Stellar evolution is the sequence of rapid changes that stars experience in the process of life.According to the mass of stars, the life span of stars ranges from the most massive star only a few million years to the smallest star billions of years longer than the age of the universe.All stars come fromMolecular cloudGas and dust collapse.In the course of millions of years,ProtostarTo reach a state of balance, settle down and become the so-calledMain sequence star。
Most of the life span of starsnuclear fusionThe state of producing energy.At first, the main sequence star fused hydrogen into helium at the core to generate energy, and then,Helium nuclei It has an advantage in the core.Stars like the sun will start from the core layer by layerspherical shellFuses hydrogen into helium.This process will gradually increase the size of the starSubgiant Until reachingRed GiantStatus of.Stars with a mass no less than half of the sun can also generate energy by fusing hydrogen in the core into helium, and stars with heavier mass can be sequenced intoConcentric circleProduce heavier elements.When a star like the sun runs out of fuel in its core, its core will collapse and become compactWhite dwarfAnd the outer layer will be driven awayPlanetary nebula。A star whose mass is about 10 times or more than that of the sun collapses into a very denseneutron starOr a black holeSupernova。Although the age of the universe is not enough to make the lowest massRed dwarfEvolving to the end of their life,Stellar modelThink they're depleting the corehydrogen fuelThe former will gradually become brighter and warmer, and then become a low-quality white dwarf star.The changes of stars are very slow, and even no changes can be detected for centuries, so it is impossible to observe a star alone to study how it evolves.Therefore, astronomyphysical scientistBy othersAlternative methods, such as observing many stars in different life stages, and using computer simulation to inferStellar structure。
Chinese name
Stellar evolution
Foreign name
Stellar evolution
Concept
The process of stars from the main sequence star stage to aging and dying in different ways
Due to the evolution of starsDensity mapMove up, but keep onDotted lineInside the box.Figure A3 is an enlargement of the box area, showing the main thermonuclear reactions in the center of stars at different stages of evolution.
Due to the control of gravity, the general trend of star evolution is that the density increases (moves downward in the figure), while the mass is lost（Stellar windLoss or companionaccretion ）, fragmentation, instability, explosion and other phenomena reduce its mass (moving to the left in the figure).The evolution of stars must end in one of three possible cold states:White dwarf，neutron star, black hole.
Stellar evolutionismIn astronomy, it is a theory about the evolution of stars during their lifetime.
Since the evolution of a single star usually lasts for billions of years, it is impossible for humans to observe it completely, and only computers can be used for the time beingModel simulationThe evolution of stars.
Some kinds of stars simulated by computer
Evolutionary stage
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be born
Herotu
The evolution of stars beganGiant sub cloud。OneGalaxyThe density of most of the void is about 0.1 to 1 atom per cubic centimeter, but the density of the giant particle cloud is millions of atoms per cubic centimeter.A giant sub cloud contains hundreds of thousands to tens of millionsSolar mass, 50 to 300 light-years in diameter.As the giant molecular cloud rotates around the galaxy, some events may cause itsGravitational collapse。The giant molecular clouds may collide with each other or pass through the dense part of the spiral arm.AdjacentSupernovaThe high-speed material thrown by the explosion may also be one of the trigger factors.last,Galaxy collisionThe resulting nebula compression and disturbance 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 break down into smaller fragments.The mass is less than about 50Solar massThe debris of will form stars.In this process, the gas is heated by the potential energy released, andConservation of angular momentumIt will also cause the nebula to start spinning and then form the original star.
Star formationThe initial stage of the nebula is almost completely covered by dense nebular 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 temperature of primitive stars with very small mass (less than 0.08 solar mass) will not reach enough to start nuclear fusion, and they will becomeBrown dwarf, slowly cooling in hundreds of millions of years.Most of the higher quality protostarsCenter temperatureIt will reach 10 millionKelvin degreeAt this time, hydrogen will start to fuse into helium, and the star will start to glow.Nuclear fusion in the core will produce enough energy to stopGravitational collapse, reaching onestatic equilibrium。From then on, the star entered a relatively stable stage.If there are still giant remnants near the starMolecular cloudDebris, then these debris may continue to collapse on a smaller scale, becoming planetsasteroidandcometIsoplanetary objects.If the stars formed by the fragments of the giant molecular cloud are close enough, they may formKonductraAnd multi satellite system.
