White dwarf[13], also known asDegenerate dwarf）It is a star with low luminosity, high density and high temperature.White dwarfs areA compact star maintained in equilibrium by the pressure of degenerate electrons against gravity.It got its name because most of the early discoveries were white.[13]With a surface temperature of 8000K, it usually emits white light and can have a life span of several billion years.[1]
In 2022, astronomers at the University of Warwick, UK, found a faint white dwarf 90 light-years away from the Earth, and the debris of the planetary system circling it. They have a history of more than 10 billion years.[14]
White DwarfDegenerate dwarf）Is a kind of lowluminosityHigh density and high temperaturefixed star。Because of its white color and small size, it is named White Dwarf Star.A white dwarf is a star at the end of its evolution, which is mainly composed of carbon, covered with hydrogen andhelium。White dwarfs have gradually cooled and darkened over hundreds of millions of years. They are small in size, low in brightness, but high in density and mass.The white dwarf star list published in 1982 shows that there were 488 white dwarfs found in the Milky Way at that time, all of which were near the sun.With the rapid development of observational astronomy in recent decades, especially the implementation of large-scale sky survey projects, the number of newly discovered objects has increased sharply, especially the spectral sky survey of SDSS and the sky survey of Gaia satellite have found hundreds of thousands of white dwarfs.
In the main sequence star stage, the middle and low mass stars will undergo helium fusion in the core after the hydrogen fusion reaction, that is, every three helium nuclei will converge into a carbon nucleus, and the carbon nucleus will capture another helium nucleus to form an oxygen nucleus, which will expand into a red giant star.
When the radiation pressure of a red giant cannot balance the gravity, the outside expands outward and keeps getting cold, while the internal helium core shrinks and collapses under the action of gravity, and the compressed material keeps getting hot, and the final core temperature will exceed 100 million degrees, so helium begins to converge into carbon.After millions of years, the helium core has been burned out, and the structure and composition of the star are no longer so simple: the shell is still a mixture of hydrogen, but there is a helium layer below it, and there is a helium layer buried insideCarbon sphere。nuclear reactionThe process becomes more complex, and the temperature near the center continues to rise, eventually transforming carbon into other elements.At the same time, unstable pulsating oscillations begin to occur outside the red giant star: the radius of the star increases and decreases from time to time, stableMain star orderThe star becomes a huge unstable fireball, and the nuclear reaction inside the fireball becomes more and more unstable, sometimes strong, sometimes weak.At this time, the density of the inner core of the star has actually increased to about 10 tons per cubic centimeter. We can say that at this time, a white dwarf star has been born inside the red giant star.When the unstable state of the star reaches the limit, the red giant star will explode, throwing the matter outside the core away from the star body, and the matter will diffuse outward intoNebulaThe remaining core is the white dwarf we can see.So white dwarfs are usually composed of carbon and oxygen.But it is also possible that the core temperature can reach the temperature of burning carbon but still not enough to burn neon, and then a white dwarf star with a core composed of oxygen, neon and magnesium can be formed.Occasionally, there are white dwarfs composed of helium, but this is caused by the mass loss of conjoined stars.
There is no matter inside the white dwarf anymoreNuclear fusion reactionSo the star no longer has energy.At this time, it is no longer resistant to gravity collapse by the heat of nuclear fusion, but produced by extremely high-density materialsElectronic degeneracy pressureTo support.In physics, for a white dwarf without rotation, the maximum mass that the electron degeneracy pressure can support is 1.4 times the mass of the sun, that isChandraseka limit。manyCarbon oxygen white dwarfThe mass of the white dwarf is close to this limit. Sometimes it is transferred by the mass of the companion star, and the white dwarf may explode into aIa supernova。
White dwarfs form at very high temperatures, but because there is no source of energy.Therefore, it will gradually release its heat and become cooler (temperature decreases), which means that its radiation will gradually decrease from the initial high color temperature over time and become red.After a long time, the temperature of the white dwarf will cool to the point where its luminosity can no longer be seen, and it will become a cold black dwarf.However, the current universe is still too young (about 13.7 billion years old). Even the oldest white dwarf still emits thousands of K of temperature, and it is impossible to have a black dwarf.[2]
Crystal nucleosome
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Astronomy teaches usuniverseHowever, the physical explanation of the strange event that occurred in is unimaginable. Recently, scientists found that the mysterious "crystalline" nucleus may appear inside the white dwarf star.
