Stars are made of lightPlasma——Mainlyhydrogen、heliumAnd trace heavy elements - a giant sphere.On a sunny night, there are always countless light spots in the night. Except for a few planets, most of them are stars.The sun is the nearest star to the earth, and almost all the stars that can be seen at night are in theGalaxyInside.[1]Of the 300 billion stars in the Milky Way system, only a small part can be observed.[2]Human beings have been observing stars for a long time, and there are many observation methods.Those brighter stars are divided into constellations andConstellationSome stars have special names.The brightness of a star is calledthe magnitudeThe brighter the star, the lower the magnitude.Astronomers also compiledCatalogueTo facilitate research.
The star will be in the corenuclear fusionTo generate energy and transmit it outward, then radiate from the surface to outer space.Once the nuclear reaction of the core is exhausted, the life of the star will end soon.At the end of life, stars will also containDegenerate matter。Different sizes and masses of stars will lead to different outcomes:White dwarf、neutron star、black hole。
Two or more stars bound by gravity can form double stars or poly stars. When such stars are in relatively close orbits, the material exchange between them can have a significant impact on their evolution.
In 2023,Astronomical research indicates that planets located in the habitable zone of metal poor stars may be the best place to search for potential life.[42]In 2024, Italian astronomerIt is the first time to calculate the weight of water vapor around the star.[45]
The observation of stars has a long history.The ancient EgyptiansSiriusAt the moment when the eastern horizon appears, predictthe NileThe flood of.ChinaShang DynastySpecial officials will be set up to observe the Great Mars（Antares ）The appearance in the East determines the time of the beginning of the year;Song dynastyOfSi TianjianFound a supernova in Taurus during observation——Tianguan Guest Star；[3]Sailors in the Ming Dynasty usedNavigation Nine StarTo judge the direction.
Many ancient astronomers believed that stars were fixed in the eternalcelestial sphereAnd will never change.By making conventions, astronomers form groups of starsconstellationperhapsConstellationsAnd use them to locate the movement of planets in the sky.The cycle of the sun's movement in the sky background is used to createcalendarAnd provide seasonal guidance during farming.[4]It is now used almost all over the worldGregorian calendarIt is based on the sun, the star closest to the earth.[5]
Copernicus' heliocentric system - Bruno insisted on this theory and even gave his life.
The oldest star map with exact dates appears in ancient Egypt in 1534 BC.[6]Islamic astronomers still use Arabic names for many stars today. They also invented many astronomical instruments that can measure and calculate the position of stars.However, people have misunderstood stars for a long time.In 1584,Gi ordanoBruno DevelopedNicolaus Copernicus The heliocentric theory of“heresy”。Ancient Greek philosophersDemocritusandEpicurusHe once put forward the same idea as him.seventeenth centuryNewtonfindUniversal gravitationLater, people's misunderstanding of stars gradually disappeared.Bessel First used in 1838parallaxTechnology to detect a star（61 Cygni ）The distance is 11.4 light years, which reveals the vastness of space and the distance between celestial bodies.William Herschel He was the first astronomer to try to determine the distribution of stars in the sky.In the 1780s, he made a series of measurements in 600 directions with a measuring instrument, calculated the number of stars visible along the line of sight, and drew the first star map of the Milky Way (silver disk).[7]
Joseph FraunhoferandAngelo Secchi They pioneered scientific astrospectroscopy, and found that the dark spectral lines in the stellar spectrum are caused by the absorption of wavelengths at specific frequencies by the atmosphere.In the 20th century, star research began to turn to physics.In 1913, Heroto came out, which promoted the research of stellar physics, and the explanation of the internal structure of stars and the model of star evolution were successfully proposed.becausequantum mechanicsThe problems in stellar spectra can also be well solved.In today's world, due to the rapid development of science and technology, all kinds of telescopes have been built continuously, and the research on stars has become more and more detailed.
In 2023, the research team of Massachusetts Institute of Technology reported the process of a planet (ZTF SLRN-2020) being "swallowed" by a host star, or as direct evidence that a star can swallow one of its circling planets。[43]
In 2024, Italian astronomers found water vapor in the disk around a young star. The water content in the inner disk of HL Tauri, a young sun like star in Taurus 450 light years away from the earth, is at least three times that of all the sea water on the earth.This is a scientistIt is the first time to calculate the weight of water vapor around the star.[45]
Nomenclature
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Chinese Star Officer
The naming of stars in ancient China is to divide (mainly in the north) the sky into several star officials.[8]The number of stars in each star officer varies from one to dozens.The coverage of the sky varies.In the eyes of the ancient people, there should be a state and society in the sky as well as in the earth, so they compared the situation on the ground and gave the stars in the sky corresponding to the things on the earth: the Son of Heaven, the princes, the army, etc.It is also mixed with many mythical elements.The stars in the northern sky can be roughly divided into three walls:Purple Placanticline, Oriental starryTaiweiyuanAnd the northern starsTianshiyuan。There are many stars in each wall, and they share different "duties" according to the imagination of the ancients: Ziweiyuan is the place where the Emperor of Heaven lives, Taiweiyuan is the place where the Emperor of Heaven handles government affairs, and Tianshiyuan is the place where transactions take place.Each wall contains various things or officials composed of stars.[9-10]
The ecliptic and nearby stars form 28 constellations.These constellations were set up by the ancients for the convenience of measuring the movement of celestial bodies.At the same time, the 28 constellations corresponding to the southeast, northwest and northwest are dividedFour images。The division of stars and constellations is very helpful for the timing of agricultural production activities.[9]
The concept of constellations has existed since the Babylonian period. The ancient stargazers combined the more prominent stars with specific scenes such as nature or myths, and imagined them into different shapes, and the nature or myths of their related images.The modern international constellation division can be traced back toBabylon, Cuba。Ancient Babylon had the earliest constellation name 5000 years ago.In the 13th century BC, they had divided the 12 constellations of the zodiac, called "the 12 zodiac", which means the 12 palaces in the annual movement of the sun.The 12 constellations in the zodiac are the basis of astrology.Many distinct individual stars are also given exclusive names, especially those in Arabic and Latin.[12]Later generations gradually expanded and named more constellations.In the second century AD, astronomersPtolemy On the basis of summarizing the previous knowledge, a table containing 48 constellations was compiled.16th to 17th centuriesGreat Geographical DiscoveryIt also adds some constellations in the southern sky.At this time, the concept of constellation is just a combination of bright stars visible to the naked eye, and there is no clear boundary between constellations.With the development of astronomical telescope technology, more and more dark stars have been found and studied in depth, but it is difficult to determine which constellation they belong to and how to label and call them.
In 1928, for the needs of astronomical research,International Astronomical UnionThe conference held in Leiden, the Netherlands clearly divided the whole sky into 88 constellation regionsRight ascension、DeclinationThe line zigzags the boundary, retains the traditional constellation name, uses Latin to stipulate its academic name and its abbreviation symbol composed of three clear capital and lowercase letters, which is used uniformly throughout the world.Later, the Chinese Astronomical Society determined the Chinese translation of constellation, which became the official academic name.[13]
In 1603, German astronomerJohann Bayer Created a combination of Greek alphabet and constellationBayer designation , name each star in the constellation.Then British astronomersJohn FranstideThe Franstide naming method based on the value of right ascension was invented.The Bayer naming method is based on the brightness of each star in the constellation, and it is named in Greek alphabetical order.for exampleSiriusCanis Major is the first bright star, so it is called Canis Majorα，VegaIt's Lyraα，RigelOrionβ。When you run out of Greek letters, use numbers or Roman letters.[8]While Franstide is ordered according to the right ascension of stars, and only stars visible in the northern hemisphere have numbers.
Pseudo color images of the sun superimposed on different wavebands taken by the Solar Dynamics Observatory.
