Open star clusters are hundreds to thousands of stars connected by weak gravityfixed starThe diameter of the celestial body is usually only tensLight year。The density of stars in open clusters is different, butGlobular clusterThe density of stars in open clusters is much lower than that in highly dense mesostars.Open clusters are only found in regions where stars are actively formed, includingspiral galaxy Swivel arm andIrregular galaxy。Open star clusters are generally very young, only millions of years old, younger than many rocks on Earth.
Younger open star clusters may still contain remnants of the molecular clouds that formed when they were formed. The light generated by the star clusters makes them formIonized hydrogen region。Molecular cloudGenerated in star clustersRadiation pressureGradually spread under the influence.ObservationStellar evolutionOpen star clusters are rare objects.This is because the members of the same open cluster are very similar regardless of age or chemical composition, and it is easy to observe the slight differences among the members of the cluster.
Since the gravitational correlation of cluster members is not very strong, after circling the spiral galaxy for several weeks, it may be caused by the surrounding objectsgravitationThe influence is scattered.
includePleiades(M45), the brightest open star clusters have been known since ancient times.Many others looked like blurry spots before the telescope was invented.Open star clusters are irregular in shape, including relatively few stars and relatively uniform distribution in the sky.Because almost all gathered inGalaxyIn the equatorial plane, open star clusters are sometimes called“Galactic cluster”。It has long been found that stars in open clusters are closely related to each other.
In 1767, John MichellministerThrough calculation, it is found that the probability of star clusters like the Pleiades cluster forming randomly is only 1 in 496000.[1]along withAstrometryWith the improvement and development of accuracy, astronomers found that members of the cluster have similar self motion, and by analyzing the spectrum, they found that all members maintain the same visual speed, which proves the unity of motion in the cluster.
Although open clusters andGlobular clusterThere are many differences. Relatively small globular clusters and large open clusters do not look different.Some astronomers believe that there are twoclusterThe basic formation process of is exactly the same, except that there are a lot offixed starstayGalaxyAnd gradually become rare.
Since open star clusters contain hundreds or even thousands of stars with different colors and brightness in a relatively small area, they are good observation targets for astronomers.And, open clusterslight pollutionSevere areas can also be covered by small telescopes, or evenBinocularsObserved.
Star cluster formation
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In the universescattered all over like stars in the sky or men on a chessboardIt is composed of fine particles such as gas and dustMolecular cloud。The density of these molecular clouds is very low, and the main components arehydrogen。Molecular clouds can be extremely large and have great mass, which is equivalent to 10 to 1000 suns.Because only molecular clouds whose mass is several times that of the sun will be affected by their own gravityCollapse, and such a heavy molecular cloud cannot collapse into a star, so all members of the open cluster areMulti satellite systemFormed in.
The infrared image shows a forming open cluster.
Without interference, these molecular clouds can remain unchanged for thousands of years.However, when molecular clouds are affected byGalaxy collision, produced by being in a galaxyDensity wave、SupernovaThe density of the explosive shock wave will change slightly.These slight changes will cause the molecular cloud to shrink (collapse) by gravity, thus forming some spheres called protostars.At the initial stage of the formation of open star clusters, because nuclear fusion has not yet taken place in the core of the protostar, they still cannot be called real stars.
Once stars begin to form, the stars with the highest temperature and mass will emit a lot of ultraviolet rays, ionize the nearby molecular clouds, and formIonized hydrogen region。From massive starsStellar windAnd radiation pressure will drive those gases away.In millions of years, the cluster will have its first supernova explosion, which will also drive away the surrounding gas.After tens of millions of years, the star cluster will lose all the gas, and there will be no newStar formation。Previously, only 10% of the original gas in the cluster would form stars.
In the Milky Way, on average, a new open cluster is born every 1000 years or so.[2]
Sometimes multiple open clusters can be generated in the same molecular cloud;For example,Large Magellanic GalaxyInHodge 301Cluster (Hodge 301) andR136All star clusters aretarantula nebula Formed in.Through traceabilityGalaxyThe motion of mesostars, astronomers foundHyades(Hyades) andGhost constellation(Praesepe) was formed in the same cloud about 600 million years ago.
