Supernovae is the largest explosionVariable star。The energy released during the explosion is generally up to 10forty-one—10forty-fourJ. And all or most of the material is blown up,[39]In the process, most or even almost all of its substances can belight speedSpeed of[2]Such explosions are extremely bright, and the sudden electromagnetic radiation in the process can often illuminate the whole area where they areGalaxyAnd may last for weeks to months before gradually declining.During this period, the radiant energy released by a supernova can be equal to the total radiant energy of the sun in its lifetime[1]。The star explodes andInterstellar matterradiationshock wave[3]。This shock wave will lead to the formation of a shell structure composed of expanding gas and dust, which is calledSupernova remnant。A supernova is a galaxyGravitational wavePotential strong source[4]。Supernovae accounts for a large proportion of primary cosmic rays[5]。
A supernova is more active than a nova.The English name of a supernova is supernova. nova means "new" in Latin, which means it looks like a new bright star on the celestial sphere.The term supernova was proposed by Walter Budd and Fritz Zweiki in 1931[6]。
Hipparchus The interest in observing stars may be related to the observation of a supernova[7]。The earliest recorded supernova observed by humans is SN 185, which was seen by Chinese astronomers in 185 A.D., and the brightest supernova recorded is SN 1006.Both Chinese and Islamic astronomers have detailed accounts of this[8]。SN 1054, the supernova observed most frequently, formedCrab Nebula。The supernovae SN 1572 and SN 1604 are the last two supernovae in the Milky Way that have been observed with the naked eye so far. The discovery of these supernovae has a significant impact on the development of European astronomy, because these discoveries are used to refute the invariability outside the moon and planetsAristotleCosmic viewpoint[9]。Johanne Kepler The supernova SN 1604 was observed on October 17, 1604, when it reached its peak brightness, and its brightness was estimated during this period, until the brightness became dim to be invisible to the naked eye in the next year.It was the second supernova observed at that time (afterTycho Brahe OfCassiopeiaSN 1572).
With the development of telescopes, people can observe supernovae not only in the Milky Way, but also in other galaxies.It was observed in 1885 thatAndromeda galaxyThe supernova S Andromeda.American astronomers Rudolf Minkowski and Fritz Zweiki launched the modern supernova classification plan in 1941[10]。In the 1960s, astronomers found that the maximum intensity during the supernova explosion can be used as the astronomical distanceStandard candleTo measure the distance of the celestial body[11]。Recently, some distant supernovae have been observed to be darker than expected, which supports the view that the expansion of the universe is accelerating[12]。In order to reconstruct the observation of supernovae without written records, new technologies have been developed to observe supernovae.From the explosion date of supernova Cassiopeia A, people have detected the return light event from the nebula[13]。From the measurement of its temperature and theγX-ray decay, estimated the age of supernova remnant RX J0852.0-4622.In 2009, by matching the nitric acid content of Antarctic ice sediments, people learned about the time of past supernova events.
Discovery process
Supernova
Because supernovae are rare events in a galaxy,GalaxyIt happens about every 50 years. In order to get a good sample of supernovae, many galaxies need to be detected regularly.Supernovae in other galaxies cannot be predicted accurately.Usually, when they are discovered, the process has already started.The most scientific research on supernovae (such as measuring distance as standard candle) needs to observe its peak brightness.Therefore, it is very important to find them before they reach their peak.The number of amateur astronomers greatly exceeds that of professional astronomersOptical telescopeObserving some nearby galaxies and comparing them with previous pictures have played an important role in searching for supernovae.
By the end of the 20th century, astronomers were increasingly turning to computer controlAstronomical telescopeAnd CCD to find supernovae.This kind of system is very popular among amateur astronomers, and there are also large facilities, such asKatzman automatic imaging telescope（KAIT）。neutrinoIt is a large number of subatomic particles produced by supernova explosion, and it is not absorbed by interstellar gas and dust in the Milky Way.
The search for supernovae falls into two categories: some focus on relatively recent events, while others look for earlier explosions.Due to the expansion of the universe, the distance of a remote object with known emission spectrum can be estimated by measuring its Doppler frequency shift (or red shift).On average, objects that are far away weaken at a greater speed than objects that are near, so they have a higher redshift.Therefore, the search is divided into high redshift and low redshift, and its boundary is about z=0.1 – 0.3 - where z is the dimensionless dimension of the spectral frequency shift.
The search for high redshift usually involves the observation of the supernova photometric curve, which is important for generatingHubble diagramAnd the standard or calibration candle used for cosmological prediction is very useful.The spectrum of a supernova at the low redshift end is more practical than that at the high redshift end, and can be used to study the physics and environment around the supernova.The low redshift can also be used to measure the Hubble curve at the near end, which is used to describe the relationship between the visible galaxy distance and redshift. See Hubble's Law.
2011The nobel prize in physicsAnnounced: American professor Permat, American Australian professor Brian Schmidt and American professor Reiss won the prize. They found that the universe is accelerating expansion and cooling by studying supernovae, saying that the entire universe may eventually become ice.For the first time, the medical prize was awarded to a deceased scholar.The chemistry prize, peace prize, literature prize, economics prize and other awards will be announced in succession.The 2011 Nobel Prize is still 10 million yuanSwedish krona(about 1.46 million US dollars).
In November 2011, NASA（NASA）New infrared observations made with telescopes have confirmed that the first recorded supernova explosion was the first time that there was a celestial anomaly recorded during the Eastern Han Dynasty in China and a guest star invaded.