Modern astronomers study stars. The main source of information is the electromagnetic radiation from stars.Although modern humans can launch probes in the solar system or even land on the planet, the distance between stars is so far that humans can hardly detect stars other than the sun at close range.Although in addition to electromagnetic waves,Gravitational wave、cosmic rayThey can also capture the clues of distant stars, but these observation technologies are relatively less mature than electromagnetic wave observation.So far, electromagnetic radiation is still the main channel to obtain the information of stars (except the sun).[14]
electromagnetic waveIt is generated when the electric charge in the atom moves at a variable speed.The stars have extremely high temperatures, so the atoms move very violently, and the electromagnetic radiation is also very strong.Wavelength range fromlong wavereachγradialBoth.When a star's radiation passes through the earth's atmosphere, many wave bands are absorbed by atmospheric molecules.This shielding effect of the atmosphere is the guarantee of life on the earth. Without such shielding, life on the earth will be threatened.But this is a hindrance to astronomy.Fortunately, there are two transparent windows: optics and radio, which provide necessary information channels for the development of human astronomy.
For ground observation, the absorption of the atmosphere to electromagnetic waves of different wavelengths is different.
The optical window is a wave band with a wavelength of 0.35-22 μ m, includingvisible lightAnd part ofinfrared。Among them, 17-22 microns are translucent, and 1.1-17 microns are discontinuous windows, that is, a number of small slits can pass through radiation.The radio window has a wavelength of 1mm to 30mradioband.A portion of 1-40 mmmicrowaveIt is also a translucent window.[15]Therefore, ground telescopes are usually optical telescopes andradio telescope。For example: Ounan StationVery large telescope(VLT), 500m aperture spherical radio telescope（FAST）Large sky area multi-target optical fiber spectrum telescope（LAMOST）Etc.
Star magnitude, brightness, luminosity
The brightness of a star is called apparent brightness.Astronomers divide brightness into several grades, which are called apparent magnitude.As early as the 2nd century AD, ancient Greek astronomersHipachasThe thousands of stars seen by the naked eye are divided into six grades.The brightest one is level 1, and the darkest one is level 6.If the brightness of two stars is 100 times different, their apparent magnitude is 5 degrees different, which is very consistent with the physiological feelings of the human eye.The relation between magnitude and brightness is:
。The magnitude of stars varies from bright to dark.The limit magnitude of human visual perception is generally 6.5, and with the help of large telescopes andElectronic eyepieceYou can see stars below 20 degrees.
The apparent magnitude of a star cannot reflect the true luminosity of the star, because the distance between the star and the earth is different.Seen from the earth, the sun is the brightest star, but Vega is 60000 times brighter than the sun. Because it is very far from the earth, its apparent magnitude is only 0, much darker than the sun.[16]In order to compare the true luminous gap of stars, astronomers defined the absolute magnitude: the apparent magnitude of stars at the 10 second gap.Set the star in
The apparent magnitude of the sun is - 26.7, but the absolute magnitude is only 4.75.Although Sirius' apparent magnitude is -1.46, its absolute magnitude is 1.41.
The true luminous power of a star is usually expressed in luminosity.The luminosity of a star is defined as the illuminance generated by the starlight on the plane perpendicular to the line of sight at the observation point.[15]。Illuminance is the degree to which an object is illuminated, in lux (lx).1 lux is equivalent to the illuminance generated by a standard candle at a distance of 1 meter.Photometric measurement includes photoelectric photometryCMOS、CCDmetering.[17]In additionHot magnitude，Radiant powerThey can also represent the luminous power of stars.
To determine the position of a star on the earth, you only need to determine its position in thecelestial sphereAnd the distance from the earth.
To determine the coordinates of stars on the celestial sphere, it is usually necessary to specify the celestial coordinate system.GenerallyHorizontal coordinate system、Equatorial coordinate system、Ecliptic coordinate systemandGalactic coordinate systemEtc.All celestial coordinate systems specify the basic axis, basic point and measurement direction range.Now with large-scale sky survey data, it is easy to obtain the celestial coordinates of stars.The difficulty lies in measuring the distance between stars.
Triangular parallax method refers to comparing photos taken in the same sky area at different times (usually half a year), measuring the annual position change of stars caused by the revolution of the earth, and then calculating the distance by solving the triangle.Since the angle measurement accuracy is limited by 0 ''. 01, the trigonometric parallax method is only applicable to stars with a distance less than 100 seconds.In recent years, the launch of some high-precision astronomical observation satellites can further improve the accuracy.There are about 10000 stars with trigonometric parallax.The spectroscopic parallax is to use the spectroscopic technology to obtain the spectrum of the star, then use some features in the spectrum to deduce the absolute magnitude of the star, and finally use the distance modulus formula to calculate the distance.The width of the hydrogen Palmer line in the stellar spectrum and the spectral line intensity of some metal elements such as lithium, calcium, potassium, magnesium are all related to the absolute magnitude.For distant stars, the accuracy of the spectral parallax method is good.There are about 60000 stars in the measured spectral parallax data.[18]
Cepheid parallax method usesCepheid variableDistance measures the distance of a star cluster or galaxy.Cepheid variables are a class of strictPeripheral light relationA variable star.The distance of a Cepheid variable can be calculated from its luminosity and period.When measuring the distance of extragalactic galaxies, as long as one Cepheid variable is found, the distance of the galaxy can be easily estimated.[16]
The standard candlelight method uses Ia type supernovae to measure the distance of distant galaxies.The luminosity of Ia type supernova is constant, so as long as Ia type supernova is found in extragalactic galaxies, the distance of this galaxy can be easily measured.For further galaxies (beyond the 15G second gap)Hubble relationshipThe distance measurement is relatively accurate.If there are more distant galaxies, astronomers are unable to measure their distances at present.
Stellar motion measurement
The motion of stars in three-dimensional space needs three parameters to describe.The movement along the line of sight is called the visual velocity;The motion on the plane perpendicular to the line of sight (the celestial sphere) is called self motion.
Due to the distance, the self motion of the star can be considered as a uniform linear motion, and the unit is taken asAngular second/Years.The magnitude of the self value is obviously related to the distance of the star.The fastest known selfBarnard star, is 10.3 angular seconds/year,[19]It is 5.9 light-years away from the earth.[18]It is very difficult to accurately determine the self motion of stars. Astronomers use the same method to take pictures of the same sky area at different times, with time intervals ranging from several years to decades.
Figure displaying red shift and blue shift.The displacement of the absorption line can be seen.
There are two possibilities for the radial motion of stars to approach and away from each other.Astronomers use the Doppler effect to determine the displacement of certain spectral lines in the stellar spectrumblue shiftorred shiftSize of.
Dispersion of light: Newton discovered that white light is composed of seven colored light.
In the visible light range, light of different wavelengths gives people different visual perception.Light of various frequencies mixed together gives the human eye the feeling of white light.If the light wave of a certain frequency takes up a large proportion, the mixed light will show a certain color.The sunlight is yellow white light, and the blue of the sky is caused by the scattering of blue light by the earth's atmosphere.The difference in the color of a star is also the difference in the proportion of its radiation wavelength.The radiation will be refracted when passing through the medium, and the refractive index will be different with different frequencies.When a beam of parallel sunlight shines into a small water drop in the air, the red light with the shortest wavelength has the smallest refractive index in the water drop, making its outgoing light form the largest angle with the incoming light, while the purple light with the shortest wavelength has the largest refractive index in the water drop, which forms a rainbow.[20]Similarly, we can get the seven color light of the sun by using a prism, which is called spectrum.The earliest spectrum was obtained by placing a prism in front of a telescope.
Flame reaction: emission spectra of different elements are different.
