Red superstar, yesfixed starIt is a relatively short unstable stage in the later stage of combustion. When the star passes through its long young and middle-aged period - the main sequence star stage, it will enter the old age period.according toStellar massThe duration is only millions of years, which is very short compared with the stable period of stars for billions of years or even tens of billions of years.
The surface temperature of stars during the red giant period was relatively low, but they were extremely bright because of their huge size.stayHerotuThe red giant star is a huge non main sequence star, and its spectrum belongs to K or M type.The reason why it is called a red giant star is that it looks red and has a huge volume.TaurusA desperate battleandHerdsmanOfArcturusIt's a red giant,OrionOfBetelgeusethen isRed supergiant。
Chinese name
Red Giant[8]
Foreign name
red giant[8] red giant star
Classification
fixed star；Non main sequence star (Herlottu);Giant star branch orSupergiantbranch
stayHerotu(Hertzsprung Russell diagram), the red giant stars are distributed in a relatively dense area at the upper right of the main sequence area, almost in a horizontal direction.
The stars are burning by thermonuclear fusion inside them.nuclear fusionAs a result, every four hydrogenNucleusCombine into a helium nucleus and release a large amount ofatomic energy, forming radiation pressure.A star in the main star sequence stage whose nuclear fusion is mainly at its center（core）Partly, the radiation pressure is in balance with its own shrinking gravity, and the burning of hydrogen in the star is extremely fast. A helium core is formed in the center and is increasing.As time goes on,Helium nucleusThere is less and less hydrogen around, and the energy generated by the central nucleus is not enough to maintain its radiation, so the balance is broken, and gravity has the upper handheliumNuclear andhydrogenThe outer star shrinks and collapses under the action of gravity, making its densitypressureAnd temperature rise sharply,hydrogenThe combustion of the helium core is pushed into a shell around the helium core.After thatStellar evolutionThe process is: the core shrinks and the shell expands - the helium core inside the combustion shell shrinks inward and becomes hot, while the stellar shell expands outward and keeps cooling, greatly reducing the surface temperature.This process only lasted for hundreds of thousands of years, and the star became a red giant in its rapid expansion.Helium fusionThe final result is to form aWhite dwarf[2]。
The core of most red giant stars is unfused helium, and the energy is provided by the hydrogen combustion envelope outside the helium core. They form the branch of red giant stars (RGB stars) on the figure.In addition, the core is carbon and other heavier elements, and the outside is burning helium cladding and hydrogen cladding, which form the horizontalAsymptotic giant bifurcation(AGB star).stayStellar atmosphereMedium carbon content is higher than oxygen contentCarbon starThe spectral types of AGB stars are generally C-N to C-R.
evolution
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The stars whose mass is between 9 and 40 times that of the sun will burn out the hydrogen fuel in the core and then move to the hydrogen layer outside the core.Since the inert helium core itself has no energy, it shrinks and is heated due to gravity, and the hydrogen on it will shrink together, so the fusion speed will increase, generating more energy, leading to the stars becoming brighter (1000~10000 times brighter than the original) and expanding the volume.The extent of volume expansion exceeds the increase of luminous capacity, so the effective temperature of the surface decreases.The decrease of surface temperature makes the color of stars incline to red, so they are called red giant stars.In theory, the main sequence stars with stellar spectra from A to K will evolve into red giant stars andRed supergiantAnd stars of type O and B will becomeBlue supergiant(There are many differences from the evolution of red giant stars).
When the core of the star continues to shrink to the density and temperature conditions sufficient to ignite the 3 helium process, helium fusion will start.
For stars with mass less than 2.5 times the sun,heliumThe core needs to shrink continuously to resist the helium accumulation of more and more cores, and the only way to resist gravity isElectronic degeneracy pressure。Therefore, when the temperature rises to the ignition temperature of~100 million degrees, it is already a degenerate compact core similar to a "white dwarf".Such helium combustion can not transmit energy through thermal expansion in time, and there will be helium flashes out of control. In about one minute, most of the helium nuclei will fuse into carbon nuclei (and subsequent oxygen nuclei), and transmit huge amounts of energy to the outer layer of the star, causing the star to suddenly brighten and last for a short period.Then, the core no longer generates energy, and the outer layer of hydrogen continues to fuse into helium in a more complex way at a shallower position.The stellar core slowly accumulates helium again. After a long period of time, similar helium flashes occur again in the helium envelope outside the carbon oxygen rich core.At this time, the star is located on the asymptotic giant branch on the Herot chart. After each helium flash, it branches from one red giant to another.