In March 2016, a scientific research team led by Peter Garnavich, an astronomer at Notre Dame University, spent three years analyzing the spectra of 50 trillion stars observed by Kepler, and found two supernovae, one named KSN2011a, which is equivalent to nearly 300 suns in size and about 700 million light years away from the Earth;The other one, KSN2011d, is about 500 suns in size and 1.2 billion light years away from the Earth.
Researchers observed shock bursts for the first time on larger supernovae, but not on smaller supernovae.They speculate that this may be because the small supernova is surrounded by gas, which blocks the generated shock wave.Garnavich said in a statement: "The flash of a shock wave storm lasts about one hour, so if you want to catch one of these flashes, you must either be very lucky or you must continuously observe millions of stars." NASA's statement called this discovery a "milestone" in astronomical observation.
In March 2024, scientists of the Weizmann Institute of Science in Israel used multiple telescopes to draw the most complete supernova explosion process so far by observing the data of the supernova SN 2023ixf.This achievement will help scientists further reveal the mechanism that leads to the end of stellar life and the formation of new things.The related paper was published in Nature on March 27.[41]
Historical discoveries
On December 7, 185, the second year of Zhongping in the Eastern Han Dynasty, Yichou, Chinese astronomers observed Supernova 185, the first supernova in human history.The supernova shone in the night sky for eight months.According to the Chronicles of Astronomy in the Later Han Dynasty, "In October of the second year of Zhongping (185), the guest star came out of the south gate, as big as a half feast, with five colors of joy and anger, and slightly smaller, until June of the next year".[14]
April 30, 1006: SN 1006 burst in the constellation of Jackal. It may be the most luminous supernova ever recorded. It is inferred that its brightness has reached - 9.According to modern astronomers, "In the spring of 1006, people may even be able to read in the middle of the night with the help of its light." In the Song Dynasty in Chinese history, this supernova was discovered by Sitianjian, Zhou Keming and others, so it was called Zhou Boxing.In the fifth and sixth volumes of the Chronicles of Astronomy in the History of the Song Dynasty, it is recorded that: "In April of the third year of Jingde, when Wu Yin and Zhou Boxing saw each other, they left the southern part of the country and rode once to the west of the country. They looked like half moons with awned horns. They could learn things from each other and go to the east of the library. In August, they went into the turbid state along with the heavenly wheel. In November, they came back to the country. Since then, they often saw the east in November, and went into the turbid state in the southwest in August."
July 4, 1054: A supernova explosion that produced the Crab Nebula, this time the appearance of the guest star was recorded in detail by astronomers in the Song Dynasty in Chinese history. Volume 176 of the Long Edition of the Xunzizhi Tongjian contains: "By May of the first year of He He, the guest star could be several inches out of the southeast of the Tianguan in the morning (not since March of the first year of Jiayou)." Japanese and American aborigines also have observation records.[15]
At the beginning of November 1572 (probably between 2 and 6): the supernova in Cassiopeia (Tycho supernova) erupted. The Danish astronomer Tycho had observed it, and therefore published De Nova Stella, which is the source of the Latin name nova of nova.It is estimated that the absolute magnitude of this supernova is - 15.4, 7500 light-years away from the Earth;Its highest apparent brightness is - 4, which can be compared with Venus.[16]
October 9, 1604: The supernova in Ophiuchus (Kepler supernova) has been observed in detail by the German astronomer Kepler. It is the last supernova found in the Milky Way so far, with an apparent magnitude of - 2.5 and 6000 light-years away from the Earth.It was used by Galileo to refute the Aristotelian school's theory that heaven is always the same.
August 19, 1885: The supernova SN 1885A (Andromeda S) located in the Andromeda galaxy was discovered by Irish amateur astronomer Issac Ward in Belfast. This is the first time that humans have discovered a supernova in an extragalactic galaxy, and it is also the only supernova ever found in the Andromeda galaxy.[17]
February 24, 1987: Supernova 1987A located in the Large Magellanic Cloud was discovered within a few hours after the explosion, which is the first opportunity for modern supernova theory to be compared with actual observations.It is about 51400 seconds away from the earth, and its apparent magnitude at its brightest is 3.
September 18, 2006: SN 2006gy, a supernova 238 million light-years away from the Earth (once assumed to be unstable to supernova, but not confirmed), is the strongest supernova explosion ever observed.[18]
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In the 1930s, Walter Budd and Fritz ZweikiMount Wilson ObservatoryHour[19]The initial work believed that this was just a new type of nova.The term "super novae" was used by Bud and Zweiki in 1931California Institute of TechnologyWas used for the first time in a speech in 1933American Physical SocietyIs used by the public in the meeting.In 1938, the hyphen was canceled and became a modern form of appearance and use[20]。Because supernovae are relatively rare events in galaxies, which occur only three times every century in the Milky Way. To obtain good research samples, we need to monitor many galaxies regularly.
Supernovae in galaxies[21]Usually, they have already happened when they were found.The most interesting aspect of science for supernovae is distance measurement - for example, as a standard candle - which requires observing their peak brightness.Therefore, it is essential to find them as early as possible, preferably before reaching the maximum brightness.The number of amateur astronomers is far more than that of professional astronomers, and they have played a great role in searching for supernovae.Usually, we use optical telescopes to observe some nearby galaxies, and compare earlier photos to find[22]。
At the end of the 20th century, more and more astronomers used computer-controlled telescopes and CCD to discover supernovae.Amateur astronomers also like this device and have professional settings, such as the Katzman Automatic Imaging Telescope.Recently, SNEWS (Supernova Early Warning System) has begun to use the network neutrino detector to give early warning to supernovae.Neutrinos are particles that will be produced in large quantities when supernovae explode, and they will not be obviously absorbed by interstellar gas and dust in the galactic disk.