Most stellar cores burn through hydrogen nuclear fusion, transforming mass into energy, and generating light and heat. When the hydrogen fuel inside the star is consumed, helium fusion reaction begins, and heavier carbon and oxygen are formed. This process is relatively short for stars like our sun, and white dwarfs composed of carbon and oxygen are formed,If its mass is greater than 1.4 times the mass of the sun, a type Ia supernova explosion will occur.
The observation of GD 518 white dwarf by the 2.1 meter telescope of MacDonald Observatory found that its surface temperature reached 12000 degrees, about twice the sun's temperature and 1.2 times the sun's mass. According to the star evolution model, its main components are oxygen and neon.Judging from the changes in the brightness of GD 518 white dwarf, it is actually undergoing "pulse" expansion and contraction, which means that there is instability in its interior. Scientists predict that there has been crystallization or solidification in its interior, forming a "small crystal ball" with a certain radius, which is a very inconceivable result,Scientists believe that continuing to investigate this white dwarf star will help provide evidence for other types of supernova explosions and better measure the large-scale scope of the universe.[3]
Stellar parameter
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Surface gravity
White Dwarf (Artistic Imagination)
The surface gravity of a white dwarf (Sirius B, the neighbor of Sirius) with the same size as the Earth is about 180000 times that of the Earth.Under such high pressure, no object exists, evenatomIt's all crushed and the electrons are detachedAtomic orbitalBecomesfree electron。[4]
volume
Small in size, its radius is close to the radius of the planet, with an average of less than 10000 km.
luminosity
Luminosity is the total energy radiated by a star per second, that is, the luminous power of the star.The luminosity of white dwarfs is very small, and the span is large, about one thousandth to one thousandth of the luminosity of the sun.
quality
The mass is 0.2-1.4 solar mass.
temperature
The surface temperature of white dwarfs is very high, with an average of 10four℃。
magnetic field
Some white dwarfs have magnetic fields as high as 10five-10sevenGauss.
Cooling principle
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White dwarfs have gradually cooled over billions of years.Therefore, the coldest white dwarf can be used to predict the age of the universe (i.eCosmochronology，cosmochronology）。In order to obtain accurate results, it is necessary to establish an accurate physical model for the cooling of white dwarfs.[5]
one
How much heat energy is stored in the white dwarf;
two
How quickly energy is lost from the hot core through the thin and opaque outer layer.
Therefore, the first step is toThermodynamic calculationThe total energy contained in the white dwarf, and then solve the energy transmission problem of the whole star.This requires accurate thermodynamic description equations and the radiative and conductive opacity of the cladding.In addition, the problem of convective transport must be solved, and a more accurate atmospheric model needs to be used as the outer boundary condition.
Mestel's calculation of white dwarf cooling is regarded as a classical cooling theory, and the later models are constantly improved and developed on this basis.Mestel gives
Where, A is the atomic weight of the core material, which is divided intoAverage molecular weight, M and L represent the mass and luminosity of white dwarfs, respectively.Although the above formula is not perfect, it can be seen that the cooling time of white dwarfs is related to the core chemical composition, cladding chemical composition, mass and luminosity.Lamb made some modifications on this basis, and developed the first white dwarf evolution program, which has been continuously developed and improved, and is still improving today.
Chabrier's work in 1998 described in detail the basic principle of the cooling calculation of white dwarfs.Considering the basic physical factors and starting from the thermodynamic principle, he deduced the basic formula for white dwarf cooling calculation:
Where dq/dt is the heat loss rate per unit mass;Du/dw represents the change of unit internal energy and gravitational energy respectively;Is the neutrino emission rate per unit mass.The above equation describes the basic process of white dwarf cooling: the energy lost is equal to the sum of the change of internal energy and gravitational energy and the energy taken away by neutrinos.Although this is a simplified model, we can learn the basic method of establishing the evolution model of white dwarfs.