The sun becomes a red giant, baking the earth's surface into vegetation and smoke
Stars have different colors and sizes.From cooled red toHigh feverFrom 0.08 to 150Solar mass。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, so they burn hydrogen much faster.
After star formationHerotuAt a specific point in the main star sequence.Small and cold M typeRed dwarfIt will slowly burn hydrogen, and may stay in this sequence for 100 billion to several trillion years, while the large and hot O typeSupergiantWill leave the main sequence in just 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 star sequence, which is considered to be located in theMiddle age。After the star burns the hydrogen in the core, it will leaveMain star order。
Middle age
In middle age, they form red superstars,Supergiant。
Hertz Roth diagram reveals important laws of star evolution
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.The mass of the star is not enough to produce helium fusion to release heat energy, which gradually cools and becomes a white dwarf star.
The hot core will cause the star to expand significantly, reaching hundreds of times the size in the main star sequence stage, becomingRed Giant。The red giant stage will last for millions of years, but most red giants areVariable star ，不如Main sequence starstable.
The next evolution of stars is once again determined by their mass.
Recession period
The period from old age to death ends with one of three possible cold states:White dwarf，neutron star, black hole.
Encyclopedia x ignorance: illustrating black holes
Evolutionary morphology
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Low mass star
Red Dwarf Artistic Imagination
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 perform in the coreHelium fusion, producing an unstable and unbalanced response, and a strongsolar wind。In this case, the star will not explodePlanetary nebulaAnd will only run out of fuel to produce red dwarfs.
But less than 0.5 timesSolar massEighty five percent of stars do not produce helium reactions in their cores even after hydrogen is depleted.imageAdjacent starIn this wayRed dwarfIts life span is hundreds of billions of years. After the core reaction ends, the red dwarf starelectromagnetic waveOfinfraredAnd microwave band gradually fade down.
achieveRed GiantAt this stage, the outer shell of a star with a mass of 0.5 to 8 sun will expand outward, while the core will compress inward, resulting in the condensation of helium into carbonnuclear reaction。Fusion will regenerate energy, temporarily relieving the star'sDeath process。For stars the size of the sun, this process lasts about a billion years.
Helium combustionExtremely sensitive to temperature, causing great instability.Huge fluctuations will make the shell get enough kinetic energy to leave the star and become a planetary nebula.The core left by the center of the planetary nebula will gradually cool and become small and compactWhite dwarf, usually 0.6-1 times the mass of the sun, but only one earth size.
In gravity and electron mutual exclusionForce balanceWhite dwarfs are relatively stable.In the absence of energy sources, stars release surplus energy in the long years and gradually fade.Eventually, the white dwarf that has released its energy will becomeBlack dwarf。
Planetary nebula with a hot white dwarf at its center
Formed at the same timeKonductraOr in multi star system, interstellarQuality communicationIt may change the evolution process.Because part of the mass is obtained by other stars, the evolution of the red giant stage of the heavier stars in the system will be accelerated, while the smaller stars will absorb part of the mass of the red giantMain star orderStay longer.For example,SiriusOfcompanionIt's an old one, about oneSolar massBut Sirius is about 2.06 solar massMain sequence star。If the mass of the white dwarf exceedsChandraseka limitThe repulsive force of electrons will not be enough to resist gravity, but will continue to collapse.This will cause the star to throw out its shell, that isSupernova explosion, marking the death of the star.In other words, there will be no white dwarf star with a mass greater than 1.4 times that of the sun.If a white dwarf is composed of another starbinary star , then the white dwarf may use the hydrogen from another star for nuclear reaction and heat and throw out the surrounding material, even if the mass of the white dwarf is less than 1.4 times the mass of the sun.Such an explosion is called a nova.