In 1814, the German physicist Joseph Fraunhofer discovered that there were thin black lines on the background of the sun's "continuous spectrum".1859KirchhoffandBunsenIt is found that the light emitted by chemical elements at high temperature is not continuous. When each element is heated, its steam will produce a unique bright line.This means that each chemical element has its own characteristic spectrum, so the basic composition of any substance can be determined according to the spectrum emitted by the substance.And after the light of high-temperature elements passes through the steam of low-temperature elements, the bright lines disappear, leaving behind dark lines, which are absorption lines.Spectral analysis was born.This shows that the composition of chemical elements on the sun 150 million kilometers away can be detected on the earth.[21]
In modern astronomy, spectral analysis has been a very important research method.In addition to using prism to obtain spectrum, the most advanced method is grating spectrography.Grating is a kind of precise optical element.On a very clean optical plate, many parallel thin lines with equal spacing are drawn to form gratings.The superposition of interference and diffraction can make light pass through grating and decompose into spectrum.If the star image on the focal plane of the telescope is imported into multiple grating spectrographs using optical fiber, the spectrum of many stars can be obtained simultaneously.[22]Now, there are holographic gratings, ladder gratings and other more advanced light splitting devices.[23]
The absorption lines of different types of stars are different.[24]
Research
Study on stellar spectral type
Spectrum is the ID card of a star, and the spectrum of each star is different.At the beginning of the 20th century, the Harvard University Observatory studied the spectra of 500000 stars that had been photographed and classified them.The basis of classification includes: energy distribution of continuous spectrum, density and intensity of spectral lines, chemical elements of characteristic spectral lines, etc.This classification isHarvard Classification。The spectra of stars are divided into 7 categories, which are marked with English letters;There are also 10 subtypes of each type, marked with Arabic letters.For example, Vega is A0, Sirius is A1, and the sun is G2.But not every subtype has a corresponding star.
Characteristics of various spectral types
Spectral type
Star surface temperature/on
colour
Spectral line characteristics
example
O
40,000-25,000
blue
The continuous UV spectrum is strong.There are ionized helium, neutral helium and hydrogen lines.The secondary ionized carbon, nitrogen and oxygen lines are weak.
Orionι(Cutting III)
B
25,000-12,000
blue and white color
Strong hydrogen line, obvious neutral helium line, no ionized helium line, but ionized carbon, nitrogen, oxygen and secondary ionized silicon line.
R. The three subtypes N and S only reflect the difference in chemical composition.R. There are strong absorption bands of carbon and cyanide molecules in the N-type spectrum, so R and N type stars are sometimes called carbon stars.The S-type spectrum is similar to the M-type spectrum, but has a strong zirconia absorption band and hydrogen emission lines.Within the 450 second gap centered on the sun, type B stars account for 1%, type A stars account for 1.5%, type G stars account for 13%, type K stars account for 20%, and type M stars account for 56%.It can be seen that there are many low temperature stars than high temperature stars.
In the 1940s, American astronomers Morgan and Kennan proposed another classification (MK system classification): add a photometric data to the Harvard classification based on temperature.The luminosity has seven levels, which are represented by Roman numerals and attached to the Harvard classification of stars: I supergiant, IIBright superstar, III normal superstar, IVSubgiant, V dwarf, VISub dwarf, Ⅶ white dwarf.If necessary, add lowercase English letters after the Roman numerals to arrange the photometric order.The bright ones are giant stars and the small ones are dwarfs.sunlightThe spectral type of is G2V.The spectral type of Rigel is B8Ia, which indicates that it is a blue and white supergiant.[18]
Hero diagram is a diagram showing the relationship between the spectral type and luminosity of starsHerzpoonandRussellCreated independently.[25]The ordinate of the Herot chart is the luminosity of the star, and the abscissa is the surface temperature or spectral type of the star.It can be seen from the Herot diagram that most stars form a sequence stretching from the upper left corner to the lower right corner. This sequence is called the main sequence, and the stars in it are called the main sequence stars.The main sequence stars include 90% of the stars that we can observe.The main star sequence shows that most stars obey the law that the higher the temperature is, the stronger the luminosity isStefan Boltzmann lawThe inevitable result of the display.The other dense group is short, low in left and high in right, distributed at the upper right of the figure. This sequence is called the giant sequence.They are generally red giants and red supergiants, such as Arcturus and Betelgeuse.In addition, there are some white dwarfs scattered at the lower left of the main star sequence.Sirius's companion star is a white dwarf.The raw material of stars - interstellar clouds - is at the far right of the Herot chart.Neutron stars and black holes cannot be shown on the Herot chart because there is no optical observation data.[26]
In 2023,Astronomical research indicates that planets located in the habitable zone of metal poor stars may be the best place to search for potential life.[42]
Stable star
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The so-called stability means that the star is in hydrostatic equilibrium and thermodynamic equilibrium.In this state, the gravity and pressure on each part of the star's interior are in balance, and the energy lost by surface radiation is in balance with the energy transferred from the interior to the surface.Therefore, the spectrum, temperature, luminosity, volume and mass of the star remain relatively unchanged.
Main sequence star
The main sequence star is a star in a stable stage in its life.Stars stay at this stage for more than 90% of their life span, which is equivalent to the youth stage of human beings.The chemical composition of the main sequence stars is basically the same, and the energy mechanism is basically similar.In the early stage of star evolution, the energy mechanism of stars is not yet mature, so they are on the right side of the main star sequence.When a star evolves to a later stage, its internal chemical composition and energy mechanism have changed greatly, and the star thus diverges from the main star sequence.The luminosity of the main sequence star is approximately proportional to the 3.5-4 power of the mass, which is calledMass light relation。
Mass is the most important physical quantity of stars, which often determines the future of stars.The residence time of stars in the main sequence depends on their mass: the residence time of stars with large mass is short, and the residence time of stars with small mass is long.The sun stayed in the main sequence for about 10 billion years, and now it has been 5 billion years.A star 0.5 times the mass of the sun will stay for 200 billion years.
The mass of stars has a certain range, the maximum is not more than 150 times the solar mass, and the minimum is not less than 0.08 times the solar mass.The greater the mass, the more unstable the star is, and the strong radiation pressure will blow away the outer atmosphere of the star;The mass is too small to cause thermonuclear reactions in stars.The hydrogen in the star as the energy source is the standard to maintain the main star sequence. Once the hydrogen is exhausted, the star will leave the main star sequence and enter the old age.[18]
A binary star is two stars that orbit each other around the common center of mass.It looks like a star, but in fact it may be a double star.In 1802, William Herschel discovered and realized the first pair of binary stars:Beihe II。Since then, binary stars have become one of the important topics in astronomy.The mass of binary stars can be determined by Kepler's third law.The orbit can be calculated by the apparent ellipse or the apparent motion curve, and then the mass can be obtained by measuring its motion period.This kind of variable star is calledEclipsing variable star。
Light variation curve of Dalingwu (energy range 0.3-10keV).[27]
When the orbital plane and visual direction of a binary star are almost in the same plane, one star will block the eclipse of another star, and the starlight will be significantly dimmed.Such binary stars are called eclipsing binary stars.The most typical and earliest discovered eclipsing binary isFive Tombs(Perseusβ）。Its orbital period is 2 days, 20 hours, 48 minutes and 55 seconds.During this period, its brightness has obvious changes and has special rules.It's brightest at 2.13, darkest at 3.4, and part of the brightest time is slightly darker.The reason is that the two stars have different luminosity.When the star with low brightness blocks the star with high brightness, the total brightness is the smallest;The larger the brightness, the smaller the total brightness.[18]Typical eclipsing binaries andGradual Stage IIEtc.