Sometimes, two star clusters formed at the same time will formBinary clusterSystems, such as those in the Milky WayPerseus binary cluster。There are at least ten known binary cluster systems in the Milky Way.In largeMagellanic galaxyMany double star clusters have also been found in, because the projection effect will make the star cluster system in the Milky Way look very close.
Form and classification
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Open cluster
The number of members in open star clusters ranges from hundreds to thousands. Generally, the central part is particularly concentrated and scattered around.The diameter of the central part generally reaches three to four light years, and the radius of the entire cluster generally reaches 20 light years.Generally speaking, the density of the central part can reach 1.5 stars/cubic light year.In contrast, the density of stars around the sun is 0.003 stars/cubic light year.
Open clusters are usually based on Robert·Trumpler(Robert Trumpler)classificationClassification.The Trumpler classification consists of three digits: Roman numerals from one to nine indicate cluster density (from high to low) and separation from surrounding star fields, the second digit is Arabic numerals, from one to three (from low to high) indicates the brightness of members, and the third digit uses "p", "m" or "r" to indicate that the cluster content is low, medium or high,If "n" is added, the cluster is located in a nebula.
Using the Trumpler classification, the Pleiades star cluster is divided into I3rn (highly dense, high brightness, numerous members and located in the nebula), and the nearby Hyades star cluster is divided into II3m (relatively scattered, containing fewer stars).
Quantity and distribution
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More than 1000 open star clusters have been found in the Milky Way, but the actual number may be ten times that.stayspiral galaxy Most open clusters have the highest gas densitySwivel armAnd theStar formationThe activity is the most active.Open clusters are highly concentratedGalactic surfaceNearby.
NGC 346 is an open cluster in the Magellanic galaxy
as forIrregular galaxyWe can find open clusters everywhere in the galaxy.Open clusters are not found in elliptical galaxies becauseElliptical galaxyThe star forming activity of the first star formation stopped millions of years ago, and the original open star clusters have disappeared without trace.
stayGalaxyThe lifetime of open clusters depends on their location;Early star clusters tend to be closer to the edge of galaxies.Galactic centerThe strong tidal force ofGiant molecular cloudThere are more open clusters in the central part of the galaxy, so the life of the open clusters in the central part of the galaxy is shorter than that in the peripheral part.
Astral composition
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Open star clusters in the Large Magellanic Galaxy illuminate the Tarantula Nebula
Open star clusters are often blue stars. They are relatively young and have high quality, but their life span is only tens of millions of years.Relatively old open star clusters contain more yellow stars.Some open clusters have blue stars that are much younger than others.these ones hereBlue stragglerIt also occurs in globular clusters: after multiple stars collide in the center of a very dense globular cluster, stars with much higher temperature and mass will be formed.The density of stars in open clusters is much lower, and the collision of stars is difficult to explain the number of observed blue discrete stars.The theory is that the gravity of other starsbinary star Converge into a star
When nuclear fusion exhausts hydrogen, the outer layers of lower mass starsStellar windGradually separated, and finally evolved intoWhite dwarfAnd form planetary nebulae.Although many star clusters disperse gradually before most members become white dwarfs, the number of white dwarfs in open star clusters is still far below the expected value depending on the age and initial mass distribution of star clusters.One hypothesis is that whenRed GiantThe outer layer ofPlanetary nebulaPreviously, the uneven distribution of matter would accelerate the star itself by thousands of kilometers per second, enough to push it out of the cluster.
Ultimate fate
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Many open star clusters are very unstable and have low mass, which makes the escape speed of star clusters lower than the average speed of their members, so they will disperse rapidly within millions of years.
When the surrounding clouds dissipate, open clusters often have enough gravity to exist independently for tens of millions of years, but they will still gradually disperse.The collision of the members in the cluster often makes one of them get high enough speed and leave the cluster. Repeated collisions cause the members of the cluster to slowly "evaporate".
On average, every 500 million years, one open star cluster will be affected by external influences.When an external star approaches, the star cluster is affected by the tidal force brought by the star, forming aStellar flowAll members move in a similar direction and speed.The speed of cluster disintegration depends on the initial star density. The higher the density, the longer the cluster life.Of an open clusterhalf lifeAbout 150 to 800 million years.
When an open star cluster gradually disperses, the whole team members keep a similar track, which is calledStar AssociationPhenomenon.ursa majorSeveral members of the Big Dipper in China once belonged to an open star cluster, but now they are in a state of star consonance.Finally, the speed gap between stars in the star association gradually widened and gradually dispersed.