The search for supernovae falls into two categories: those focused on relatively recent events, and those looking for distant explosions.Because in the expanding universe, the distance (or red shift) of the known emission spectrum in the distance can be estimated by measuring its Doppler frequency shift;The farther away the object is, the greater the retrogression speed is, so it has a higher redshift than the nearby objects.Therefore, the search is divided into high redshift and low redshift, and the boundary between them is about the range of redshift z=0.1 – 0.3, where z is a unitless quantity.
The search for high redshift supernovae usually involves the observation of the light curve of supernovae.When the spectra of supernovae are used to study the physics and environment of supernovae, the low redshift ones will be more practical than the high redshift ones.The observation of low redshift also depends on the low distance end of the Hubble curve, which is used to describe the curve of distance relative to the redshift of visible galaxies.(SeeHubble's law）[23-26]
Causes
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The first stars formed in the universe are very different from those people see today.When they appeared 13.5 billion years ago, they only contained hydrogen and helium, the simplest chemical elements in nature, whose mass was estimated to be dozens or hundreds of times that of the sun. They would soon die in the explosion of supernovae, and eject heavier elements to "feed" the surrounding gas.Later generations of stars were born from this gas, and when they also died, they would eject heavier elements.[40]
fixed starCooling starts from the center. It does not have enough heat to balance the gravity of the center. The structural imbalance causes the entire star to collapse toward the center, causing the external cooling and red layers to become hot. If the star is large enough, these layers will explode violently, producing supernovae.The brightness of a massive star explosion can suddenly increase to tens of billions of times the solar luminosity, equivalent to the entireGalaxyTotal luminosity of.
Results of stellar explosion: (1) The star disintegrates into a mixture of gas and dust that expands and diffuses around, and finally diffuses intoInterstellar matter, ending the evolution of stars.(2) The outer layer disintegrates into an outward expanding nebula, leaving some material in the center to collapse into a high-density celestial body, thus enteringStellar evolutionThe late and final stages of.The supernova burst in 1054 observed by ancient Chinese astronomers (this supernova was also named as Chinese supernova internationally).In a galaxy, supernovae are rareCelestial phenomenaHowever, in the galaxy world, dozens of stars can be observed every year.On February 23, 1987, a Canadian astronomerLarge Magellanic CloudA supernova has been found in, which is the first one that can be seen with the naked eye since 1604. This supernova is named "1987A".
Cosmic scale such as supernova explosion“Violence”What's the probability?Although this probability is very small in each galaxy, many galaxies can be observedExtragalactic galaxyTherefore, a considerable number of extragalactic supernova events can be observed every year.However, since 1604, supernovae have not been observed again in our Milky Way.This may be becauseCosmic dustThe presence of a supernova blocks the light of a supernova that appears in a corner of the Milky Way.
WhenInternational Astronomical UnionAfter receiving the report of supernova discovery, they will name it.The name is composed of the year of discovery and one or two Latin letters: the first 26 supernovae discovered in a year will be named with capital letters from A to Z, such asSupernova 1987AIt was the first supernova discovered in 1987;The ones after the 26th are named with two lowercase letters, starting with aa, ab, and ac.Professional and amateur astronomers can find hundreds of supernovae every year (367 in 2005, 551 in 2006, and 572 in 2007). For example, the last supernova discovered in 2005 was SN 2005nc, indicating that it was the 367th supernova discovered in 2005.[27]
Historical supernovae only need to be named according to the year of discovery, such as SN 185, SN 1006, SN 1054, SN 1572（Tycho supernova）And SN 1604（Kepler supernova）。Since 1885, only one supernova was discovered (such as SN 1885A and 1907A).The prefix SN representing supernovae can sometimes be omitted.
The above mentioned names are common, but there are still some unfamiliar ones. There are four large observatories whose discoveries need not be reported to the International Astronomical Union. They are Nearby Supernova Factory, Catalina Real Time Transient Survey, ROTSE collaboration, and Palomar Transient Factory.These four major observatories all have independent supernovae naming rules. Sometimes some discoveries will also be named conventionally, or expressed by the coordinates of a supernova, or some supernovae will not be named.Such as the world famousPalomar ObservatoryThe supernovae discovered by Palomar Transient Factory of, all begin with PTF. Astronomers in ChinaSun GuoyouandhappyThe first amateur supernova discovered on the mainland was given the number PTF10acbu by the Palomar Observatory.
Basic classification
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Astronomers divide supernovae into several types according to the absorption lines of different elements in their spectra:
The luminosity curve of II-L supernova (the magnitude changes with time, or the luminosity changes exponentially with time) shows a "linear" attenuation.
If the spectrum of a supernova does not contain the absorption line of hydrogen, it will be classified as Type I, or Type II.One type can be subdivided according to the absorption line of other elements.Astronomers believe that these observational differences represent different sources of these supernovae.They are very positive about the origin theory of type II, but although astronomy has some opinions on how to explain the occurrence of type I supernovae, these opinions are relatively uncertain.