greater thansunlightFor stars with a mass of 2.57 times, due to faster hydrogen fusion and hotter core, helium fusion can ignite before the core shrinks to the degenerate state of white dwarf density, and the whole nuclear reaction will proceed smoothly and continuously.When the initial heavy element content of such stars is low ("metal poor" stars), they will enter the horizontal branch - the positions of these stars on the Herot chart are horizontal distribution.At this stage, the metal rich stars cluster into the red cluster on the Herot chart.[2]
Very low qualityRed dwarf(<0.5sunlightMass) is only in the troposphere, the stars are in complete convection, and the element abundance of stars is basically the same everywhere.Since the core temperature is not very high and the mass is too small, the whole star does not need to shrink too much to withstand gravity.Therefore, even if the hydrogen abundance of these stars is not very high in the late stage, they cannot shrink the helium accumulated in the core to reach the temperature of nuclear fusion, and they can not become red giant stars even after using up hydrogen.Since their main sequence star stage life is far longer than the age of our universe, the evolution of such stars is only theoretical, and there is no observational example.
High quality O and B stars (more than 25 solar masses), whose main sequence star stage is located at the top left corner of the Herot chart, belong to blue giant stars or evenBlue supergiant。They will always move horizontally at the top of the Herot, and the helium fusion may becomeHigh luminosity blue variable starIt may also be a Wolf Rayet star.Then they are classified as type II or type Ib and type IcSupernovaThe explosion ended his short life.
definition
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Red Giant
Red giant is a late evolutionaryfixed starIn a broad sense, it includes all bright stars that leave the main sequence after hydrogen combustion.They are located on the right or upper right of the Herot, belonging to the giant star branch orSupergiantUsually, most of the stars of these giant or super giant branches are K-type stars and M-type stars with large volume and luminosity, so they are low-temperature stars with red light color, so they are called red giant stars, some are blue or blue white giant stars of type O and B, and some areSubgiantType G, F, A of branchHuang JuxingOr yellow and white giant stars, white giant stars, some of which are near the main sequence are stars after the main sequence has just moved out of the main sequence, while others are in the form of evolution at a certain stage. In this star family, there are many types ofVariable star, such asCepheid variable、RR Lyrae variable Etc.In addition, some stars in the early stage of evolution also appear in this region, such as the T-type star in Taurus, but this kind of stars often have diffuse gas clouds around them, while ordinary red giant stars do not, which is a difference between the two phenomena.The phenomenon that stars of various masses transform into red giant stars is different. For stars with smaller masses (less thanSolar massHalf of),hydrogenAfter depletion, the center shrinks very slowly, and finally it is in the equilibrium state of the degenerate electron gas before causing helium combustion, so the contraction will stop, and the shell will expand slightly outward, that is, it loses the radiation ability of the visible spectrum, and turns into a cold nebula around the core material,The remaining hydrogen in the outer layer of the core is gradually extinguished because it is insufficient to support the radiation of the star, and gradually shrinks to the core of the degenerate electron gas balance.The core material of the star is transformed into aWhite dwarfAs for the dying stars with higher mass and less than 1.8-2.2 times of the solar mass, the core also shrinks to the degenerate equilibrium state of electron gas after hydrogen depletion. As the helium produced by the hydrogen combustion in the outer layer continues to be added, the mass of the helium core continues to increase, so it slowly shrinks inward. When the mass of the helium core in the center increases to 0.45 solar masses,The temperature at which the helium core shrinks causes the helium to be ignited. The core material undergoes nuclear combustion under the condition of the degenerate electron gas balance. The heat generated causes the helium core to expand, and then returns to the non degenerate state of the electron gas, and then forms a stable nuclear combustion. The stars with higher mass will directly undergo nuclear combustion in the non degenerate state.
For a star whose mass is less than 1.5 times that of the sun, its moving track on the Herot chart is a slightly zigzag upward curve at the bottom. When the star moves to the top of this curve, helium combustion occurs. Later, due to the heat escape of the stellar material, helium combustion becomes stable and the luminosity decreases, and it moves to a position slightly inclined to the left, where it is in a long-term stay state,The moving curve of stars whose mass is more than 1.5 times of the sun is mainly shown as a horizontal zigzag upward moving track. For stars whose mass is less than 10 times of the sun, helium combustion occurs when they move to the right end of the Herot chart. For stars whose mass is more than 10 times of the sun, helium combustion occurs at the left end after leaving the main sequence. The result of helium combustion is carbon generation.
This reaction is usually called a reaction. In fact, it is carried out in accordance with the above two steps. The probability of direct reaction is very small. Because the generated beryllium is radioactive and will be re decomposed into helium in a very short time, the second step of reaction must be carried out immediately after the first step of reaction in order to complete the reaction,This requires a higher density and temperature inside the star, which is quite different from the combustion of hydrogen.The burning time of helium in stars is much shorter than that of hydrogen. Stars like the sun can last for 1 billion years, while stars with mass several times to dozens of times of the sun can only last for several hundred thousand to several thousand years, which is much shorter than the life of main sequence stars. This is why stars are mostly distributed in the main sequence.