Massive star
Over 9-10Times (in starsAmount of metalIn extremely low conditions, it can be reduced to 8 times) The shell of a star with solar mass expands intoRed supergiantAfter that, its core begins to be compressed by gravity, and the rise of temperature and density will trigger a series ofFusion reaction。These fusion reactions will generate heavier and heavier elements, and the energy generated will temporarily delay the collapse of stars.
Planetary nebula
Finally, fusion gradually arrivesperiodic table of ele mentsThe lower layer of silicon begins to polymerize into iron.Before that, stars passed through thesenuclear fusionGet energy, but iron can't be fusedRelease energyOn the contrary, iron fusion needs to absorb energy.This will result in no energy to counter gravity, and the core will be generated almost immediatelyCollapse。
The next evolution mechanism of star evolution is not clear, but it will cause a violent supernova explosion in a fraction of a second.Thrown out at the same time with elements lighter than ironneutrinoForm ashock waveAfter being absorbed by the thrown material, some heavier than ironradioactive elementAmong them, the heaviest is uranium.Supernova explosion is the formation ofrelative molecular massAnother way for elements larger than iron.（Heavy elementThe main source is considered to beBig Bang）
Neutrino shock waves continue to push out the ejected material.The thrown material may becometBelt collision may form new stars, planets and satellites, or become various celestial bodies.
Modern science is not clearSupernova explosionAnd the composition of stellar debris, but two possible evolutionary endpoints are known:neutron starAnd black holes.The massive blue super giant may occurpair-instability supernova , no debris will be generated.
neutron star
In someSupernovaThe electrons are pressed intoNucleusAnd combine with protons to form neutrons.After the electromagnetic force that makes atomic nuclei repel each other disappears, the star becomes a cluster of dense neutrons.Such stars are called neutron stars.
quality requirement: The collapsed core mass is more than 1.44 times the mass of the sun and less than 3.2 times the mass of the sun（Oppenheimer limit ）。
neutron starIts size is no more than a big city, but it is extremely dense.Because mostangular momentumRemained in stars, their rotation will be extremely fast, some even up to 600 revolutions per second.The radiation of stars will be limited by the magnetic fieldMagnetic axisNearby, but rotates with the star.If the magnetic axis rotatesChina CouncilIf you aim at the earth, you may observe the radiation of a star during each rotation of the earth.Such neutron stars are calledPulsarIs the first neutron star discovered.
black hole
It is widely recognized that not all supernovae will form neutron stars.IfStellar massLarge enough, even neutrons will be crushed until the radius of the star is less thanSchwarzschild radius Light cannot be emitted and becomes a black hole.
Quality requirements: the collapsed core mass is more than 3.2 times (overall mass is about 25-30 times) the mass of the sun (greater than the Oppenheimer limit).
Stephen Hawking (Stephen Hawking) combinationGeneral relativityAnd quantum mechanics predict the existence of black holes.With the efforts of astronomers over the years, we have successfully observed the unknown planetsgravitational fieldWe can infer the existence of black holes.According to the traditional general relativity, no matter or information can escape from the black hole, butquantum mechanicsSome exceptions are allowed ("Tunnel" phenomenon occurs for substances under specific conditions, and substances can pass through obstacles through an imaginary tunnel).The existence of black holes is supported by most astronomers.
However, there are still some problems to be solved.The current supernova explosion theory is not yet perfect, which cannot explain whether the star may be compressed into a black hole without supernova explosion, whether there is a black hole formed by a supernova, and the relationship between the initial mass of the star and the evolution end point.
Evolutionary cause
In the 1930s, physicists found theoretically that,NucleusThe reaction will generate huge energy.Use this theory to studysunlightWe found that the energy of the sun can be explained by nuclear reaction.
Of all agesStellar interiorAll kinds ofThermonuclear reaction；During the evolution of stars, a series of thermonuclear reactions will occur,Light elementGraduallyHeavy elementTransformation, gradually changing the composition of the star, changing the internal state of the star.Moreover, the temperature required for these thermonuclear reactions is getting higher and higher.