Type Ia supernovae do not have helium, but have silicon.They all originate from reaching or approaching the Chandraseka limitWhite dwarfThe explosion of.One possibility is that the white dwarf is located in aClose binaryIn the system, it continuously absorbs material from its giant companion until its mass reaches the Chandraseka limit.At that timeElectronic degeneracy pressureNot enough to offset the gravity of the star itself, the collapse process can fuse the remaining carbon atoms and oxygen atoms.In the end, the shock wave generated by the nuclear fusion reaction blew the star to pieces, which is very similar to the mechanism of nova generation, except that the white dwarf corresponding to the nova does not reach the Chandrasekar limit, and carbon oxygen nuclear reaction will not occur. The energy generated by the explosion is from the fusion reaction of hydrogen or helium accumulated on its surface.
The sudden increase in brightness is provided by the energy released during the explosion. After the explosion, the brightness will not disappear immediately, but will slowly decline over a long period of time, because radioactive cobalt decays into iron and releases energy.
Ib supernovae have helium absorption lines, while Ic supernovae do not have helium and silicon absorption lines. Astronomers are still not clear about the mechanism of their generation.It is generally believed that these stars are ending their lives (such as type II), but they may have lost hydrogen before (giant stage) (Ic also lost helium), so there is nohydrogenThe absorption line of.Ib supernova may be the result of the collapse of Wolf Rayet type stars.
If a star has a large mass, its own gravity can fuse silicon into iron.Because the specific binding energy of iron atom is already the highest among all elements, iron fusion will not release energy, on the contrary, energy will be consumed.When the mass of the iron core reaches the Chandraseka limit, it will immediately decay into neutrons and collapse, releasing a large number of neutrinos carrying energy.Neutrinos transmit part of the energy of the explosion to the outer layer of the star.When the shock wave generated by the collapse of the iron core reaches the surface of the star several hours later, the brightness will increase, which is called type II supernova explosion.Depending on the mass of the core, it will become a neutron star or a black hole.
Type II supernovae also have some minor variants, such as II-P and II-L, but these only describe the differences in photometric curves (II-P curves have temporary flat areas, and II-L curves do not), and the basic principles of explosions are not very different.
There is also a kind of theoretical explosion phenomenon called "super supernova".Supernovae refers to the direct collapse of the core of some massive stars into a black hole and the generation of two jets with great energy and near light speed, which emit strong gamma rays.This may causeGamma ray burstsReason for.
Type I supernovae are generally brighter than type II supernovae.
Differences in photometric curves used to distinguish II-P and II-L supernovae
In a massive and evolving star, (a) elements fuse into onion shell layers to form an iron core (b) and reach Chandraseka mass and begin to collapse.The interior of the core is compressed to form neutrons (c), causing the collapsed material to rebound (d) and form an outward propagating shock wave (red).The alluvial wave began to lose its effect (e), but the addition of neutrinos revived the interaction.The surrounding material is dispersed (f), leaving only the degenerated remains.
Current model
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Type Ia
There are many ways to form such supernovae, but all of them share the same internal mechanism: if a white dwarf with carbon oxygen as the main component accumulates enough material and reaches about 1.38 timesSolar massOfChandraseka limit(For a star that does not rotate), it will no longer be able to balance its gravity through the electron degeneracy pressure, so it will collapse.However, today's astrophysical community generally believes that this limit is unattainable under normal circumstances: before collapse, with the increasing temperature and density of the white dwarf core, the carbon combustion process will be triggered when the white dwarf mass reaches 1% of the limit.In a few seconds, a considerable part of the material of the white dwarf will undergo nuclear fusion, releasing enough energy (1-2 × 10 joules) from it to cause supernova explosion.An outward spreading shock wave will be generated and can reach the speed of 5000-20000 km/s, which is about 3% of the speed of light.At the same time, the luminosity of stars will increase significantly,Absolute magnitudeIt can reach - 19.3, etc. (equivalent to five billion times brighter than the sun), and this luminosity hardly changes.
One of the models for studying the formation of such supernovae is a closebinary star 。The larger star in the binary star will leave the main star sequence earlier and expand into a red giant star in the evolution process.As the common orbit of the two stars gradually shrinks, the red giant will eventually eject most of its outer material until it cannot continue nuclear fusion inside.At this time, it evolved into a white dwarf composed mainly of carbon and oxygen.Then another star in the system will also evolve into a red giant star, and the mass of this red giant star will be nearWhite dwarf accretion, so that the quality of the latter keeps growing.If the orbits are close enough, white dwarfs may also accumulate mass from other types of companion stars, including main sequence stars.
Another model for the formation of type Ia supernova is the merger of two white dwarfs. At that time, the combined mass may exceed the Chandraseka limit, but the probability of such a situation is lower than that of the former.
Type Ia supernova has a characteristic luminosity curve, and its luminosity is a function of time after the explosion.The light radiation it emits comes from the energy released from the radioactive decay of nickel - 56 through cobalt - 56 to iron - 56 inside.It is now generally believed that the photometric curves of type Ia supernovae formed by single mass accretion generally have the same photometric peak, which makes them can be used as standard candles in astronomy to measure the distance from their host galaxies.However, recent observations indicate that the average width of their luminosity curves will also evolve to some extent, which means that the intrinsic luminosity of Ia type supernovae will also change, although this change is more significant on a larger redshift scale.