Causes
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Red Giant
STAR STARTnuclear reactionLater, in the protracted struggle against gravity, its main weapon was nuclear energy.Its core is a bigNuclear bomb, exploding there.It is precisely because this nuclear power can adjust itself almost precisely to balance with gravity that stars can remain stable for billions of years.Thermonuclear reactionOccurring at extremely high temperatureNucleusTherefore, it involves the basic structure of matter.At the center of a star like the sun, the temperature reaches 15 million degrees Kelvin, and the pressure isearth300 billion times the atmospheric pressure.Under such conditions, not only did the atom lose all electrons but only the nucleus remainedMovement speedIt is also so high that it can overcome the repulsive force of the electric platoon and combine. This isnuclear fusion。
Stars are in hydrogenMolecular cloudProduced by the center ofhydrogenform.Hydrogen is the simplest chemical element. Its nucleus is a positively chargedprotonThere is also a negatively charged electron rotating around the nucleus.The temperature inside a star is so high that all electrons are separated from protons, which move in all directions like molecules in gas.Since the same kind of charges repel each other, protons are protected by an electrical "armor", thus keeping a distance from other protons.However, under the temperature of 15 million degrees Kelvin in the core of a young star, protons move so fast that when they collide with each other, they can break through the "armor" and stick together instead of bouncing off like rubber balls.Four protons converge to form a helium nucleus.Helium is the second most abundant element in the universe.The mass of the helium nucleus is less than the sum of the four protons on which it forms.This quality difference is only seven thousandths of the total quality, but this ismass loss Into huge amounts of energy.A star like the sun has a huge nucleus, where 600 million tons of hydrogen are converted into helium every second.Huge nuclear energy towards the outside of the star can be stopped by violent impactGravitational contraction。
However, the "constant" evolution process will come to an end. When all hydrogen becomes helium, the core fire will not have enough fuel to maintain. The quiet days of stars in the main sequence phase will end, and the period of great turbulence will come.Once the fuel is used up, the rate of thermonuclear reaction immediately decreases, the balance between gravity and radiation pressure is broken, and gravity takes the upper hand.The stars with helium cores and hydrogen shells begin to shrink under their own gravity, and the pressure, density and temperature rise accordingly. As a result, the unused hydrogen in the outer layer of the stars starts to burn, resulting in the expansion of the shell and the contraction of the core[3]。
At a high temperature of about 100 million degrees, the helium nuclei in the star core fuse into carbon nuclei.Every three helium nuclei converge into a carbon nucleus, and the carbon nucleus captures another helium nucleus to form an oxygen nucleus.The speed of these new reactions is completely different from that of slow hydrogen fusion.They burst like lightning（Helium flashoverThe star has to adjust its structure accordingly.After about one million years, the outflow of nuclear energy gradually stabilized.Over the next few hundred million years, the stars were temporarily stableheliumIn the gradual consumption, the combustion of hydrogen is more and more advanced to the outer layer.However, there is a price to be paid for the adjustment. At this time, the star will expand greatly to make its structure adapt to the increase of luminosity.Its volume will increase one billion times.During this process, the color of the star will change, because its outer layer is far away from the hot core, and the temperature will be lower.Stars in this state are called red giants.
According to general theory, a red giant should have a very thick convective envelope.It is generally believed that many stars may lose their outer layer material (this material may formPlanetary nebula）And then become a white dwarf.It seems that red giant stars are an important stage of evolution for most stars, but to understand the evolution process before and after red giant stars, many practical and theoretical problems need to be solved.
sunlight
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In about 5 billion years, the sun will become a red giant star. After scientists' calculations, the sun will become extremely huge,Inner planet。However, the gravity of the sun will also be weakened due to the reduction of its mass, so Mars and all outer planets will move outward.At this timeMercuryEven Venus will be swallowed up by the sun.The fate of the earth is not very clear.If notTidal forceThen the Earth's orbit will escape to about 1.3 to 1.7Astronomical unit。However, the research found that because the earth and the sun have tidal force, the earth will still be swallowed up by the sun's outer atmosphere.But before that, when the sunhydrogenWhen exhaustedbiosphereIt will be destroyed, and the additional solar energy will also cause evaporation of the earth's oceans.After 3 billion years, the surface of the earth will become as hot as Venus.After another 4 billion years, the earth's air has dissipated into outer space, and finally the earth has become a scorched planet.