Research on stellar evolution
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Open clusterHerotuIt shows that most stars areMain sequence star。Some stars with the highest mass gradually began to deviate from the main sequence band and became red giant stars. By analyzing the gradually deviated positions, astronomers can calculate the age of the star cluster.
Because the distance and age of the members of the open star cluster from the Earth are roughly the same, their magnitude difference only comes from their different masses;When comparing members, many parameters are fixed.Because of this feature, open clusters are suitable for studying the evolution of stars.
For the lithium andberylliumThe research of can make astronomers know more about its evolution and internal structure.Although hydrogen nuclei can only be fused into helium at a temperature of 10 million K, lithium and beryllium no longer exist at 2.5 to 3.5 million K.This property means that the elements contained in a star are closely related to the mixing degree of its internal elements.By studying the internal elements, astronomers can accurately estimate the age and chemical composition of stars in open clusters.
Research shows that the content of lighter elements in open cluster stars is much lower than the predicted value.Although the reason cannot be fully explained, one possibility is that the convection inside the star will invade areas with high radiation energy.
distance
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M11 is close toGalactic centerAn open cluster of stars.Measuring distance is an important step in studying stars, but most stars are too far away from the earth to be measured directly.Using a series of indirect methods related to each other is the only way to measure distant stars, and open star clusters are an important part of this series of methods.
There are two ways to measure the distance from the nearest open star cluster to the Earth.First, the accurate distance can be obtained directly by measuring the parallaxIndependent starAll apply.Several open star clusters within 500 light years from Earth, includingPleiadesAnd the Hyades cluster.EbaguweiThe star also accurately measured the distance of some other open clusters.[3]
Another way to measure distance directly is called“Moving clusterBy virtue of the consistency of the motion of the members in the star cluster.By measuring the orbits of stars in star clustersVisual movementBy comparison, you can find the vanishing point.Then, by studying the spectrum, the radial velocity of the star can be obtained according to the Doppler effect, and then the distance of the star can be obtained by a simple trigonometric method in combination with the self trajectory.Through this method, the accurate distance from the Hyades cluster to the Earth is 46.3 seconds, which is also the nearest open cluster to the Earth.
After finding the distances of several star clusters closest to the Earth, the distances of the further stars can be obtained through indirect methods.By comparing the Herloths of two open clusters, the distance of the far cluster can be calculated.The farthest known open star cluster is Berkeley 29, which is about 15000 seconds away from Earth.Local galaxy groupMany open star clusters have been found in many galaxies in.
The accurate data of open cluster distance is useful for the study of Cepheid variablesPeripheral light relationIt's critical, and Cepheid isStandard candleAn important part of a sequence.After the accurate distance of Cepheid variable can be obtained, the research on the distance of celestial bodies can be extended to the nearest galaxies in the local galaxy group.
Related information
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Open star clusters are irregular in shape, including dozens to two or three thousand stars. The member stars are loosely distributed, and it is easy to separate the member stars one by one when observed with telescopes.A few open star clusters can be seen with the naked eye, such as those in TaurusPleiadesandBistar cluster, CancerPraesepe wait.
More than 1000 open star clusters have been found in the Milky Way.They are highly concentrated on both sides of the galactic plane, and the distance from the galactic plane is generally less than 600 light-years.Most have known that open clusters are within 10000 light years of the sun.There is no doubt that further open star clusters exist. They can not be identified in the dense background of the Milky Way, or can not be seen because they are blocked by interstellar dust clouds.It is estimated that there are 10000 to 100000 open star clusters in the Milky Way.
The diameter of open clusters is mostly in the range of 3 to 30 light-years.Some open clusters are young and associated with nebulae (e.gPleiades）Some even form stars.
Cancer(Cancer) old open star cluster M67 or NGC2682.The distance is 2600 light-years, the brightness is 6.9 magnitude, the age is more than 5 billion years, the right ascension is 8h50.4m, and the declination is+11 ° 49 '(2000.0).
Open clusters in the center of the Milky WayArches。The mass is very large and the density is very high. It is composed of thousands of stars.HST shooting.
Quintuplet, an open star cluster in the center of the Milky Way.It is a young star cluster with very large mass and high density. It is not older than 4 million years and is composed of red giant stars and Wolf Rayet stars.HST shooting.