Type Ib and Ic
The formation mechanism of these two types of supernovae is probably similar to the process of type II supernovae formed by the depletion of nuclear reaction fuel in a massive star;However, the difference is that the stars that form type Ib or Ic supernovae lose their outer layers of hydrogen due to strong stellar wind or interaction with their companion stars.Type Ib supernovae are considered to be the products of collapse of massive Wolf Rayet stars.In addition, there is some evidence that a small number of type Ic supernovae are the cause of gamma ray bursts, but there are also views that any type Ib or type Ic supernovae with the outer layer of hydrogen element stripped may generate gamma ray bursts if the geometric conditions of the explosion permit.
Type II
Massive stars with a mass no less than nine times the mass of the sun have quite complex evolutionary styles.The hydrogen element in the core of a star continuously generates helium through nuclear fusion, and the energy released will generate outward radiation pressure, thus ensuring theHydrostatic balanceAnd avoid the collapse caused by the huge gravity of the star itself.
When the hydrogen element in the core of a star is exhausted and cannot generate enough radiation pressure to balance gravity, the core collapses. During this period, the temperature and pressure of the core will rise sharply and canheliumElements ignite.The helium element in the core of the star begins to fuse into carbon, and can generate considerable radiation pressure to stop the collapse.This causes the core to expand and cool slightly. At this time, the core has an outer layer of hydrogen fusion and a center of helium fusion with higher temperature and pressure.(Other elements such as magnesium, sulfur and calcium will also be produced and in some cases burned in the subsequent reaction.)
The above process will be repeated several times, each time the core collapse will be stopped by the fusion process of the next heavier element, and constantly produce higher temperature and pressure.As a result, the star has become a layered structure like an onion, and the elements closer to the outer layer are more likely to undergo fusion reaction.Each layer depends on the heat energy and radiation pressure generated by the fusion reaction of the next layer inside to stop the collapse until the fusion fuel of this layer is exhausted;And each layer is hotter and burns faster than the outer layer - the combustion process from silicon to nickel takes only one day or a few days.
In the later stage of this process, the increasing heavy elements participate in nuclear fusion, and the binding energy of the generated related element atoms is also increasing, resulting in the continuous reduction of the energy released by the fusion reaction.And at higher energy, the kernel willPhotoinduced metamorphosisAnd electron capture process, which will reduce the energy of the core and generally accelerate the nuclear fusion reaction to maintain balance.The continuous synthesis of this heavy element ends at Ni-56, and there is no energy release in this fusion reaction (but it can produce Fe-56 through radioactive decay).As a result, the nickel iron core can no longer generate any outward radiation pressure that can balance the gravity of the star itself, and the only thing that can play a certain balance role is the electron degeneracy pressure of the core.If the mass of the star is large enough, the mass of the core may eventually exceed the Chandraseka limit, so the electron degeneracy pressure is not enough to balanceGravitational collapse。Finally, under the strong gravity of the star itself, the innermost layer of the core willNucleusThe forces separated from each other could not support, and the star began to collapse destructively. At this time, there was no fusion reaction that could prevent the collapse.
Core collapse
The collapse speed of the supernova core can reach 70000 kilometers per second (about 0.23 times the speed of light). When the mass of the original star is less than about 20 times the mass of the sun (depending on the intensity of the explosion and the total amount of material falling back after the explosion), the residual product after the collapse is a neutron star;For stars above this mass, the residual mass will continue to collapse into a black hole because it exceeds the Oppenheimer Volkow limit (this collapse may be one of the reasons for gamma ray bursts, and with the release of a large number of gamma rays, another supernova explosion may also occur in theory),In theory, the upper limit of this situation is about 40-50 times the mass of the sun.
For stars with more than 50 times the mass of the sun, it is generally believed that they will skip the process of supernova explosion and collapse directly into black holes, but this limit is quite difficult to calculate due to the complexity of the model.However, according to recent observations, stars with extremely high mass (140-250 solar mass) and low proportion of heavy elements (relative to helium) may form unstable supernovae without leaving black hole traces.The formation mechanism of such rare supernovae may not be the same (and may be partially similar to the eruption of type Ia supernovae), so the existence of iron core may not be required.The typical representative of this type of supernova is type II supernova SN 2006gy, which is estimated to have 150 times the mass of the sun. Its observation shows that the explosion of such a massive star is fundamentally different from the previous theoretical prediction.
The process will lead to a sharp increase in the temperature and density of the core.This energy loss process of the core terminates in the balance between the outward degenerate pressure and the inward gravity.Under the effect of light induced transformation,γradialThe iron atom is decomposed into a helium nucleus, which releases neutrons and absorbs energy at the same time;Protons and electrons are trapped by electrons (irreversibleβDecay) merges, producing neutrons and runaway neutrinos.
In a typical type II supernova, the initial temperature of the newly generated neutron nucleus can reach 100 billion Kelvin, which is 6000 times the temperature of the solar core.Most of such high heat needs to be released to form a stable neutron star, and this process can be completed by further neutrino release.These "hot" neutrinos form a neutrino anti neutrino pair covering all flavors, and are several times as many as neutrinos formed by electron capture.About 10 joules of gravitational energy - about 10% of the remaining mass of the star - will be converted into a neutrino burst lasting about 10 seconds, which is the main product of this event.Neutrino bursts will take away the energy of the core and accelerate the collapse process, while some neutrinos may also be absorbed by the outer material of the star to provide energy for the subsequent supernova explosion.