Some distant binaries cannot be recognized by telescopes even if they move to the farthest position from each other. Such binaries cannot be recognized in optical observation.But it can be seen from the spectrum that these are the spectra of two stars.Such binary stars are called spectroscopic binary stars.They may show periodic red and blue shifts in the spectral lines, or they may be the spectral superposition of two stars.It is called bispectral light splitting binary star when the spectrum of two stars is superposed. It is called monospectral light splitting binary star when only one star's spectrum is seen, but it shows periodic blue shift and red shift.There is a kind of spectroscopic binary called symbiotic star, and its children are hot and cold.For example, Andromeda Z is composed of a red giant star and a main sequence star or white dwarf star.[28]
The schematic diagram of close binary stars shows the material exchange between the two stars.
The two sub stars of a close binary have frequent material exchanges.A typical star is W. Ursa Major.Its light variation curve is like a "W" type. Its two sub stars have a common convection envelope, which encloses two sub stars, both of which are main sequence stars or sub dwarf stars. This structure leads to its intense stellar activities, such as sunspot clusters, flare bursts, etc. There are also complex magnetic field interactions and angular momentum transfers between them.[28]
When three or more stars gather together, they are called aggregators.The nearest Centaurus neighbor to the sun is a member of a trio.“Orion Quadrilateral”It's a group of quads.The more stars there are, the rarer they are.
Unstable star
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An unstable star is a star whose physical parameters change.The form of change may be periodic pulse, or irregular burst or explosion.Among various parameters, the change of brightness is the easiest to observe.Stars with brightness changes (generally referring to changes in electromagnetic radiation, not limited to visible light) are called variable stars.Although the brightness of the eclipsing binary also changes, it is not caused by the physical reason of the star itself, so it is not considered as an unstable star.
Pulsating variable star
Pulsating variable starIt is named because its brightness changes like a person's pulse.The reason for the brightness change of pulsating variable stars is the rhythmic expansion and contraction of stars.This is a phenomenon of structural instability after the main sequence star breaks away from the main sequence star.This kind of expansion and contraction only occurs in the outer layer of the star, and deep matter does not participate in expansion and contraction.Generally from the center
The closer to the surface, the greater the amplitude.This expansion and contraction is similar to a gas piston.The gravity and pressure on each part of the star's interior are unbalanced, just as the balanced piston expands and contracts after being suddenly compressed (without friction).[18]chaosAfter the development of the theory, the irregular chaotic pulsation of pulsating variable stars can also be well explained.[29]
The volume expansion and contraction of pulsating variable stars are not permanent.Previous studies believed that due to the existence of friction, the pulsation degree of stellar matter participating in the pulsation became smaller and smaller, and eventually tended to be stable.Theoretically, most pulsating variable stars should stop after 5000-10000 pulsations without additional energy supplement.However, in fact, no pulsation attenuation phenomenon has been observed in all types of pulsating variable stars, especially Cepheid variable stars, whose light variation period and amplitude are very stable.In the 1950s, the Soviet scientist Zhwagin put forward an explanation. He believed that there was an ionization zone composed of hydrogen and helium ions under the atmosphere of pulsating variable stars. The materials in the zone stored energy when the stars contracted and released energy when they expanded.This mode of energy supply is called valve effect.The conditions for the valve effect are very strict, so pulsation is only a stage behavior in the process of star evolution.
Long-period variable
The light variation range of Guigao type variable stars is large, the magnitude of variation may reach 6 degrees, and the light variation period varies from 80 days to 1000 days.They are generally late giant stars with amorphous spectral type.Typically蒭 Gaozeng'er(Cetaceano), the brightness varies from 1.7 to 10, with an average period of 332 days.
All pulsating variable stars with light variable period of 1-135 days and very stable period are classified as long-period Cepheid variable stars.They are yellow giant stars or supergiant stars, and their mass is several to dozens of times that of the sun.Such variable stars are commonly found inGalaxyIn the spiral arms of, it is generally believed that young giant stars pass through the instability zone to form.TypicallyCepheid I(Cepheusδ）。The light cycle of Cepheid I is 5 days, 8 hours, 46 minutes, 38 seconds, the brightest is 3.6, the darkest is 4.3, etc.Cepheid variables can also be found in extragalactic galaxies.It is worth mentioning that the person who first measured the light variation period of Cepheid variable stars and Daling variable stars isGoodrich。He was a deaf mute amateur astronomer who initiated the photometric measurement of variable stars and made outstanding contributions in the field of variable stars. He died at the age of 22.
At the beginning of the 20th century, H.S. Leavitt, an American female astronomer, discovered the cycle light relationship of Cepheid variable stars: the longer the light change period, the greater the luminosity.In this way, as long as the circumference light relationship of Cepheid variable is known, its distance can be known by using the distance modulus formula.This method is particularly suitable for measuring the distance of extragalactic galaxies.Cepheid variable stars are therefore known as the ruler.
The light variation period of such variable stars is less than 1 day, which is common in silver nuclei, halos and globular clusters in the Milky Way.It is generally considered as the product of the unstable core productivity of low-quality stars at the end of evolution, but it still has hundreds to thousands of times the solar luminosity.Their light curves are different from those of long-period Cepheid variables.[30]
The number of such stars accounts for about a quarter of the total number of pulsating variable stars.Its light variation period is 0.05-1.2 days, and its amplitude is not more than 1-2, etc.The characteristic of this type of variable star is that its luminosity is quite stable, but after a long time, the period and light curve will change.[31]
The variation amplitude of the variable star light does not exceed 0.25, and the period is less than 1 day. Each period of the light variation curve is different.Such stars are generally A0-F5 main sequence stars or giant stars.Typical is shield seatδ。This type of star is usually classified as a variant of RR Lyra.[31]
Special pulsating variable
The pulsation of special pulsating variable stars is not the pulsation of star volume expansion (radial pulsation), butAstroseismologyMeaningfulNon radial pulsation。Since the 1960s, astronomers have found various oscillation phenomena of the sun when observing the sun. For example, any point on the sun's surface will vibrate up and down for about 5 minutes with an average speed of 0.5-1 km/s.Later research showed that this vibration (the sun's 5 minute vibration) was 10sevenThe superposition of the above non radial pulsation modes.This non radial pulsation affects the magnetic field, luminosity and spectrum of stars.After the development of astroseismology, humans have observed some non radial pulsation information of stars, which is very helpful for the study of stellar structure.
The brightness of this variable star remains unchanged, but its magnetic field and spectrum have obvious periodic changes.Typical is Beagleαtwo。Its magnetic field intensity changed from+0.162 Tesla to -0.142 Tesla in a 5.469 day period.Another typical star is Deer Leopard 53, which has a magnetic field ranging from - 0.4 Tesla to+0.4 Tesla in a 7.8 day period.This type is also called a magnetic variable star.The reason for their magnetic field and spectrum changes is that the rotation axis has a large tilt, and different elements gather in different areas of the surface. With the rotation of the star, the observer faces different star surfaces, and the magnetic field and spectrum will change.Such stars also contain very strong metal absorption lines.
Wolf Rayet star (WR star)
WR star WR31a is surrounded by stellar material blown out by stellar wind.[32]
This kind of star was discovered by French astronomers Wolfe and Rayet in 1867.Their spectra are almost emission lines (the spectra of ordinary stars are absorption lines), which is easy to identify. More than 200 stars have been found in the Milky Way and nearby galaxies.Their absolute magnitude is estimated to be - 4, and the rate of stellar wind mass loss is very fast (about 10-5Solar mass/year), so the life span is quite short.Typical is WR124, located in the constellation Arrowhead.
B-type emission star (Be star)
This kind of star was discovered by Italian astronomer Saiqi in 1866.Its spectrum has both emission and absorption lines, and its intensity changes with time.The rotation speed of this star is so high that the surface centrifugal force is greater than the gravity, so the mass loss rate is also large (about 10-7Solar mass/year).Their absolute magnitude is between - 3-0.The typical Be star is Water Commission I (Eridanusα）。Be star is considered as a fast rotating neutron star or a close binary star.
