The theory of stellar evolution is a theory in astronomy about the evolution of stars during their life cycle.Since the evolution of a single star usually lasts for billions of years, it is impossible for humans to observe it completely, and the theory is still partially speculative.Astrophysicists mainly use the observation of a large number of stars to judge the different stages of their life cycle, and use computer models to simulate the evolution of stars.
Horsehead Nebula, the bright spot at the base is generating new stars
The evolution of stars began with giant molecular clouds.The density of most empty spaces in a galaxy is about 0.1 to 1 atom per cubic centimeter, but the density of giant particle clouds is millions of atoms per cubic centimeter.A giant sub cloud contains hundreds of thousands to tens of millions of solar masses, with a diameter of 50 to 300 light-years.
As the giant molecular cloud rotates around the galaxy, some events may cause its gravitational collapse.The giant molecular clouds may collide with each other or pass through the dense part of the spiral arm.The high-speed material thrown by the adjacent supernova may also be one of the trigger factors.Finally, the compression and disturbance of nebulae caused by galaxy collisions may also form a large number of stars.
The conservation of angular momentum during the collapse process will cause the fragments of the giant molecular cloud to be continuously decomposed into smaller fragments.Fragments less than about 50 solar masses form stars.In this process, the gas is heated by the potential energy released, and the conservation of angular momentum will also cause the nebula to start to spin and form the original star.
The initial stage of star formation is almost completely covered by dense nebula gas and dust.Usually, the star generating source will be observed by creating shadows on the surrounding bright gas cloud, which is calledBok globule。
The life cycle of the sun
The temperature of very small protostars cannot reach enough to start the nuclear fusion reaction of hydrogen, and they will become brown dwarfs.The exact mass limits of stars and brown dwarfs depend on their chemical composition,MetalThe more components (elements heavier than helium in comparison), the lower the limit.The limit of protostars whose metal composition is similar to that of the sun is about 0.075 solar mass.With mass greater than 13 Jupiter mass (MJ)Brown dwarfSome astronomers think that such stars can be called brown dwarfs, and objects larger than planets but smaller than brown dwarfs are classified as sub stellar objects.These two types, whether they can burn or notdeuteriumIts luminosity is dim and gradually cools down in hundreds of millions of years, and slowly steps towards death.
LH 95 is a stellar nursery in the Large Magellanic Cloud.
Higher qualityProtostarThe core temperature can reach 10 million K, and the proton proton chain reaction can be started to fuse hydrogen into deuterium first, and then into helium.In a star whose mass is slightly greater than that of the sun,Carbon nitrogen oxygen cycleIt contributes a considerable amount to the generation of energy.The beginning of nuclear fusion will lead to the temporary loss of hydrostatic balance, which is the balance between the "radiation pressure" outward from the core and the "gravity pressure" caused by the star's mass to prevent the star from further "gravity collapse", but the star rapidly evolves to a stable state.
LH95 YesLarge Magellanic CloudStar nursery in.Newly born stars come in various sizes and colors.Spectrum typeIts mass ranges from the lowest 0.085 solar mass to more than 20 times the solar mass.The brightness and color of a star depend on its surface temperature, which is determined by its mass.The newly born star will fall onHerotuThe main order of is marked with a specific point.Small and cold red dwarfs burn hydrogen at a slow rate, which can remain in the main sequence belt for tens of billions of yearsSupergiantIt can only linger on the main preface belt for millions of years.Stars in the middle of the size like the sun have stayed in the main sequence belt for about 10 billion years.The sun is thought to be at the midpoint of its life, so it is stillMain order bandOn.Once the star consumes most of the hydrogen in the core, it will leave the main sequence belt.
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Sagittarius is a star field with a large number of stars
Stars have different colors and sizes.From hot blue to cool red, from 0.5 to 20 solar masses.The brightness and color of a star depend on its surface temperature, which depends on its mass.Massive stars need more energy to resist the gravity on their shells and burn hydrogen much faster.
After star formation, it will fall at a specific point in the main star sequence of the Herot chart.Small and coldRed dwarfIt will slowly burn hydrogen and may stay in this sequence for hundreds of billions of years, while the large and hot supergiant will leave the main sequence after only a few million years.Medium stars like the sun will stay on this sequence for 10 billion years.The sun is also located in the main sequence, which is considered to be in adulthood.After the star burns the hydrogen in the core, it will leave the main sequence.