The core will eventually collapse into a sphere with a diameter of about 30 kilometers, and its density is equivalent to that of an atomic nucleus. Later, the collapse will be caused by the strong interaction between nucleons andNeutron degeneracy pressureSudden termination.Due to the sudden stop of the movement of the inward collapsing material, the material will rebound to a certain extent, which will stimulate the outward propagating shock wave.The results of computer simulation indicate that this outward spreading shock wave is not the direct cause of supernova explosion;In fact, the energy consumption caused by the disintegration of heavy elements in the outer region of the core makes the shock wave only exist for milliseconds.This requires the existence of a process that has not yet been understood, which can make the outer area of the core regain about 10 joules of energy, thus forming a visible burst.The current research mainly focuses on the combination of neutrino warming, spin and magnetic field effects, which are the basis of this process.
Photometric curve
Due to the existence of the Balmer absorption line in the hydrogen spectrum, the photometric curve of type II supernovae has obvious characteristics: compared with the photometric curve of type I supernovae, the photometric curve of type II supernovae decreases by 0.008 per day on average, much lower than the former.According to the characteristics of the photometric curve, type II supernovae can be divided into two subcategories, one has a flat plateau area on the photometric curve (type II-P), and the other has only linear attenuation on the photometric curve (type II-L).Thus, the overall decay rate of type II-L supernova is 0.012 per day, which is higher than that of type II-P supernova, 0.0075 per day.For type II-L supernovae, the reason for this difference is that most of the outer hydrogen element in the original star has been ejected.
The plateau region in the photometric curve of II-P supernova is due to the change of its outer layer opacity.The shock wave generated in the explosion ionizes the hydrogen atoms in the outer layer, preventing the photons generated by the internal explosion from escaping through the outer layer, thus significantly improving the opacity of the outer layer.When the outer hydrogen ions are cooled and reassembled into atoms, the transparency of the outer region will rise again.
Anomalous spectrum
Among the many anomalous characteristics of the spectrum of type II supernovae, type IIn supernovae may be born from the interaction between ejecta and the matter around the star, while type IIb supernovae may be the result of a massive star in its companion starTidal forceUnder the action, most (but not all) of the outer layer of hydrogen element is lost.With the expansion of the IIb supernova ejecta, the outer layer of the remaining hydrogen element will soon become transparent and can reveal the inner structure.
Asymmetry
For a long time, a mystery surrounding the research of supernovae is how to explain that the residual dense matter generated after explosion has such a high speed relative to the core.(As a neutron starPulsarIt has a very high speed. Theoretically, black holes also have a very high speed, but it is difficult to verify it through isolated observations.)In any case, the force that can push matter to produce such a speed should be considerable, because it can make an object whose mass is greater than the sun produce a speed of 500 km/s or more.Some explanations suggest that this driving force includes the convection during the collapse of stars and the jet generated during the formation of neutron stars.
This composite picture of X-ray and visible light describes the process fromCrab NebulaElectromagnetic radiation from the core area.The speed of particles released from a pulsar near the center can be close to the speed of light.The velocity of this neutron star is about 375 km/s. Specifically, the large-scale convection generated above the core can cause localElement abundanceChange, resulting in an uneven distribution of nuclear reaction during collapse, which will cause explosion after rebounding.The jet interpretation believes that the accretion of gas by the central neutron star will formAccretion diskAnd produce highly directional jets, which eject the material at a high speed, and generate transverse shock waves to completely destroy the stars.These jets may be important factors leading to supernova explosion.(A similar model is also used to explain the generation of long gamma ray bursts.)
But over time the explosion will become more symmetrical.This asymmetry can be detected by measuring the polarization of the outgoing light in the initial state.
Type Ia nuclear collapse
Because Ib, Ic and many types of Type II supernovae have similar mechanism models, they are collectively called nuclear collapse supernovae.The basic difference between type Ia supernovae and nuclear collapse supernovae is the energy source of the radiation released near the peak of the photometric curve.The original stars of nuclear collapse supernovae all have extended outer layers, and the expansion amount required for such outer layers to reach a certain transparency is small.Most of the energy required for the light radiation at the peak of the photometric curve comes from the shock wave that heats and ejects the outer layer material.
In contrast, the original star of type Ia supernova is compact and much smaller than the sun (but still much larger in mass). Therefore, if this compact star wants to become transparent, it needs to undergo a large expansion (and cooling).The heat generated by the explosion is consumed during the expansion of the star, which can not promote the generation of photons.In fact, the energy radiated by type Ia supernovae comes entirely from the decay of radioactive isotopes produced in the explosion, which mainly includes Ni-56（half life6.1 days) and its decay product cobalt-56 (half life 77 days).The gamma rays radiated from radioactive decay will be absorbed by the ejected materials, which will therefore be heated to incandescent state.
In nuclear collapse supernovae, as the ejected material expands and cools, radioactive decay will eventually become the main source of light radiation.A bright type Ia supernova can release nickel 56 of 0.5 to 1 times the solar mass, but the nickel 56 released by a nuclear collapse supernova is usually only about 0.1 times the solar mass.
Supernovae is a key source of elements heavier than oxygen.Among these elements, iron-56 and lighter elements are generated from nuclear fusion, while heavier elements are generated from nuclear synthesis during supernova explosion.Despite the controversy, supernovae are indeed the most likely candidate sites for the r-process, which is a fast form of nuclear synthesis at high temperature and high neutron density.A large number of highly unstable atomic nuclei are generated in the reaction, and these atomic nuclei contain excess neutrons.These states are unstableβDecay to a more stable state.