SS433 star
Schematic diagram of the hypothetical structure of SS433.
SS433 is an intense radio source located in the constellation Aquila.Its spectrum has both red shift and blue shift, and there are spectral lines that do not shift, with a period of about 164 days.Observations show that it is a group of eclipsing binary stars, the host star may be a black hole or neutron star, and it also has an X-ray jet.The companion star is a blue giant, whose material is sucked away by a neutron star or black hole, piled up into a disk and rotated at high speed, resulting in a jet.The blue shift and red shift in the spectrum result from the motion of these two jets relative to the Earth.
In a very short period of time, the brightness of the star suddenly increases, and the star whose increase rate exceeds 0.3 equal/minute is called a flare (UV type star in Cetus).The typical star is UV Cetus.In 1948, W.J. Layten found that the brightness of this star increased 12 times in three minutes, even faster than the supernova explosion. This phenomenon is called brightness.Later, about 200 similar stars were found, all located within the 20 second gap near the sun, including CentaurusAdjacent star。They are all red dwarfs, most of them are M type.
Although the solar flare activity is similar to the brightness, their energies are very different.The energy released by a flare does not exceed 1% of the normal solar radiation, and the energy released by the flare in a few minutes is more than ten times of the normal radiation energy.There is no special rule for the shining time. It usually happens once every few hours, and then returns to normal after several decades of minutes.The brightness is also accompanied by the enhancement of radio radiation and X-ray radiation.X-rays shine for the shortest time and have the longest radio frequency.Radio is the first to shine, followed by optics, and X-ray is the latest.The brightness frequency is related to the brightness. The smaller the brightness, the more frequent the brightness.It is generally believed that the brightness is caused by the large-scale flare activity of young stars.The sun has already entered its middle age, and its flare activity is much calmer than before, so it will not shine.
The energy of stars is produced by nuclear fusion.The issue of stellar energy has always been the focus of human debate.In 1926, British astronomerEddington Put forward the problem of star energy.He firmly believes that the energy generated by star fusion is enough to make the star reach the balance of gravity and gas pressure.However, physicists at that time did not think so.They felt that the fusion reaction inside the star could not be carried out.Fortunately, the development of quantum mechanics（Tunnel effect) solved this problem.
In 1938, American physicistHans BateAnd German physicist Von Weitzzek independently discovered the specific way of nuclear fusion in stars, that is, through "proton proton reaction" and“Carbon nitrogen oxygen cycle”The hydrogen in the star can be fused into helium and release energy.
Proton proton reaction consists of the following three reactions:
Proton proton reaction diagram.
oneH+oneH——>twoD+e++ν （1）
twoD+oneH——>threeHe+γ （2）
threeHe+threeHe——>fourHe+2oneH （3）
(1) Two hydrogen nuclei collide and fuse into a deuteron (hydrogen 2), releasing a positron and a neutrino.The new deuteron formed will react with the adjacent hydrogen nucleus (2), fuse into a helium 3, and release aγRay photons.The two helium 3 nuclei conduct (3) reaction, combine into a real helium nucleus (helium 4), and release two hydrogen nuclei.It is easy to see that a simplified proton proton reaction can be obtained by adding two times of reaction (1) and reaction (2) and reaction (3):
fouroneH——fourHe+2ν+2e++2γ，The positron is very easy to connect with the nearby electronAnnihilationBecomesγPhotons.
The carbon nitrogen oxygen cycle consists of the following six reactions:
Diagram of carbon nitrogen oxygen cycle.
twelveC+oneH——>thirteenN+γ （1）
thirteenN——>thirteenC+e++ν（2）
thirteenC+oneH——>fourteenN+γ （3）
fourteenN+oneH——>fifteenO+γ （4）
fifteenO——>fifteenN+e++ν（5）
fifteenN+oneH——>twelveC+fourHe （6）[33-34]
The carbon and nitrogen nuclei in this group of reactions are catalysts for thermonuclear reactions.The total result is that four hydrogen nuclei are combined into one helium nucleus, and three photons, two positrons and two neutrinos are generated.The number of neutrinos emitted from a star's interior is two-thirds of the number of photons.Neutrinos do not interact with any matter and move directly at the speed of light.
Photos of the Pillars of Creation.The rich interstellar material makes this area become the incubator of stars.
Stars originate from interstellar matter.Interstellar matter fills the space with an average density of about 10-24G/cm3.The temperature of interstellar space is about 10-100 Kelvin.Nebulae form where interstellar matter is relatively concentrated.The systematic theory of star formation isKant andLaplace。They put forward the nebula theory of star formation.The first person to use scientific methods to study the origin of stars was the British astronomer James Kings, who proposedPotential force theoremIt is believed that after a certain amount of interstellar matter is subject to gravitational disturbance (such as supernova explosion), as long as certain conditions are met（Kings quality）The gravitational force between interstellar matter will play a leading role, causing them to collapse into more dense nebulae.When the density reaches a certain level, the nebula will split into several clusters.The density of agglomerates is higher.Since then, each lump continues to collapse and split, until several smaller lumps have a mass of 0.08-150 solar mass, they will no longer split, but will gather more and more.Finally, the pressure of gravity ignited the thermonuclear reaction in the lump.Thermonuclear reactions increase the pressure inside the star, thus preventing collapse.When the thermonuclear reaction can maintain the dynamic thermal balance of the star itself, the star will be completely born.This is the standard model of star birth.However, the actual star formation models must be classified and discussed according to mass.Generally speaking, low mass stars are less than 2-3 times the mass of the sun, high mass stars are more than 8 times the mass of the sun, and medium mass stars are in the middle.
Molecular Cloud: Incubator of Stars
In various gaseous interstellar media, the molecular cloud with the highest density and the lowest temperature is the key place for star formation.Molecular clouds in the Milky Way vary in size and shape.Small molecular clouds have only a few times the mass of the sun, which is the area where isolated stars form, while large molecular clouds can reach millions of times the mass of the sun.Where the density of molecular cloud is higher, it is clumps, and where the density is higher, it is called molecular nucleus.The molecular cloud itself is the result of the condensation of interstellar media.According to modern astronomy, molecular clouds are condensed by neutral hydrogen clouds under the influence of gravitational waves or shock waves generated by supernova explosions.In addition to being full of gas, molecular clouds also contain a lot of interstellar dust.These dust can absorb high-energy photons in the environment to protect the molecular cloud from being attacked and broken.Moreover, interstellar dust is rich in chemical elements, which is also conducive to the formation of stars.[35]
Small and medium mass star formation
The standard model is in good agreement with the formation process of small and medium-sized stars.Soon after the formation of the molecular cloud, the clusters in the cloud began to split and collapse.A molecular cloud can split into many small blocks, and each small block can form a star.Refer to the collapse of one of the small lumps: the collapse of the lumps starts from the inside out, the matter falls to the center of the lumps at high speed, and its potential energy is converted into kinetic energy.After reaching the center, the material decelerates, and the kinetic energy is converted into internal energy, forming radiation.Because the molecular cloud is relatively thick, the radiation is not easy to emit and is absorbed by other substances instead.With the falling of a large amount of material, the central temperature and radiation pressure increase continuously, and finally reach the equilibrium state.At this time, the gravitational and radiation pressure on the material that has not fallen is balanced, and they no longer fall. A stellar embryo（Protostar）It was born.Compared with the real star, this stellar embryo has smaller mass and density, and larger volume, so it cannot be counted as a star.
The slender gas beam of the Herbig Harrow 110 celestial body.
There is an accretion disk around the protostar, and the protostar enriches itself by disk accretion.This is to transfer the accretion materialangular momentum。Since the molecular cloud rotates, according to the conservation of angular momentum, the farther away the matter is from the center of rotation, the greater the angular momentum.If direct accretion occurs, the excessive angular momentum may tear the original star. The accretion disk can transfer these angular momentum, and the star itself can also throw out the material with excessive angular momentum with the help of the magnetic field, forming a jet.When the jet interacts with interstellar matterHerbig Harrow object。This object was discovered by American astronomer George Herbig and Mexican astronomer Gilmo Harrow.