The energy generated by the thermonuclear reaction inside the star is transmitted by convection, conduction and radiation.Since most stellar matter is gaseous,heat conductionIt has little effect, only extremely dense insideSpecial star(e.gWhite dwarf）, the internal heat conduction is more significant.Most stars mainly rely on radiation to transmit the energy generated by nuclear reaction inside, and the transmission speed is quite slow. For example, the sun places it at the center of 700000 kilometers deepEnergy transmissionTo the surface, it will take 10 million years.ConvectiveSpeed ratioThe radiation is much faster, but stars of different masses,troposphereThe position and thickness of are very different.The star at the upper left of the main star sequence has a large mass, a small convection core in the center, and radiation outsideCladding。The stars in the middle and lower parts of the main star sequence, with small massRadiation layerVery thick, with only a thin troposphere on the surface.The star at the lower right of the main star sequence is very small in mass, and the whole star is convection.The energy generated inside a star determines itssurface temperature And luminosity.Laws of physicsThe movement, energy generation, energy transmission and consumption of the star's interior are related to its temperature, pressure, density, composition and other factors.The change of one factor will cause the change of other factors.Therefore, to study the evolution of celestial bodies is to explain how various factors change harmoniously under the constraints of physical laws.
According to the mass andchemical compositionUsing the laws of physics, we can calculate the internal structure at different times, that is, the temperature, density, pressure, energy flow andStellar radiationTotal luminosity and surface temperature ofphysical quantitySo that it can be determined that the starHerotuPosition on;In this way, we can also get the changes of the structure and physical parameters of the star with time, so we can also get the evolution process of the star, and we can also see the position movement of the star on the Herro diagram.This is the basic method for studying the evolution of stars.
The nuclear reaction theory is applied to the stellar evolution, and the calculated results are just in line with the observed data, which proves the correctness of this theory and its application.So, the starEvolutionary theoryIt began to develop.
Evolutionary results
Subrahmanyan Chandrasekhar
Chandraseka, an American astronomer, predicted that stars with a core mass less than 1.44 times (1.38 times) of the sun would evolve intoWhite dwarf。The core mass is greater than 1.44 times of the solar mass and less than 3 times of the solar mass, and the overall mass is 9-20 times of the sun, which will evolve intoneutron starThe core has more than three times the mass of the sun and has evolved into a black hole.
Research History
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In the early 1920s, British astronomersEddington (A.S. Eddington, 1882-1944Volume shrinkage, the density increases, and it evolves into a dense white dwarf star.In 1925, astronomers discovered the first white dwarf star in their observations.
In 1939, the American physicist R. Oppenheim (1904-1967) proposed that because of the huge gravity of a massive star, its final destination would not be a white dwarf, it would continue to shrink, and its atoms and nuclei would be crushedpositive electricityProtons and bands ofnegative electricityUnder the action of strong gravity, the electrons of are combined into neutral neutrons, and the huge star shrinks into a small ball with extremely small volume, mass and density - neutron star.In the same year, S. Chandrasekhar (1910-1995), an Indian American astronomer, predicted that stars with masses less than 1.44 (or 1.38) times of the sun would evolve into white dwarfs;A star with a mass greater than 1.44 times that of the sun will either become a white dwarf after discarding part of its mass in a big explosion, or continue to shrink and evolve into a neutron star or black hole with higher density through a supernova explosion.
In 1967, the British radio astronomer A. Hewish (1924 -) and his graduate student J. Bell (1943 -) discovered the first neutron star.
In the 1950s, the American astronomer M. Schwarzschild (1912 -) predicted that:High qualityAfter the star bursts, it shrinks continuously. When its gravity is strong enough to prevent light from escaping, it will become a "black hole".In 1974,britainTheoretical physicistHawking(S. Hawking, 1942-2018) proved that black holes will produceAntiparticle pair, positive energy particles will escape, forming the black hole "evaporation" phenomenon.On this basis, astronomers have now discovered several objects that may be black holes, but they can not completely confirm them.
The research on the evolution process of stars has not been completed, and the exploration will continue.