Supernovae and its explosion
The r-process may occur in the explosion of type II supernovae, in which about half of the rich elements heavier than iron are generated, including plutonium, uranium, californium and other elements.Other processes that can produce heavy elements are comparable to the s-process that takes place in the aging red giant star, but this process is much slower and cannot produce elements heavier than lead.[28-29]
The supernova remnant N63A in the Large Magellanic Cloud is located in a cluster of gas and dust. The remnant after the supernova explosion includes a central compact star and the material rapidly diffused outward due to shock waves.These substances sweep through the surrounding interstellar matter in a state of rapid expansion, which can last for two centuries.Then they will undergo an adiabatic expansion process, and then gradually cool down and mix with the surrounding interstellar matter in about 10000 years.[30]
Supernova remnant N 63A
According to the standard theory in astronomy, the Big Bang produced hydrogen and helium, and possibly a small amount of lithium;All other elements are synthesized in stars and supernovae.The supernova explosion filled the interstellar material around it with metals (for astronomers, metals are all elements heavier than helium, which is different from the concept in chemistry).These synthetic metals enrich the element composition of the molecular cloud that forms stars, so the composition of each generation of stars (and planetary systems) is different, from pure hydrogen and helium to full of metals.[31]
Supernovae is the main mechanism of redistribution of heavy elements generated in stellar nuclear fusion in the universe. All components of different elements have a great impact on the life of a star, even the existence of planets around it.The kinetic energy of the expanding supernova remnant can compress and condense the nearby molecular cloud, thus starting the formation of a star.If the gas cloud cannot release too much energy, the increased turbulent pressure can also prevent star formation.[32]
In the explosion of a supernova near the solar systemradio isotopeThe evidence provided by the decay products of, can understand the composition of elements in the solar system 4.5 billion years ago, and these evidences even show that the formation of the solar system may also be initiated by the explosion of this supernova.After the heavy elements produced by supernovae have gone through an astronomical period of time, these chemical compositions finally make the birth of life on Earth possible.
Affect the earth
If a supernova explodes so close to the Earth that it can have a significant impact on the Earth's biosphere, such supernovae are called near Earth supernovae, and their distance to the Earth is roughly 100 light-years.Supernova pairterrestrial planetThe main reason for the negative impact is gamma rays: for the earth, gamma rays can cause chemical reactions in the upper atmosphere, transforming nitrogen molecules intonitrogen oxide And destroy the ozone layer, exposing the earth's surface to harmful solar radiation and cosmic rays.It is believed that the gamma ray burst caused by a near Earth supernova may be the cause of the Ordovician Silurian extinction event, which caused the disappearance of nearly 60% of the earth's marine life at that time.[33]
The prediction of near Earth supernovae usually focuses on the massive stars that may form type II supernovae, and within the range of several hundred light years from the sun, there are indeed several major stars that may become supernovae in as short as 1000 years;A typical example is Betelgeuse, which is 427 light years away from the earthRed supergiant。It is worth noting, however, that it is generally believed that these predicted supernovae will have almost no impact on the Earth.Its distance from the earth needs to be less than 8 seconds (26 light years).The results of such prediction are mainly related to the model established for the atmosphere, and the radiation flux used in it comes from the measurement of SN 1987A, a type II supernova in the Large Magellanic Cloud.Current predictions of the probability of supernova explosions within the 10 second gap around the Earth vary widely, from once every 100 million years to once every 10 billion years.
If the eruption of type Ia supernovae is close enough to the Earth, they are considered to be a potential great danger. This is because they are formed from ordinary dim white dwarfs, so a type Ia supernova may explode in a star system that has not been carefully studied under unexpected circumstances.Some theories believe that Ia supernovae affect the Earth within 1000 seconds (3300 light-years).[34-35]
In 1996, astronomers at the University of Illinois at Urbana Champaign theorized that it might be possible to detect the trace of the Earth's past supernova influence from the metal isotopes in the strata.Immediate menstruationMunich University of TechnologyThe researchers reported that the enrichment of iron - 60 caused by near - earth supernovae was detected in deep-sea rocks in the Pacific Ocean.[36-38]
Observational significance
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Except for those observed in the visible light areaSupernova remnantIn addition, through theArtificial satellite"Einsteinobservatory”People have discovered many X-ray sources in the sky, more than 30 of which are X-ray supernova remnants.Longqing appeared in 1572cometThat is, the ancient nova, which left X-ray traces.The supernova shock wave makes the interstellar medium temperature reach several million Kelvin and radiates strongX-ray。This is a typical type I supernova.
Supernova brightness increase phenomenon
useradio telescopeSupernova remnants composed of only the rarest gas can be found.For example, yesRadio astronomyHome was the first to find outCassiopeia AThis supernova remnant was also found its extremely weak counterpart in the optical band later.
Supernova explosion andcosmic rayThere is also a certain relationship between the production of.The velocity of particles in the interstellar medium is generally in the range of tens of kilometers per second, but there are some special cases - some particles can move at the speed of light, which is called cosmic rays.Cosmic rays are composed of some material particles such as electrons and protons, which are completely different from electromagnetic waves in essence.Generally speaking, because the earth's atmosphere absorbs cosmic rays, cosmic rays must be detected outside the atmosphere.If you go up to 50km high by balloon, you can use negative film to shoot the track of cosmic rays.Only a few extremely energetic cosmic rays can reach the earth's surface.However, when high-energy cosmic rays interact with the earth's atmosphere, it will trigger a flash effect and generate secondary cosmic rays at the same time. It is relatively easy to detect secondary cosmic rays on the earth's surface.