As accretion proceeds, protostars become more and more massive, their brightness begins to increase, and their temperature rises.Meanwhile, protostellarStellar windVery strong, it can blow away the surrounding materials, thus slowing down the quality growth.At the same time, protostars are also shrinking, heating the internal material.When the matter around the protostar is almost blown away and there is nothing in the accretion disk, the protostar evolves intoPrecursor of main sequence, it can be observed in the optical band.Japanese astronomerChushiro Hayashi It is found that the leading stars of the main sequence are concentrated in a narrow vertical region on the right side of the Herot——Linzhong Silang Line——Inside.Here, the whole star is in a convection state, the internal temperature rises to 1 million Kelvin, and the nuclear reaction (mainly deuterium as fuel) begins in the center.The content of deuterium in the center is small, and convection will bring the deuterium on the surface to the core.At this time, the star stops shrinking, and the radiation pressure and gravity reach balance.However, the deuterium is only enough for about 100000 years. After the deuterium is used up, the radiation pressure decreases, the star before the main sequence shrinks, and the core temperature rises, so the star is out of the evolution process of Lin Zhongsilang.When the core temperature rises to 7 million Kelvin, hydrogen begins to be used as fuel.At this time, the brightness of the star before the main sequence remains unchanged, and the surface temperature rises, entering the Henye evolution process.This was proposed by American astronomer Henye.The nuclear reaction of hydrogen tends to be stable after the main sequence star has walked a certain distance along the Henye evolution, and the star stops shrinking and enters the equilibrium state again.At this time, a star was born, and it officially entered the main sequence.
For massive stars, the Heyer evolution process is relatively long, the evolution process of Lin Zhongsilang is relatively short, and for low mass stars, the opposite is true.[35]
Time taken for stars with different qualities to reach the main sequence
Mass of leading star in main sequence: solar mass
Time to reach main sequence/year
thirty
30 thousand
ten
300 thousand
four
1 million
two
8 million
one
30 million
zero point five
100 million
Massive star formation
The standard model can beautifully explain the formation of small and medium mass stars, but can not do anything about the formation of large mass stars.[36]First, the massive stars are very far away from the earth, and the nearest massive star forming region——Orion nebula (M42) - It is 1500 light years away from the earth.Secondly, the environment of the massive star forming region is relatively complex, which is full of turbulent gas flow, interstellar dust with complex composition elements, and violent stellar wind.Although the Orion Nebula is very beautiful, it is difficult to observe stars due to its complicated environment.In addition, the life span of massive stars is very short, basically less than 100 million years, which leads to few samples to be studied.Finally, this kind of star formation is very fast. It takes only a few hundred thousand years from birth to the main sequence, which is a flash in the pan.
Modern astronomers support the collapse accretion model.Of course, there are other models, for example, and they think that the massive stars are merged from the small ones.However, the collapse accretion model of massive stars less than 40 times the mass of the sun is consistent.
The nebula around the Pleiades cluster is the ionized hydrogen region created by young, massive stars.
Massive star formation occurs fromInfrared dark cloudThe stage begins.The infrared dark cloud is a molecular cloud with extremely low temperature (about 10 degrees Kelvin). Its morphology shows a fibrous structure. The fiber contains a cold molecular nucleus with high mass and density (about one million molecules per cubic centimeter), which is the predecessor of a massive star.After that, the nucleus began to collapse and heat, and the density could reach 10 million molecules per cubic centimeter.At this time, a medium mass celestial body has been formed in the nuclear center and thermonuclear reaction has started, and violent accretion has taken place.The accretion friction generates radiation, heats the surrounding gas and dust, causing the temperature of the molecular cloud to rise (up to 150 degrees Kelvin), and there is also a faint jet, but the molecular cloud is too thick to be seen in the optical band.Accretion continues. When the ultra dense ionized hydrogen phase begins, due to the high core temperature, the medium mass objects in the core have burned hydrogen on a large scale, and the surrounding accretion materials have not been completely absorbed, the intense stellar wind can't wait to break them up.The surrounding of the star is also full of hydrogen ions ionized due to high temperature. This area is calledIonized hydrogen region。Because of gravity, ionized hydrogen cannot be too far away from the star, and accretion can also continue for some time.Since then, stars have become larger and larger, and it is possible that accretion disks can be swallowed by stars.The stellar wind is getting stronger and stronger, blowing away a large amount of material, and the density of ionized hydrogen region is getting higher and higher (it may reach 10000 electrons per cubic centimeter), and the range is increasing.But as the matter around the star is blown away, the size of the star will no longer expand, and the density of the ionized hydrogen region decreases with the expansion of the range, thus forming the ordinary ionized hydrogen region.The strong stellar wind and radiation of massive stars can change the structure of the surrounding space, and the shock wave caused by expansion can compress the interstellar medium.The interaction of these factors can eventually lead to the re collapse of the interstellar medium, triggering the formation of more stars.Due to the short life span of a massive star, it may become a supernova when a new star is immature. However, the huge energy of a supernova can lead to star formation, as well as the formation of heavy elements.Therefore, massive stars are essential key objects in the universe.[37]
Main order evolution
The evolution of stars in the main sequence star is characterized by nuclear fusion with hydrogen as the fuel inside the star.The main sequence star is a stable star, and the difference in spectral type and luminosity of different main sequence stars is completely determined by their quality, so the main sequence star is simple.
Because the energy of the main sequence star is generated only by nuclear fusion of hydrogen, as long as the total mass and the contents of hydrogen, helium and heavy elements are known, various physical parameters of the main sequence star can be calculated, and even its evolution can be predicted.Eddington said, "There is nothing simpler than a star."That's what we mean. If we give some assumptions about the main sequence star:
1. The geometric structure of the star is spherically symmetric;
2. The interior of a star is composed of multiple layers, and the temperature, pressure, density, opacity and composition of each layer are uniform.
3. The interior of the star is stable and meets the hydrostatic equilibrium conditions everywhere;
4. The magnetic force, tidal force and rotation of stars are not considered.
From these assumptions, we can deduce five equations of the main sequence star: (The derivation process is omitted)
1. Mass equation
Consider a certain shell of a star with radius r, thickness dr and densityρ(r) , the mass of the sphere from the sphere to this layer is:
2. Balance equation
Let the pressure at radius r be p (r), the mass of the sphere be M (r), and the universal gravitation constant be G, then the following equation can be obtained:
3. Photometric equation
Set L as luminosity,εIs the productivity power per unit mass inside the star, then the following equation is obtained:
4. Radiation transfer equation
Let T (r) be the temperature inside the spherical shell, and K (r) be the absorption coefficient or opacity coefficient,σIs the Stepan constant, then there is the following equation:
5. Equation of state
Let the interior of the star be an ideal gas,μ(r) Is the average molecular weight, mH is the mass of hydrogen atom, and k isBoltzmann constant, c is the speed of light, then there is the following equation:
If a series of assumptions are removed, the formula and calculation will be quite complex.[38]Of course, the physical parameters of stars can also be easily calculated through high-performance computers. After comparison, the results are in good agreement with the reality.For stars in the main sequence stage, their mass is only slightly lost, so the changes of physical parameters are quite slow.When the mass changes, the star will enter another stage.