Pleiades in Taurus.It is 417 light years away and consists of more than 1000 stars.Hyades in Taurus.It is composed of more than 300 stars, and the whole cluster moves collectively in space, so it is also called a moving cluster.
Perseus cluster.Double open star clusters in Perseus.
Open cluster
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Members of the cluster
Many open star clusters can be seen with the naked eye, such as the Pleiades cluster in Taurus, the Hyades cluster and most stars in Ursa Major. We are too close to the Ursa Major cluster to make it one of its membersSiriusIt seems to be in a completely different part of the sky. But you should know that the sun is neither a member of this star cluster nor a member of other known star clusters
Bistar cluster
Without exception, all open star clusters are close to the central plane of our Milky Way Galaxy. This prominent configuration makes it difficult for us to identify open star clusters that are far away from the sun. These remote open star clusters are completely submerged by the dense background stars in the Milky Way Galaxy
However, the members of the star cluster can be identified by their movements. If the star cluster is close to the Earth, the stars that belong to the cluster physically can be identified by using the common self motion. If a star in the cluster has a completely different self motion, it may be located behind the cluster.It may be somewhere between the star cluster and the earth. Although each member of the cluster also moves relative to the whole cluster, its speed is much smaller than the speed of the cluster's space movement. Among all open clusters, the Hybrids cluster is the most suitable for explaining the common principle of motion, because the cluster is not only close to us, but also shows itself clearly, and has a compact structure,One picture can capture the whole star clusterTaurusMiddle shelledA desperate battleIt is a binary star. However, its own motion indicates that it does not belong to the Hyades cluster. We have measured that its parallax gives a distance of only 58 light-years, while the Hyades cluster is 130 light-years away, so it is indisputable that Aldebaran is not a member of the Hyades cluster. All the stars belonging to the Hyades cluster gather in the space with a diameter of about 33 light-years
For more distant clusters, the method of identifying cluster members by research itself is no longer practical, because their own motion will be reduced once they are far away. However, at this time, the determination of apparent velocity can be used to identify cluster members, for example.The apparent velocity of Aldebaran is+30 miles per second instead of+21 miles per second, and the latter value is the apparent velocity of the Hyades cluster
Color I photometry
Because the brightness and color of stars can be measured with a photoelectric cell with extremely high accuracy, it is not only convenient but also useful to compile a Herrault map of star cluster members. Those stars in the field that meet the expected pattern are truly members of star clusters;Stars whose positions in the Herot diagram are quite different from those of the members of the cluster can be selected from the clusterSpectral parallaxIt can also be used to judge the members of star clusters that are difficult to determine. The use of phototubes to determine the color of stars can replace the Herot spectrum sequence. Because the spectral type and color of stars are both related to temperature, these two methods are often used alternately. However, because the spectral classification also depends on the atmospheric pressure of stars, the color of giant stars anddwarf starIt should belong to slightly different spectral type
Spectral temperature color index
B0 25000K -0.32
B5 15600K -0.16
A0 11000K 0.00
A5 8700K +0.15
F0 7600K +0.30
F5 6600K +0.44
G0 6000K +0.60
G5 5520K +0.68
K0 5120K +0.82
K5 4400K +1.18
M0 3600K +1.43
The color of a star is evaluated by its color index. This value represents the relative brightness of the blue and yellow regions of the spectrum. By adding an appropriate filter in front of the photocell, it can be measured that the color of the star is close to that estimated by visionApparent magnitudeThis magnitude is calledVisual magnitude5. Use another filter.It can also measure a magnitude close to that established by photographic method;This magnitude is called photographic magnitude B, which is equivalent to the magnitude determined by blue light.The visual star magnitude depends on the color sensitivity of the eye, which reaches a maximum in the green yellow range of the spectrum. The photographic star magnitude depends on the sensitivity of the commonly used photographic emulsion, which is most sensitive to blue light, ultraviolet light and near ultraviolet light. A blue star appears brighter in the photographic star magnitude system than in the visual star system;For a red star, the opposite is true. According to international agreements, these two systems have been calibrated to ensure that the magnitude of an AO star in the two systems is exactly the same