Astronomical discovery
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Volume reduction
Research has found that the size of the red giant star Betelgeuse, located in Orion, has shrunk by 15% in 15 years, but astronomers cannot explain the mystery of shrinking.Betelgeuse is one of the ten brightest stars observed by astronomers in the universe so far (2012). It is a familiar astronomical observation target for astronomers, and also a super massive star observed by astronomers for the first time. This red giant star ishubble space telescope This is the first star whose surface state can be photographed by the Hubble Space Telescope.[4]
Carbon element
The evolution of the sun towards red giant stars
August 22, 2012, AustriaUniversity of ViennaThe bulletin said that the researchers of the Astronomical Research Institute of the university found that a red giant star was wrapped in a cloud of soot like material.Researchers useEuropean Southern ObservatoryOfVery large astronomical telescopeWhen observing the red giant star named "R Fornacis", it was found that it was surrounded by clouds similar to soot dust.Due to the low surface temperature and rich carbon elements of red giant stars, complex hydrocarbons and solid matter dust may appear.These substances may form planets suitable for life.
Solar evolution
In August 2012, the international team of astronomers found evidence that the red giant star swallowed up the inner orbital objects, suggesting that the sun in 5 billion years' time would also destroy the Earth.About 5 to 7.5 billion years later, it will be enough to devour the inner planets in the solar system (2012), including Earth and Mars.finalMercuryVenus, Mars and even the earth will be swallowed up by the sun.[5]
CW Leo
CW Leo，It is a sun like star, whose mass is 4-7 times that of the sun when it was born, but after a great mass loss, its mass is 1.2 times that of the sun, only 490 light-years away from the earth, and it is one of the red giant stars closest to the earth.At present, it is expanding into space and releasing mass into the end of its life.In the process of expansion, the star will slightly lose dust, until it is observed that it releases mass at a constant flow, forming a smooth layer of expanded dust and gas, which looks like a ring of clear tree rings.
In the past decade, the CW Leo stars observed by astronomers are different.New York UniversityPatrick Huggins said: "This is the star closest to the Earth, which continuously ejects and loses mass."
Huggins and Nicolas Mauron of the University of Montpellier in France accidentally found that the dust layer surrounding CW Leo star formed a peanut shape, with a coaxial dust arc (local spherical shell) expanding outward, and the expansion range was 25000 times the distance between the star and the Earth.According to the expansion ratio of the star, the history of the star can be traced back to 8000 years ago.
Deason said that CW Leo star is surrounded by at least a dozen dust arcs. Their thickness and location indicate that the distance between the star and the adjacent dust arc shell is changing.Those giant stars do not lose mass at a constant rate, they are in a state of fluctuation.Sometimes stars lose mass from around the poles, sometimes from around the equator.
Dust arcs similar to tree rings can reveal the time of their formation, and the dust shells released from star expansion suggest the formation of dust arcs in this process.The distance between the dust arcs of stellar jets is about 500-1700 years apart.Deason said that changes in the temperature of the star's surface may cause dust to compress in colder regions and then expand outward.
Dyson pointed out that other red giant stars may also form dust arcs similar to annual rings, but it is difficult to observe their dust arcs in detail because they are too far away from the Earth.EmittingHerschel Space Observatory Perhaps more similar phenomena can be observed in the future.
Deason thinks it should be like this.She said: "In the next 5 billion years, the sun will expand into a red giant star, and correspondingly there will be similar spectacular space phenomena. Unfortunately, it is impossible for us to measure and analyze the sun's annual rings. In the process of the sun becoming a red giant star, the earth will be swallowed by the sun."[6]
The final stage of dying red giant
According to foreign media reports, recentlyastronomerUtilization of scientific research teamAtacama millimeter/submillimeter wave array telescope(ALMA) observed the spiral shape of the gas around a red giant named LL Pegasi and its companion star 3400 light-years away from the Earth.UCLAMark Morris, professor of physics and astronomy, said that the spectacular scene we observed was actually the final death stage of a dying red giant star, releasing a large amount of gas in the form of wind.After comparing the observation results with the computer simulation, astronomers believe that the gas morphology around the system is caused by highly elliptical orbits.
Morris said that due to the orbital motion of the red giant star, which has lost a lot of mass, the cold molecular gas that makes up the wind is ejected from the star like the water jet from the garden sprayer.ALMA，This powerful telescope is jointly managed and used by many countries, including the United States, and can measure extremely short wavelength radio radiation.
With this unique device,scienceThe family was able to create a 3D image of the molecular gas ejected by LL Pegasi and the spiral shape caused by the companion star.This image reflects many complete evolutionary processes of the spiral pattern, which provides a lot of information for scientists to study the binary system for more than 5000 years.Morris said that this rare system provides us with a new perspective on how such systems evolve when stars lose a lot of mass[7]。