Celestial characteristics
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number
More than 1000 white dwarfs have been observed.SiriusThe companion star of (Sirius) is the first white dwarf discovered by people, and also the brightest white dwarf (star of magnitude 8) observed. Its density is about 10 million tons/cubic meter, its volume is not much larger than the earth, but its mass is about the same as the sun.
The white dwarf star list published in 1982 shows that,GalaxyThere are 488 white dwarfs, all of which are near the sun.According to the statistics of observation data, about3%The white dwarf stars account for about 10% of all stars according to theoretical analysis and calculation.
screw
White dwarf collision
About 1600 light years away, a very famousbinary star Two compact white dwarfs rotate around their respective orbits every 321 seconds.ChandraobservatoryAstronomerX-rayThe analysis of band data refutes an already impressive view: the short orbital period of these two white dwarfs is in a stable state, and the closer their spirals are, the shorter their period is.Even if they are two stars separated by 80000 kilometers (the distance between the earth and the moon is 400000 kilometers), they are destined to merge.According to this artist like point of view, the reason for the spiral destruction of the famous J0806 system is the sameEinstein's Theory of RelativityAs predicted in: white dwarf due toGravity waveAnd finally lose its orbital energy.In fact, J0806 may be one of the brightest light sources of gravity waves in our Milky Way Galaxy, which can be captured directly by gravity wave tools set up in space in the future.[6]
Celestial evolution
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White dwarfs belong to stars that have evolved to the late age. In the later stage of evolution, stars eject a large amount of material. After a large amount of mass loss, if the mass of the remaining core is less than 1.44 solar masses, this star will evolve into a white dwarf.As for the formation of white dwarfs, some people also believe that the predecessor of white dwarfs is a planetary nebula (a ring or disk shaped material composed of high-temperature gas, a small amount of dust, etc. in the universe). Its center usually has a star with high temperature - the central star. Its nuclear energy has been basically exhausted, and the entire star began to slowly cool, crystallize, until the final "death".
electron degeneracy pressure The strong gravitational balance with the white dwarf maintains the stability of the white dwarf.When the mass of the white dwarf further increases, the electron degenerate pressure may be unable to resist its ownGravitational contraction, the white dwarf willCollapseMore dense objects:neutron starorblack hole。For a single satellite system, since there is noThermonuclear reactionTo provide energy, the white dwarf emits light and heat, but also cools at the same speed.After hundreds of billions of years, the old white dwarf will gradually stop radiating and die.Its body becomes a huge crystal harder than a diamond——Black dwarf。
For multi star systems, the evolution process of white dwarfs may be changed (such as binary stars).[7]
Discovery History
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The first white dwarf to be discovered is 40 Eridanus, a trinity star whose members areMain sequence starAnd the white dwarf star 40B and the main sequence star 40C in the constellation of Eridanus 40A and Eridanus 40B, which form a conjoint star at a distance.The conjoined stars 40B and 40C were discovered by William Herschel on January 31, 1783. They were observed again by Friedrich George Wilhelm Struve in 1825 and Otto Wilhelm von Struve in 1851.
In 1910, Henry Norris Russell, Edward Pickering and William Fleming found that he had a dim and inconspicuous companion star, while the spectral type of Eridanus 40B was A or white.[8]
In 1892, Alvan Graham Clark discovered the companion star of Sirius.According to the analysis of star data, the mass of this companion star is about one solar mass, and its surface temperature is about 25000 K, but its luminosity is about one ten thousandth of that of Sirius, so according to the relationship between luminosity and surface area, it can be inferred that its size is equivalent to that of the Earth.Such density is beyond the reach of materials on earth.In 1917, Adriaan Van Maanen discovered the white dwarf star Van Maanen, which is currently known to be the nearest to the sun.
