it "star", which means "eternal star".But today we know that they are constantly moving at a high speed,such asThe sun moves around the center of the Milky Way with the whole solar system.But other stars are so far away from us that we can hardly detect their position changes.
The ability of stars to emit light varies from strong to weak.Astronomy uses "luminosity" to express it.The so-called "luminosity" refers to the power radiated from the surface of stars in the form of light.The surface temperature of stars varies from high to low.Generally speaking, the lower the surface temperature of a star, the redder its light;The higher the temperature, the bluer the light.The higher the surface temperature, the larger the surface area, and the greater the luminosity.Scientists can extract a lot of useful information from the color and luminosity of stars.
Historically, astronomer Herzpoon andphilosopherRussellFirst, the relationship between star classification and color and luminosity is proposed, and the stellar evolution relationship known as "Herr Roth diagram" is established, revealing the secrets of stellar evolution.In the "Hershey Rao diagram", from the high temperature and strong luminosity area at the upper left to the low temperature and weak light area at the lower right is a narrow star concentration area, where our sun is also located;This sequence is called the main star sequence, and more than 90% of the stars are concentrated in the main star sequence.Above the main sequence area isSuperStarAnd supergiant regions;The lower left is the white dwarf region.
Stars are born from interstellar dust in space (scientists call them "nebulae" or "interstellar clouds").
The "youth age" of a star is the longest golden stage in its life - the main sequence stage, which accounts for 90% of its entire life span.During this period, the stars give off light and heat with almost constant brightness, illuminating the surrounding space.
After that, the star will become turbulent and become a red giant star;Then, the red giant will complete all its missions in the explosion, throwing most of its own matter back into space, leaving behind debris, perhaps white dwarfs, perhaps neutron stars, or even black holes
In this way, stars come from nebulae and return to nebulae to finish their glorious life.
The gorgeous stars will always be the most beautiful scenery in the night sky.
Detection results
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May 2023In September, French and Italian scientists jointly used the European Space Agency's Very Large Telescope to discover the "ash" left by the first batch of star explosions in the universe for the first time: they detected three distant gas clouds whose chemical composition matched the scientists' expectations for the first batch of star explosionsWith.[1]