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The death of a star with mass similar to the sun will become a planetary nebula
After millions to hundreds of billions of years of formation, stars will consume the hydrogen in their cores.High mass stars will consume the hydrogen in their cores faster than low mass stars.After the hydrogen in the core is consumed, the nuclear reaction in the core will stop, leaving a helium core.
After losing the nuclear reaction energy against gravity, the shell of the star began to collapse gravitationally.The temperature and pressure of the core rise as in the process of star formation, but at a higher level.Once the temperature of the core reaches 100 million degrees Kelvin, the core will start to conduct helium fusion, and generate energy through nuclear fusion again to resist gravity.If the star mass is not enough to produce helium fusion, it will release heat energy and gradually cool down to become a red dwarf star.
The hot core will cause the star to expand significantly, reaching hundreds of times the size of its main star sequence stage, and become a red giant star.The red giant stage will last for millions of years, but most red giant stars are variable stars, not as stable as the main sequence stars.
The next evolution of stars is once again determined by their mass.
Old age and death
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Low mass star
The end point of the evolution of low mass stars is not directly observed.The age of the universe is thought to be more than 10 billion years, which is not enough for these stars to exhaust the hydrogen in their cores.Current theories are based on computer models.
Some stars will conduct helium fusion in the core, producing an unstable and unbalanced reaction, as well as a strong solar wind.In this case, the star will not explodePlanetary nebulaAnd will only run out of fuel to produce red dwarfs.
But stars less than 0.5 times the mass of the sun will not produce helium reaction in the core even after hydrogen depletion.Red dwarfs like Bling star have a lifetime of hundreds of billions of years. After the core reaction ends, the red dwarfs gradually fade in the infrared and microwave bands of electromagnetic waves.
Medium mass star
When a star with mass similar to the sun dies, it will becomePlanetary nebula, likeCat's Eye Nebula。In another case, the acceleration of nuclear fusion reaction in several hydrogen containing shells around the core will immediately cause the star to expand.Because these are several layers at the periphery of the core, they are subject to lower gravity, and their expansion rate will be faster than the energy increase, which will result in a temperature drop and make them redder than at the stage of the main sequence band.Stars like this are calledRed Giant。According to the Herot chart, red giant stars are giant stars that are not in the main sequence belt. The star classification is K or M, including stars inTaurusAldebaran andHerdsmanThe Arcturus is a red giant star.
Stars with mass within several solar massesElectronic degeneracy pressureUnder the support of, the helium core will be developed, which is still surrounded by hydrogen.Its gravity compresses several layers of hydrogen directly on the helium core, which causes the reaction rate of hydrogen fusion to be faster than that of stars with the same mass on the main sequence belt.This in turn makes the star brighter (brightness increases 1000 to 10000 times) and expands;The degree of expansion exceeds the increase of luminosity, thus leading to the decrease of effective temperature.
The expansion of stars is in the peripherytroposphereThe material is carried to the surface of the star from the region near the nuclear fusion, and mixed with the material on the surface through turbulence.For all stars except those with the lowest mass, the materials for nuclear fusion in the interior are deeply buried in the interior of the star before this point, and the products of nuclear fusion can be seen on the surface of the star for the first time through convection.In this stage of evolution, the result is very subtle. The biggest effect is the change of hydrogen and helium isotopes, but it has not yet been observed.What works is surfaceCarbon nitrogen oxygen cycle, lower 12C/13C ratio and changed carbon and nitrogen ratio.These were created bySpectroscopic opticsIt has been found on and measured on many evolving stars.
Schematic diagram of star evolution demonstration
Schematic diagram of stellar evolution demonstration with mass similar to the sun
The star is born from its shrinking gas cloud (1), becomes a protostar (2) after the shrinking stage, and then enters the main sequence band (3).Once the hydrogen in the core is depleted, it expands intoRed Giant⑷, and then its shell dissipates into a planetary nebula, and its core metamorphoses into a white dwarf (5).When the hydrogen around the core is consumed, the core absorbs the generated helium, further causing the core to shrink, and making the residual hydrogen perform nuclear fusion faster, which will eventually lead to helium fusion (including the 3 helium process) in the core.In a star with a mass greater than 0.5 solar mass,Electronic degeneracy pressureIt may delay the helium fusion for millions to tens of millions of years;In heavier stars, the total mass of the helium core and the gas superimposed on the outer layers will make the electron degeneracy pressure insufficient to delay the process of helium fusion.When the temperature and pressure of the core are enough to ignite the helium fusion of the core, if the electron degeneracy pressure is the main force supporting the core, helium flash will occur.In the core with greater mass, the electron degeneracy pressure is not the main force to support the core, and the combustion of helium fusion will be relatively calm.Even if helium flash occurs, the time for rapid release of energy (108 orders of magnitude of solar energy) is relatively short, so the surface layer that can be observed outside the star will not be affected [2].The energy generated by helium fusion will cause the expansion of the core, so the hydrogen fusion rate superimposed on the outer layer of the core will slow down, reducing the generation of total energy.So, the star will shrink, although not all will return to the main sequence belt, it willHerotuIt migrates on the horizontal branch of, gradually shrinks on the radius and increases the surface temperature.