In 1917, Van Manan discovered a lonely white dwarf star, calledFan Ma Nanxing。These three white dwarfs were first discovered as the so-called classic white dwarfs.Finally, many dim white stars were found. They all have high self motion, indicating that they are low luminosity objects close to the Earth, so they are all white dwarfs.William Ruiden seemed to be the first person to use the term white dwarf when he wanted to explain this kind of celestial body in 1922. Later, the term was popularized by Arthur Eddington.
Developed by Max Planck and others in the early 20th centuryQuantum TheoryLater, in 1926, Ralph H. Fowler established aFermi Dirac statisticsA theory that explains the density of white dwarfs.
Despite various doubts, the first nonclassical white dwarf was not recognized until about the 1930s.In 1939, 18 white dwarfs had been discovered. In the 1940s, Ruiden and others continued to study white dwarfs. By 1950, more than 100 white dwarfs had been discovered. By 1999, the number of white dwarfs had exceeded 2000. After that, Sloan's several surveys found more than 9000 white dwarfs, and most of them were new discoveries.[8]
In April 2014, astronomers found a 11 billion year old white dwarf star in the vast universe. Its low temperature has crystallized its carbon and turned it into a“Diamond Planet”。The white dwarf discovered this time is about 900 light-years away from the Earth, in the direction of Aquarius.It is estimated that this white dwarf star is about the size of the Earth and has a life span of 11 billion years, about the same as that of the Milky Way.It is the coldest and darkest white dwarf star ever discovered.As the temperature drops, the carbon that makes up this white dwarf star has crystallized, making it a "diamond planet".Previously, scientists had found thatCentaurOne named“BPM37093”A white dwarf star with a diameter of 4000 kilometers and a weight equivalent to 10thirty-fourCarat.Scientists infer that its core has crystallized from its pulsating oscillation.However, although the molecular structure is similar, this kind of "diamond" in the universe is not exactly the same as the commonly said diamond, and just in terms of weight, it is not affordable for the human body.Therefore, although this "diamond planet" is priceless, its most suitable location is still the twilight in the vast universe.[9]
Location coordinate of Sagittarius nova
On February 13, 2015, using the observation facilities of the European Southern Observatory and the telescopes on the Canary Islands, scientists at the Madrid National Astronomical Observatory in Spain surprised to find two white dwarfs at the center of the planetary nebula Henize 2-428, which are close binary stars composed of white dwarfs.The two white dwarfs are rotating around each other and getting closer and closer. When the two stars merge into one in about 700 million years, they will have enough material to trigger a violent supernova explosion.The two white dwarfs discovered this time, with a total mass of about 1.8 times that of the sun, orbit each other every 4 hours.These two stars are close enough. According to Einstein's general theory of relativity, they will hover closer and closer due to the radiation of gravitational waves, and eventually merge into a star in the next 700 million years.This is the most massive white dwarf binary discovered so far. In the future, when these two white dwarfs merge into one, they will have a runaway thermonuclear explosion, producing the first case of an Ia type supernova.The merged star is too massive and will exceed the theoretical upper limit of white dwarf. Nothing can prevent it from collapsing under its own gravity and then exploding into a supernova.This observation supports the theory that the central binary may explain somePlanetary nebulaBut a more interesting result follows.Further observations made with telescopes in the Canary Islands allow scientists to determine the orbits of these two stars and calculate their respective masses and distances between them.[10-11]
The Artistic Imagination of White Dwarf Absorbing the Material of the Companion
On March 15, 2015, Australian astronomers found a bright star at the center of Sagittarius (also known as Sagittarius) with a brightness of about+6. After ruling out the possibility of asteroids and stars, it was determined to be a new star.On March 18, 2015, when Japanese astronomers observed the new star again, its brightness was+5.3, so it can be inferred that its brightness is still increasing.
"Nova" is not a newborn star from scratch, but originally in the sky, just relatively dim, not observed.When it explodes, its brightness will suddenly increase, which is considered as a new star, hence its name.The explosion of nova originates from the material exchange generated by the binary system composed of white dwarf and companion stars.For most binary systems, the heat of hydrogen combustion is unstable, and will quickly convert a large amount of hydrogen into other elements, resulting in thermonuclear reactions.This process will release a large amount of energy, causing the white dwarf to explode extremely brightly, and blow away the remaining gas on the surface.