The color index of a star is the difference between its photographic magnitude and its visual magnitude. In calculation, the color index is obtained by subtracting the self viewing magnitude V from the photographic magnitude B. Therefore, the color index of O or B stars will be negative;These two types of stars appear brighter in the camera magnitude system, so their camera magnitude is numerically lower than the visual magnitude. Any star later than the AO type (such as a G type star) has a positive color index because it appears darker in the blue region,Therefore, their values on the photographic magnitude scale are higher than those on the visual magnitude scale. The right table lists the color indices of main sequence stars of various spectral types. This table is compiled according to the surface temperature of the sun (G2 type), which is 5730K everywhere. In order to be consistent with the temperature of stars, this temperature value has been corrected for the edge darkening effect. The color index of the sun is+0.65
Herotu
Since the determination of the color index is faster and more objective than the determination of the spectral type, the color photometry map is usually drawn for star clusters in practice. Essentially, the color photometry map is the same as the spectrum photometry map (i.e. Herot map)
The color photometry can not only enable astronomers to identify members of distant star clusters, but also provide valuable information about the evolution of stars in many star clusters. The left figure is the color photometry of several open star clusters. All star clusters haveMain star orderAnd the stars on the main branch, but the branches occupied by different star clusters are different. The double star cluster in Perseus has many blue hot bright stars. The hottest star in the Pleiades star cluster, although very bright, is not an O-type star. The hottest star in the NGC188 is not much hotter than the sun. It is inferred that these differences are due to the different age of the star cluster. The youngest star cluster NGC2362,Its age may be less than one million years. But in NGC 188, even a medium star is evolving from the main star sequence to the red giant stage;Therefore, it must be the oldest star cluster on this map. According to estimates, the age of the Hyades cluster is 900 million years
C. The Pleiades star cluster is 400 light-years away from the sun, because it contains early B starsAstronomical timeIn terms of scale, it is in its infancy. If the B-type star with the mass of nine suns shrinks toMain star orderIt is estimated that it will take about 21 million years to exhaust the hydrogen in its core and start to expand to the red giant star. Therefore, this value should be the age of the open star cluster. However, only the color photometry of the Pleiades star cluster clearly shows that it only contains 0.2Solar massThe stars ofZero age main star orderUp. As recently asStellar evolutionTheoretically, it is estimated that the time required for a star with a mass of 0.2 sun to shrink to the zero age main sequence is about 600 million years. So, what is the age of the Pleiades star cluster?Is it 21 million years or 600 million years?
M45 Pleiades
In fact, the contradiction is not as sharp as it seems. Herbisch believes that before the formation of massive stars, low-mass stars have been formed from nebular matter. If the late type main sequence stars are formed first, they willEarly type starshrink toMain star orderArrive within the same time ofZero age main star orderAnd then burn the hydrogen in their cores and begin to expand towards the red giant stage. This theory seems to be supported by observationsStar formationAnother early evidence is Taurus IAurigaDark nebulaThese nebulae contain a large number of faint red stars, but not bright blue stars.
All this shows that the Pleiades star cluster with about 300 stars began to form 600 million years ago, and continued to form type B stars. These very bright stars radiate extremely rich ultraviolet rays. They have ionized and completely blown away the gas, leaving only traces of gas in the Milky Way,The process of star formation tends to stop.Think about itThe Rosette NebulaInterestingly, there is a group of very bright stars in its center. This nebula may be formed by this process of blowing gas from the center outward. This assumption may explain the ring structure of this extremely interesting gas and star assembly
NGC2237 Rosette Nebula
d．Star AssociationGuanganStar formationAll these theories ofOrionThe support of the complex. As mentioned earlier, there are some Herbig Harrow objects and too manyT Tauri stars There are many O and B stars in the Orion sky, which form a very loose community, the so-called star association. The star association is not a star cluster,However, there are many stars of the same type in the same region of the Milky Way. Stars in the Orion region are mixed together in a huge and thin hydrogen cloud 1500 light-years away from the sun. Only a small part of this hydrogen cloud can be seen by the naked eye, which is often saidOrion nebula It is just a group of very hot and bright stars in the central area of this hydrogen cloud. The diameter of the entire hydrogen cloud is more than 300 light-years, while the diameter of the visible part is only about 20 light-years. Radio observations show that the gas is distributed in a symmetrical pattern around this group of hot blue stars. The uneven brightness and darkness in optical photos are due to the presence ofInterstellar cloudAs a result
M42 Orion Nebula