After the star has consumed the helium in the core, the fusion continues near the hot core containing carbon and oxygen.As the star enters the asymptotic giant branch on the Herot chart, it evolves parallel to the original red giant, but the energy generation is fast (and therefore the duration is short) [3].
Changes in energy output cause periodic changes in star size and temperature.The energy output itself reduces the frequency of energy emission, accompanied by strong stellar wind and violent pulsationMass loss rateIncrease of.Stars at this stage are calledLate type star、OH-IR starorMira type star。The expelled gas comes from the inner part of the star, and also contains relatively rich elements to be created. In particular, the abundance of carbon and nutrients is related to the type of star.The gas shell of expansion device composed of gas is calledCircumstellar envelope(circumstellar envelope), and will gradually lower the temperature as it is far away from the star, allowing the formation of fine dust and molecules.In an ideal situation, the high-energy infrared ray from the core will be excited to form a stroboscopic radiation after it is input into the ring star packet.
The burning rate of helium is extremely sensitive to temperature, which will lead to great instability.The huge pulsation combination will eventually throw out several layers of gas shells outside the star with enough kinetic energy, forming a potential planetary nebula.The core of the star still in the center of the nebula will gradually decrease in temperature and become a small and compact white dwarf.
Massive star
Crab Nebula
Crab NebulaIt is the scattered debris of a supernova that exploded about 1000 years ago.In massive stars, before electron degeneracy pressure can become mainstream, the core is large enough to ignite helium produced by hydrogen fusion.Therefore, when these stars are expanding and cooling, their brightness will not be much greater than that of low mass stars;But they will be much brighter than the initial stage of low mass stars, and also brighter than the red giant stars formed by low mass stars, so these stars are calledSupergiant。A star with a very large mass (about 40 times the mass of the sun) will be very bright and have a very fast stellar wind.Before they expand into red giant stars, because of the strong radiation pressure, they tend to peel off the outer gas shell first, so their mass loss is also very fast, which causes them to maintain high temperature on the surface (blue white color) at the stage of the main sequence band.Because the shell of a star will be stripped by extremely strong radiation pressure, the mass of the star cannot exceed 120 solar masses.Although lower quality can makeHousingThe speed of being stripped is slowed down, but if they are near enough conjoined stars, when they expand and their shells are stripped, they willcompanioncombination;Or because of theirrotationIf it is fast enough, convection will bring all the materials to the surface, resulting in complete mixing. Without the core and shell that can be separated, it can avoid becoming a red giant orRed supergiant[4]。
As hydrogen is obtained from the base of the shell and fused into helium, the core gradually becomes hotter and denser.In massive stars, the electron degeneracy pressure is not enough to prevent gravity collapse alone. As for each element consumed in the core, lighting the fire of heavier element fusion can also temporarily prevent gravity collapse.If the core of the star is not too heavy (its mass is about 1.4 times less than that of the sun, considering that many mass losses have been generated before), it may be able to form a white dwarf star (possibly surrounded by planetary nebulae) with a lower mass as described earlier. The difference is that this white dwarf star is mainly composed of oxygen, neon and magnesium.Before the collapse of the core, the core structure of a massive star was arranged in layers like an onion (not in proportion).Above some masses (estimated to be 2.5 times the mass of the sun, and the mass of the original star is about 10 times the mass of the sun), the temperature of the core can reach the temperature of local destruction (about 1.1GK) and begin to form oxygen and helium, and helium will immediately merge with the residual neon to form magnesium;Oxygen then fuses to form sulfur, silicon, and a small amount of other elements.Finally, when the temperature reaches the high temperature level where any element will be destroyed locally, it will usually releaseAlpha particle(Helium nucleus), and immediately fuses with other atomic nuclei, so a small number of atomic nuclei will become heavier after sorting, and the net energy released is increased, because the energy released by breaking the parent atomic nucleus is greater than that from fusionNucleusEnergy required.