In February 2015, it was found that there are two white dwarfs in the center of the planetary nebula Henize 2-428, whose mass is smaller than that of the sun, but the two white dwarfs are approaching each other, and they will merge once in about 700 million yearsLa type supernovaThe two white dwarfs will finally disappear in a super explosion.[12]
In 2022, astronomers at the University of Warwick, UK, found a faint white dwarf 90 light-years away from the Earth, and the debris of the planetary system circling it. They have a history of more than 10 billion years.Astronomers found that the "red star" WDJ2147-4035 has a history of about 10.7 billion years, of which 10.2 billion years was in the white dwarf cooling state, using the spectral and photometric data from Gaia Space Observatory, the Dark Energy Survey and the European Southern Observatory's X-Sagittarius.[14]
Solar evolution
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White dwarf
Most of the hydrogen on the sun is gradually burning into helium, which can be said that the sun is in the most stable main sequence star stage.For stars of such mass as the sun, the main sequence star stage can last for about 11 billion years.The star slowly shrinks due to the light it emits. In the process of shrinking, the density of the central part will increase, and the pressure will also increase, making the hydrogen burn more severely. In this way, the temperature will rise, and the brightness of the sun will gradually increase.Since the sun entered the main sequence star stage 4.5 billion years ago, the brightness of the sunlight has increased by 30%. It is expected that it will continue to increase in the future, making the earth's temperature rise continuously.6.5 billion years later, when the main sequence star stage of the sun ends, it is expected that the brightness of the sun will be 2.2 times as high as today's, and the average temperature of the earth will be about 60 ℃ higher than today's.At that time, even if there is still seawater on the earth, I'm afraid it will be vaporized.If only from the average temperature, Mars would be the most suitable planet for human habitation.In the main sequence star stage, the inward force caused by the star's own gravity contraction and the outward force caused by combustion will pull each other to achieve balance.However, after 6.5 billion years, the hydrogen in the central part of the sun will burn out, and only the spherical shell around it will be left to burn.In the region that no longer burns in the spherical shell, due to the weakening of the outward force that counteracts gravity, it starts to shrink rapidly. At this time, the sun will become brighter and brighter, and the outer part of the spherical shell will be affected, causing the temperature to rise and begin to expand, which is the beginning of another stage - the red giant stage.The red giant stage will last for hundreds of millions of years, during which the brightness of the sun will reach 2000 times today, Jupiter andSaturnThe surrounding temperature will also rise, and Jupiter's icy moon and Saturn's characteristic ring will be evaporated without trace. Finally, the outer part of the sun will even expand to the currentEarth orbitNearby.
On the other hand, gas will be continuously emitted from the outer part, and the final mass of the sun will be reduced to 60% of the main sequence star stage.As the sun's gravity weakens, the planets begin to move away from the sun.When the sun's mass is reduced to 60% of the original, the distance between the planet and the sun will be 70% larger than now.In this way, althoughMercuryIt is very likely that Venus and Mars will be swallowed up, but before the outer part of the sun arrives, the earth should extend its distance and survive. Mars and Jupiter type planets (Jupiter, SaturnUranus、Neptune）Will survive.
A planet as massive as the sun will only shrink to a certain extent in the central part where its density has become very high, that is, the temperature will only rise to a certain extent, and the fire in the central part will gradually disappear.The sun gradually loses its light, and the expanded outer part will shrink and cool into a compact white dwarf.The planets that survived the test of the red giant era will continue to orbit the sun, and everything will be frozen. At last, the solar system will greet the end of silence.
If a star like the sun turns into a white dwarf, it will rotate once a second.The density shall be at least 1.41 × 10elevenkg/mthree
world record
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The largest diamond in the universe:In February 2004, the first direct evidence that white dwarfs crystallize into huge diamonds was announced.The observation of BPM 37093 pulsation enables astronomers from the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts, to infer this.Carbon white dwarfs have crystallized into diamonds about 4000 kilometers (2500 miles) in diameter.(Guinness World Records)