wholeOrionThe research results of the complex show that it is expanding. The hot blue stars in its center are not too old. According to their movement, these hot blue stars were formed in a relatively small space about 10000 years ago. The research on gas movement and ionization degree shows that the age of the nebula itself is about 10000 to 20000 years. It is also possible that,It started a million years agoDark nebulaFor a long time, some stars with smaller mass have been formed during this period, which are now dark red stars. However, once those giant stars are formed in the central region, because their surface temperatures are between 20000K and 50000K, the gas will be ionized and begin to expand. As the expansion process continues,The star formation process in the central region must slow down
OrionStar AssociationIt can be seen from the color photometry map that stars with very large mass have not only developed toMain star order, and has already burned off quite a lot of hydrogen, thus starting the process of evolution to the right in the Herot chart. Medium mass stars areZero age main star orderHowever, stars with very small mass have not yet reached the zero age main sequence. Because their contraction is extremely slow, stars with very small mass will reach the main sequence at the latest even if they start to form first. This is due to their unfavorable conditions. In the Orion complex,The fact that a large number of young stars are mixed in the hydrogen matrix strongly supports the idea that stars are generated from gas clouds
However, it is still unclear thatStar AssociationWhy does it decompose immediately or eventually become an open star cluster soon after its formation? One view is to study the percentage of cloud clusters condensed into stars. It believes that the cloud clusters originally gathered by the gravitational action of the gas itself that makes up it will most likely condense into stars, while only a small amount of gas will escape. In this case,Most of the mass still exists, and its gravitational field is still enough to attract these stars to a more permanent star cluster. On the contrary, if most of the gas in the cloud cluster has not condensed into stars, it is likely that it will be blown away by the particles of the formed stars. Therefore, the total gravitational field will weaken, and the star association may soon collapse
Latest findings
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German scientists found nine open star clusters near the solar system
Open cluster
University of Heidelberg, GermanyNineteen percent of astronomers found that there are nine new open star clusters around the solar system, which will change our understanding of the stellar system near the solar system.Studying star clusters is an important step to understand the evolution history of the Milky Way. More than 1000 similar structures have been found in the Milky Way, some of which are giant molecular clouds.But scientists are still looking for more open star clusters, especially near the solar system.Sigfried Rother, an astronomer from the University of Heidelberg in Germany and the Strasbourg Observatory in France, and others observed 2.5 million bright stars in the night sky. These nine star clusters are distributed 1500 light-years away from the sun.
The study of star clusters is an important part of understanding the formation of stars and the history of the Milky Way.Stars developed from the same giant molecular cloud sometimes form a collective loosely bound by gravity, which is what we call open star cluster.Recent observations and studies show that older open star clusters are very rare within 3000 light-years from the sun.The low-density distribution of older open star clusters in space is the subject of a survey by Heidelberg University.In order to find the target star, the researchers analyzed the Tycho-2 catalog andUS Naval ObservatoryAutomatic Astrometric Catalogue (URAT1).
The Tycho - 2 catalog is an astronomical reference catalog, which contains the location and self motion information of the brightest 2.5 million stars in the sky, as well as the results of two-color photometric measurements of stars.In order to test how the star's self motion data more accurate than Tycho-2 can help the research of star clusters.Scientists also used the United States Naval Observatory's Automatic Astrometry Catalogue (URAT1), which contains location data of about 22.8 million stars with magnitudes ranging from 3.0 to 18.5.Researchers believe that we combined the Tycho-2 catalog and the US Naval Observatory's automatic astrometry catalog to infer the star's own motion, and found nine open star clusters in the vicinity of the solar system between 70 million years and 1 billion years old. The distance between these star clusters and the sun is within 200 to 500 seconds.[4]
On May 4, 2022, 541 new open star clusters, named and numbered "CWNU" (West China Normal University), which were discovered by Dr. He Zhihong of West China Normal University, were officially published in the top international journal Astrophysical Journal Supplement.The "CWNU star cluster catalog" has become the first large sample catalog named after Chinese universities and research institutes in the world.[5]
From May 8 to 10, 2023, Venus will perform a "moment of intimacy" with another open star cluster M35 (also known as shoelace cluster), which is an opportunity to photograph planets and deep space objects.[6]