On the main sequence belt, from the upper left corner to the lower right corner, the mass of stars gradually decreases, which is consistent with the observation
The chemical composition of the main sequence stars is basically the same: hydrogen accounts for 70% - 75%, helium accounts for 24% - 25%, and the rest are heavy elements (mainly carbon, oxygen, etc,Metal rich starThere are more heavy elements).Most helium isBig BangCreated and preserved in the early stage.According to statistics, in the Milky Way, the number of main sequence stars accounts for about 90% of the number of stars.The higher the mass of the star, the higher the internal temperature, the more intense the thermonuclear reaction, and the lower its life.Low mass stars have a long life span.A star 30 times the mass of the sun can only stay in the main sequence for 1 million years, while a star 0.5 times the mass of the sun can stay for 100 billion years.The mass of the main sequence star is also limited.The upper limit is still uncertain, but the lower limit - 0.08 times the solar mass - is certain.The "stars" with mass less than the lower limit are not qualified to become stars because their internal temperature is too low to allow thermonuclear reaction. They are classified asBrown dwarf。Due to the complex structure of massive stars, it is still unclear how large they can be.
After main sequence
When the thermonuclear fuel hydrogen in the star is gradually converted into helium, hydrogen fusion cannot be maintained, and the structure of the star will change significantly.At this time, a star has gone through a long main sequence and come to its old age.
More thermonuclear reactions
Temperature reaches 10eightWhen the above is turned on, the helium nucleus will be used as the fuel, and eventually the oxygen element will be generated.This step is called helium combustion.Temperature to 8 * 10eightWhen it is turned on, carbon and oxygen are burned to form magnesium, silicon, phosphorus and sulfur.Temperature to 3.5 * 10nineWhen on, magnesium nucleus and silicon nucleus undergo photofission reaction to generate aluminum, neon and oxygen elements, and simultaneously emit protons, neutrons and helium nuclei（αParticles).The helium nucleus reacts with silicon and other elements to form sulfur, argon, calcium, titanium, chromium, iron and nickel（αprocess）。Iron and silicon are the slag of silicon combustion, and the slag of fusion reaction in all stars.On the whole, the energy generated by thermonuclear reaction is less and less.IronSpecific binding energyMaximum, which means that no matter to the heavier element or to the lighter element, we should absorb energy rather than release energy.Therefore, the formation of iron marks that stars (generally massive stars) are dying and will end their lives in extremely heroic explosions.The specific process of each reaction is as follows:
1. Carbon combustion
——————{twentyNe+fourHe
twelveC+twelveC—>{twenty-threeNa+H (indicating that three reactions occur simultaneously, the same below)
When a star whose mass is less than 2.3 times the mass of the sun evolves to the late main sequence, the hydrogen in the core has been burned out.When the temperature has not reached the point where helium reacts, it does not have enough radiation pressure to maintain its equilibrium state, so the core inevitably shrinks.This contraction converts the gravitational potential energy into internal energy to increase the temperature, so that the core temperature can lead to helium fusion, even if the reaction is too violentHelium flashoverPhenomenon.At the same time, the excess heat energy radiated by the core makes the outer material pushed away, causing the core to shrink and the outer layer to expand.After the core begins to fuse, hydrogen fusion will restart under radiation heating because there is still unused hydrogen around the core.At this time, the core of the star is helium fusion, and the periphery of the core is hydrogen fusion, so the star can glow again.The energy generated by the core makes the outer layer of the star expand continuously, and the surface temperature decreases again and again until it is below 4000 Kelvin.At this time, the star emits red light, which is huge and calledRed Giant。Beihe III(Geminiβ）、A desperate battle(Taurusα）、arcturus(Herdsmanα）All the others are red superstars.
After becoming a red giant, the diameter of the sun can reach 2 astronomical units.
When the helium in the red giant is exhausted, carbon and oxygen are left. At this time, the star will shrink rapidly under the action of gravity as before.However, the mass of the star is not large enough, and gravity cannot make the core temperature rise to cause carbon combustion and oxygen combustion.This contraction only allows the helium around the carbon oxygen core to continue to burn.This state will not last long.After the star completely loses its vitality, the instability of the whole star increases, and the gravity causes the core density to become higher and higher.When the density reaches 6 * 10sevenAt g/cm3, due to Pauli incompatibility principle, electrons generate degenerate pressure, making the matter no longer compressed, while the temperature is maintained at 50000 Kelvin, and the volume is very small. At this time, a white dwarf star is formed in the core.At the same time, the stellar shell broke away from the constraints and expanded to a large area, formingPlanetary nebula。The planetary nebula evolves very quickly. It expands at the speed of 10-30 km/s and becomes thinner and thinner. After about 50000 years, the planetary nebula will be blown away.
The sun is a low mass star.In another 5 billion years, the sun will become a red giant star.At this time, the sun will expand, and its radius can be large enough to include the orbit of the Earth or even Mars. At that time, the Earth may have been swallowed up.Soon the core shrank again and became a white dwarf, while the outer shell became a planetary nebula.
Late middle mass stars
If the mass of a star is 2.3 to 8.5 times the mass of the sun, the core will shrink slightly after the hydrogen combustion of the core is completed, but because of its large mass, the core can smoothly enter the helium combustion phase.If the mass of the star is close to 3 times the mass of the sun or less, its surface temperature is 5000 Kelvin, which is a red giant star;If the mass exceeds 7 times the mass of the sun, the surface temperature will reach 10000 Kelvin, which is a giant star but not red.After burning the helium in the center, the core shrinks again.Due to the large mass, the temperature and density generated by gravitational potential energy are enough to trigger carbon combustion, and the core temperature rises again, but carbon combustion proceeds at a very fast rate, so that the core cannot expand in time to slow down the reaction speed, and carbon combustion is completed.This very rapid combustion of carbon is calledCarbon flashIt may lead to the unstable state of the star, or even lead to the explosion and destruction of the star.If you are lucky enough, the star will eventually become a white dwarf.
Late stage of massive stars
Diagram of onion structure of massive stars.In fact, the hydrogen layer can account for more than 80% of the diameter.
The total mass of a massive star is so huge that carbon combustion can also proceed smoothly without carbon flash.As the core carbon burns, the hydrogen and helium in the shell also burn.After the carbon in the core is burned, the temperature has reached more than 1 billion Kelvin, and the oxygen combustion begins.If the temperature rises to 2 billion Kelvin, silicon can start to burn.In extreme cases, the star has become a giant "onion": the core is composed of slag - plasma iron, and the outside is composed of various shells, each of which is undergoing different types of nuclear reactions.At this time, the star has expanded to becomeRed supergiant。The combustion time of heavier elements is shorter than that of lighter elements.For a star 25 times the mass of the sun, hydrogen can burn for 7 million years, helium can burn for 500000 years, carbon can burn for 600 years, oxygen can burn for one month, and silicon can only burn for one day.The unstable combustion after leaving the main sequence may cause the star to enter the pulsating variable state for many times.Betelgeuse IV (Orionα）、Antares 2 (Scorpioα）They are all red superstars.A massive star will end its life in a supernova explosion.
Binary evolution
Although the evolution mechanism of binary stars is the same as that of single stars, the evolution of some binary stars is far more complex than that of single stars (more complex for close binary stars) due to the frequent material exchange between two stars.There are many kinds of binary stars. It is not easy to summarize the general rules for the combination of main sequence stars, white dwarfs, pulsating variable stars, neutron stars, black holes, etc. with different masses.Because of stellar wind, matter exchange, accretion and even the explosion of one of the stars, it is more difficult to study them.There are still many imperfections in the theory and observation of binary evolution, which need further research and observation.