The core mass is too large to form a white dwarf star, and is not enough to withstand the conversion of neon into oxygen andmagnesiumThe stars ofelementBefore, it will experience the process of gravity collapse (because of electron capture) [5].Whether the temperature increases or decreases due to electron capture, it will form smaller atomic nuclei (such as aluminum and sodium) before gravity collapse, which can cause a significant impact on the generation of total energy before gravity collapse [6].This may lead to a dramatic supernova explosionIsotope abundanceAll have an impact.
Crab Nebula
Before the collapse of the core, the core structure of a massive star was arranged in layers like an onion (not in proportion).
Before the core collapse, the core structure of massive stars
Once the process of star nucleosynthesis occursTie-56, the next process will consume energy (the energy released by combining the fragments into atomic nuclei is less than the energy required to smash the parent atomic nuclei).If the mass of the core is greater than the Chandraseka limit, the electron degeneracy pressure will not be enough to support and resist the gravity generated by the mass, and the core will suddenly collapse, and the catastrophic collapse will form a neutron star or black hole (where the mass of the core exceedsTolman Oppenheimer Wakov limit).Although the process is not fully understood, the conversion of some gravitational potential energy causes these cores to collapse and be converted into type Ib, Ic or II supernovae.It is only known that when the core collapses, as observed in Supernova 1987 A, there will be a huge neutrino surge.Extremely high-energyNeutrinoIt will destroy someNucleusSome of their energy will be consumed in releasingnucleon, including neutrons, and some energy will be converted into heat energy and kinetic energy, which will cause the shock wave to converge with some materials from the core collapse and cause rebound.The electron trapping that occurs in very dense confluent matter produces additionalneutronSome rebounded materials are bombarded by neutrons, which induce some nucleons to capture, creating a series of elements heavier than iron, including radioactive uranium [7].Although the neutrons released by non explosive red giant stars in early reactions and secondary reactions can also create a certain number of elements heavier than iron, the abundance of elements heavier than iron produced by such reactions (in particular, some stable and long-lived isotopes and some isotopes) is significantly different from the supernova explosion.We found that the abundance of heavy elements in the solar system is different from both, so neither supernovae nor red giant stars can be used alone to explain the observed abundance of heavy elements and isotopes.The energy transferred from the core collapse to the rebound material not only generatesHeavy elementThey also provide the escape speed required for their acceleration and disengagement (this mechanism has not been fully understood), thus leading to the generation of type Ib, Ic or II supernovae.The understanding of these energy transfer processes is still unsatisfactory. Although computer simulation can provide partial explanation for the energy transfer of type Ib, Ic or II supernovae, it is still insufficient to explain the energy carried by the observed mass ejection [8].Some evidence obtained from the analysis of the orbital parameters and mass of neutron star conjoined stars (requiring two similar supernovae) shows that the supernovae produced by the collapse of the oxygen neon magnesium core may be different from the supernovae observed by the collapse of the iron core (there are other differences in addition to the size) [9].
The most massive star may be completely destroyed in the supernova explosion because its energy exceeds its gravitational binding energy.This rare event leads to paired instability, and the debris after the event is not even a black hole.
Stellar debris
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After the star has exhausted its fuel, according to its mass during its life, if the assumedQuark starIts remains will be one of the following three types.
White dwarf
White dwarf
The mass of a star with a solar mass after its evolution into a white dwarf is about 0.6 solar mass, and the compressed volume is similar to the size of the Earth.White dwarfs are very stable objects because their inward gravity is in balance with the electron degeneracy pressure generated by the core electrons (this is the result of the Pauli exclusion principle).The electron degeneracy pressure provides a rather loose limit to resist the further compression of gravity;Therefore, for different chemical elements, the larger the mass of a white dwarf, the smaller the volume.Without fuel, the residual heat of stars can continue to radiate for billions of years.The chemical composition of a white dwarf depends on its mass.Stars with only a few solar masses cancarbonThe fusion produces magnesium, neon and a small amount of other elements, resulting in a main component ofoxygenWhite dwarfs of neon and magnesium.Under the condition of discarding enough quality, its quality will not exceedChandraseka limit(see below);And under the condition that the carbon combustion is not strong enough, it prevented him from becoming a supernova.A star with the same mass as the sun cannot ignite the nuclear reaction of carbon fusion. The white dwarf produced is mainly composed of carbon and oxygen, and its mass is too low to cause gravitational collapse unless it can increase its mass later (see below).Stars with a mass lower than 0.5 solar mass cannot be ignited even by helium combustion (see above), so the main component after being compressed into white dwarfs is helium.