Experiments show that some cosmic rays with lower energy are affected bysolar activityImpact.For example, the solar activity has an 11 year cycle, and the observed low-energy cosmic rays also change with this cycle.In addition, when the solar activity increases, the magnetic field around the earth will increase, thus weakening the cosmic ray activity observed on the earth.On the contrary, the maximum cosmic ray flux often appears inSolar flareWhen the activity is minimum.Observations also show that the vast majority of cosmic rays come from supernova explosions in the distant depths of the universe.
Because the cosmic ray often changes its direction of motion due to the action of the interstellar magnetic field, it is difficult to determine where its radiation source is.But when cosmic rays interact with interstellar medium, they will radiateгRay;andгRay iselectromagnetic wave, the direction of motion is no longer affected by the magnetic field.NASA has launched a special space observationгX-ray satellite.The observation results show that the universeгThere is a good correlation between the distribution of X-ray and the distribution of supernovae.This, to a large extent, supports the view that cosmic rays come from supernova explosions.
There is another essential difference between a supernova event and a nova event, that is, the supernova explosion only occurs on the surface of the star, while the supernova explosion occurs in the deep layer of the star, so the scale of the supernova explosion is much larger.The materials scattered into space during supernova explosion have important contributions to the formation of new interstellar media and even new stars, but these materials come from the shell of dead stars.
Research purpose
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A supernova is at the intersection of many different branches of astronomical research.As the final destination of many kinds of stellar life, supernovae can be used to test the current stellar evolution theory.The phenomena observed at the instant of explosion and after explosion involve various physical mechanisms, such asneutrinoandGravitational waveEmission, combustion propagation and explosionNucleosynthesis、Radioactive decayAnd the interaction between shock wave and surrounding matter.And the remnants of the explosion, such as neutron stars or black holes, and expanding gas clouds, are heatedInterstellar mediumRole of.
SupernovaeHeavy elementAspects play an important role.The Big Bang only producedhydrogen、heliumAnd a small amount oflithium。Nuclear fusion in the red giant stage produced various medium mass elements (heavier than carbon but lighter than iron).The elements heavier than iron were almost synthesized during the supernova explosion, and they were thrown into interstellar space at a high speed.In addition, supernovae are the main "spokesmen" of the chemical evolution of galaxies.In the early evolution of galaxies, supernovae played an important feedback role.The loss of galactic matter and star formation may be closely related to supernovae.
Because they are very bright, supernovae are also used to determine distances.Combine the distance with the expansion speed of the supernova parent galaxy to determineHubble constantAnd the age of the universe.In this regard, type Ia supernovae have proved to be powerful distance indicators.It was initially assumed by the standard candlelight, and later by the use ofLight curve shapeAnd other parameters to calibrate the peak luminosity.AsVirgo LeagueThe calibrated peak photometric dispersion of the best distance indicator is only 8%, and it can extend to the distance V>30000 km/s.The Hubble diagram of Ia supernova (more specifically, the magnitude redshift relationship) is now the most powerful tool for studying the history of cosmic expansion: its linear part is used to determine the Hubble constant;The curved part can study the evolution of expansion, such as acceleration, and even the different material and energy components that make up the universe.The Ia supernova can also be used as a "standard candle" to study the local motion of its parent galaxy.The light curve of high redshift Ia supernova can also be used to testCosmic expansion theory。
It can be predicted thatTime dilationThe effect will be shown on the light curve of high redshift supernovae.The observational data show that the width of the light curve of Ia supernova at redshift z is (1+z) times that at z=0.This provides another strong support for the expansion universe theory.Some type II supernovae can also be used to determine distances.The expansion speed of II-P supernovae projectiles at the platform stage is related to their thermal luminosity, which is also used for distance measurement.After the above correlation correction, the dispersion of the original II-P supernova's V-band magnitude of - 1 can be reduced to the level of - 0.3, which provides another means to measure the distance independent of SN Ia.In addition, the type II supernovaRadio emissionIt also seems to have quantitative properties. For example, the 6cm light curve peak is related to the time when the 6cm peak appears after the explosion, which can also be used for distance estimation.
Candidate Nova
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The nebula surrounding Wolf Rayet star WR124 is about 21000 light-years from Earth.In the next few thousand to hundreds of millions of years, many large stars in the Milky Way are thought to be likely to become supernovae, includingRho Cassiopeiae 、Haishan II、Ophiuchus RS、Scorpio U、KPD1930+2752、HD 179821、IRC+10420、Canis Major VY、Betelgeuse、Antares andCornucopia。
Many Wolf Rayet stars, such asTiansheji, WR 104, and the members of the Pentagram Cluster are all considered as candidates for supernovae in the "near" future.
The candidate of the nearest supernova to the earth isPegasus IK(HR 8210), which is only 150 light years away from the earth.It is a close binary system composed of a main sequence star and a white dwarf star, which are only 31 million kilometers apart.It is estimated that the mass of the white dwarf is about 1.15 times that of the sun, and the white dwarf will grow to enough mass through accretion in several million years, thus evolving into an Ia type supernova.