The End of the Star
Announce
edit
Degeneracy pressure
Pauli in his student days.[39]
An atom consists of a nucleus and electrons.Physicist, 1925PauliIt is believed that the electrons in atoms are not allowed to have the same state of motion.If the electrons in one space of the atom occupy all possible states, thenPauli exclusion principleNo more electrons are allowed in this region.This force that does not allow many residual electrons to enter a certain space is the electron degeneracy pressure.For ordinary matter, the distance between atoms is very large, and generally there is no case of electrons squeezing electrons from other atoms.And the electron degeneracy pressure is very small.However, the electron degeneracy pressure has its upper limit.If the matter is further compressed, then the electrons will be forced to press into the nucleus and combine with the protons to become neutrons, and the electron degeneracy pressure will no longer exist.At this time, the atom barrier no longer exists, and the atom has been "dismembered".If the pressure is increased further, neutrons and neutrons will be pressed together closely, creating pressure between them, which is called neutron degeneracy pressure.When the pressure further increases, even the neutron degeneracy pressure cannot resist, the matter will be beyond the current cognitive range, where all known physical laws will fail.
The stars mainly rely on the radiation pressure generated by nuclear fusion to resist their own gravity to maintain their own shape.Once nuclear fusion stops, stars will inevitably collapse.But there are two important barriers - electron degeneracy pressure and neutron degeneracy pressure - to prevent this collapse.A white dwarf is a celestial body whose electron degeneracy pressure is in balance with its own gravity.Although stars are full of plasma materials, and electrons are no longer bound around atoms, the Pauli exclusion principle still works. And because stars contain a lot of materials, the electron degeneracy pressure can be very large, enough to counter gravity.
White dwarfs are usually composed of carbon and oxygen.There is no nuclear fusion reaction inside it and no energy is generated.White dwarfs form at a very high temperature, which can reach 200000 Kelvin.But because there is no energy source, it will eventually radiate most of the energy, and eventually fade into aBlack dwarf。Modern astronomy believes that black dwarfs may be a kind of dark matter.
American astrophysicistSubrahmanyan Chandrasekhar It was discovered in 1931 that white dwarfs have a maximum mass of 1.44 times the mass of the sun, which is calledChandraseka limit。If this limit is exceeded, the electron degeneracy pressure will lose its grip on gravity, atoms will be crushed, and electrons and protons will combine to become neutrons.Until neutrons are pressed together, the resulting neutron degeneracy pressure counteracts gravity, which forms a neutron star.
The diameter of the neutron star is only 10 kilometers, but the material per cubic centimeter can weigh up to 1 billion tons, so the rotation speed is extremely fast, and the period may reach one seventh of a second, forming a pulsar.Some neutron stars have strong magnetic fields, forming magnetostars.
On April 10, 2019, humans took the first picture of a black hole, located in M87 nebula[40]
American physicistOppenheimerAnd the Canadian physicist Volkov put forward the upper limit of the mass of neutron stars in 1939, known asOppenheimer limit 。If the mass of the neutron star exceeds this upper limit, no force can stop the action of gravity, causing the star to become a black hole.The Oppenheimer limit is uncertain, generally 2-3 times the mass of the sun.
Black hole is one of the most magical celestial bodies in the universe.From the 18th centuryLaplace, to the twentieth centuryschwarzschild 、Einstein, countless physicists have described the shape of black holes.The black hole itself is a point with zero volume and infinite density——singularity, which makes its gravity so strong that it existsHorizon, nothing within the event horizon radius can be observed in any way.Because of the theory of relativity, time around a black hole passes very slowly.Around the black holeJet，Accretion diskAnd so on.
Encyclopedia x ignorance: illustrating black holes
Nova
Nova occurs in a binary system consisting of white dwarfs and ordinary stars.If the white dwarf is in theRoche limitIt will continue to extract hydrogen, helium and other gases from its companion star. These gases will accumulate on the surface of the white dwarf star with high density and temperature.When the temperature reaches 20 million Kelvin, hydrogen fusion reaction will occur.This process will release a lot of energy, making the white dwarf star produce an extremely bright explosion, and blow away the remaining gas on the surface, forming a gas shell.When the light reaches its peak, it drops.According to the length of time it takes for luminosity to decline, novae can be divided into fast novae and slow novae.The nova can be very bright, for example, the Cygnus nova, which appeared in 1975, appeared in Cygnus on August 29, 1975Tianjin IVAt about 5 degrees in the north, the star magnitude reaches 2.0, which is equivalent to Tianjin IV.
The types of supernovae are mainly classified by spectral type.Type I supernovae have a continuous spectrum containing emission lines and absorption lines, and their luminosity decreases slowly.Type I supernovae can also be divided into type Ia, type Ib, and type Ic.Among them, type Ia supernova is the formation of a neutron star due to the collapse of a white dwarf due to its own mass exceeding the Chandraseka limit due to some reason (absorption of material from the companion star or white dwarf merger, etc.).Generally, the mass of this white dwarf star when it explodes is slightly greater than 1.44 times the mass of the sun, which makes its maximum burst luminosity converge, and its absolute magnitude is about - 20.This makes it a good ranging tool: as long as we know the apparent magnitude of type Ia supernova explosion in a distant galaxy, we can infer the distance of this galaxy.Therefore, Ia type supernovae are also called "standard candles".Type Ib supernovae have strong radio radiation, type Ic supernovae andGamma ray burstof
The spectra of type II supernovae are continuous and have strong hydrogen absorption lines.Type II supernovae are considered to be the final outcome of massive red supergiants.When the thermonuclear reaction of a massive star reaches its extreme, its temperature may soar to 4 billion Kelvin.At this time, in the plasma iron nucleus inside the star, the iron atomic nucleus undergoes nuclear fission and decomposes into 13 helium atomic nuclei and 4 neutrons, and then the helium atomic nucleus decomposes into protons and neutrons, releasing neutrinos at the same time.These steps not only do not release energy, but also consume a large amount of energy, which means that the radiation pressure of the core drops suddenly, and gravity quickly takes the upper hand, making the core collapse rapidly, with a speed of 10000 km/s (collapse), making the matter in the core area break through the electron degeneracy pressure and generate the meson degeneracy pressure.Once the nuclear reaction of the outer shell is terminated, the matter in these shells will fall rapidly under the core gravity, with a speed of 40000 km/s.A large amount of matter collides with the hard neutron degenerate matter at the core and bounces back.When returning, he encountered other falling matter at high speed, and the two collided strongly. A huge amount of matter carried huge energy, forming shock waves that swept everything, blowing the whole star into powder, thus forming a gorgeous supernova.This process releases more energy than the total energy released by a star in its lifetime.After the supernova explosion, what is left are supernova remnants and a neutron star or black hole.
Schematic diagram of the evolution of low mass stars (left circulation) and high mass stars (right circulation).
"Although the tortoise lives long, it still has its time. When the snake flies in the fog, it will become dust." Although the life of the star is long, it will also die.But the death of a star is also the driving force for the birth of a new generation of stars.The shock wave of supernova explosion will compress the interstellar medium and cause the molecular cloud to collapse, thus triggering star formation.All elements in the universe can only be generated in the process of supernova explosion, except iron and previous elements.Planetary nebulae and supernova explosions can also bring out the material inside stars and throw it into space, becoming the raw material for new generation stars."All things in the world live on what they have, and what they have begins with nothing." Although the end of stars is illusory interstellar matter, it is they that have created the universe we see today, and it is this "nothing" that has created mankind itself.
Bright star catalogue
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21 bright stars throughout the day (apparent magnitude greater than 1.5)
On March 30, 2022, NASA said that the Hubble Space Telescope had discovered the most distant single star ever observed by humans through the "gravitational lens" effect.This star was born less than 1 billion years after the Big Bang, about 28 billion light years away from Earth.[41]
On June 9, 2023, Science magazine published the paper of the international cooperation group of the High Altitude Cosmic Ray Observatory (LHAASO, referred to as "Lasso" in Chinese) - Trillion Electron Volts Afterglow of the 221009A Narrow Jet of the Extremely Bright Gamma Ray Burst online.Lasso, located in Sichuan Province, China, recorded the whole process of the trillion electron volts gamma ray explosion at the moment of death of a massive star for the first time, opening the veil of the explosion.[44]