In the end, all white dwarfs will become cold and dark celestial bodies. Some people call them black dwarfs.But the universe is not old enough to produce imagesBlack dwarfSuch celestial bodies.
If the mass of the white dwarf can increase beyondChandraseka limit- For white dwarfs whose main components are carbon, oxygen, neon, and/or magnesium, the solar mass is 1.4. The electron degeneracy pressure will not be able to resist gravity, and the star will collapse due to electron capture.Depending on the chemical composition and core temperature before collapse, the core may collapse into aneutron star, or it is out of control due to the combustion of carbon and oxygen.The heavier the element is, the more likely it is to collapse, because higher temperature is required to re ignite the fuel in the core, so the electron capture process that can lighten the nucleon can make the nuclear reaction easier;However, the higher the core temperature is, the more likely it is to cause the runaway nuclear reaction of the star, which will cause the star to collapse intoIa supernova[12]。Even if the type II supernova produced by the death of a massive star releases more total energy, this supernova will be several times brighter than the type II supernova.This instability, which will lead to collapse, makes it impossible for a white dwarf star with a mass greater than or close to 1.4 solar mass to exist (the only possible exception is a white dwarf star that rotates at super high speed, because the centrifugal force counteracts the mass problem).The mass transfer between conjoined stars may cause the mass of white dwarfs to approach the Chandraseka limit, thus causing instability.
If onClose binary systemThere is a white dwarf star and an ordinary star. Hydrogen from the larger companion star will form an accretion disk around the white dwarf star and increase the mass of the white dwarf star until the temperature of the white dwarf star increases and causes runaway nuclear reactions.The mass of the white dwarf star has not yet reachedChandraseka limitPreviously, this explosion would only form nova.
neutron star
Shock wave generated by supernova explosion 15000 years ago
The bubble like image is the shock wave generated by the supernova explosion 15000 years ago, which is still expanding.When the core of a star collapses, the pressure causes electron trapping, which makes most of the hydrogen turn into neutrons.After the electromagnetic force that originally separated the atomic nucleus disappears (in proportion, if the atomic nucleus is like dust, the size of the atom is like a 600 foot longFootball Field）The core of a star becomes a compact sphere with only neutrons (just like a huge atomic nucleus), and there are several layers of shell composed of degenerate matter (mainly the thin layer of iron and the material produced by subsequent reactions) outside.Neutrons also followPauli exclusion principle , using a force similar to the electron degeneracy pressure but stronger to resist the compression of gravity.Such stars, called neutron stars, are very small - only 10 kilometers in diameter, no larger than the size of a large city - and have extremely high density.TheirRotation periodBecause of the contraction of stars, they shrink dramatically very short (because of the conservation of angular momentum), some as high as 600 revolutions per second.With the rapid rotation of these stars, the earth will receive a pulse whenever the magnetic pole of the star faces the earthradiation。Neutron stars like this are calledBoshaThe first neutron star discovered is of this type.
black hole
The magnetic field swirling outside the black hole stirs the acceleration circle, forming a super strong wind of 480km/s, thus releasing energy
If the remnant of a star has enough mass,Neutron degeneracy pressureWill not be enough to prevent the star from collapsing toSchwarzschild radius The debris of this star will becomeblack hole。It is not known how much mass is required for this to happen, but it is estimated to be between 2 and 3 solar masses.Black holes are celestial bodies predicted by general relativity, and astronomical observations and theories also support the existence of black holes.According to the traditional theory of general relativity, no matter or information can be transmitted from the inside of a black hole to the outside observer, although quantum effects allow this rigorous law to produce errors.
Although the mechanism of star collapse to produce supernova has not been fully understood, it is also unknown whether the star can directly collapse to form a black hole without the visible supernova explosion;Or, after the supernova explosion, the neutron star should be formed first, and then continue to collapse into a black hole;The correlation between the initial star mass and the final debris mass is not completely reliable.To solve these uncertain problems, more supernovae andsupernova remnant 。
