Since ancient times, man has turned his gaze to the heavens, where countless star clusters shone, inaccessible but alluring with their unique beauty.
The drawings of stars that the ancient inhabitants of the Earth saw were formed into various bizarre pictures, which were assigned sonorous epic names. The Andromeda nebula, the constellation Cassiopeia, Ursa Major and Hydra are only a small part of the names that allow us to judge what associations were evoked by the distant amazing luminaries sparkling on the dark canvas of the sky. It was believed that the destinies of people are inextricably linked with the relative position of the stars, which can bring wealth, happiness and luck to those born under them, as well as bitterness, troubles and disappointments.
The importance of star clusters for astronomy
Star cluster Messier 7, ESO image
With the development of civilization, mystical and poetic ideas about the structure of the firmament have significantly changed and systematized, acquiring much more rational outlines, but historical sonorous names preserved. It turned out that stars that appear to be closely located can actually be far from each other and vice versa. Therefore, it became necessary to create a star hierarchy corresponding modern ideas about the universe. So, in astronomical classification The term “star clusters” appeared, uniting a group of stars moving in their galaxy as one whole.
These formations are extremely interesting because the luminaries included in them were formed approximately simultaneously and are located, by cosmic standards, at the same distance from the earthly observer, which gives additional features, allowing comparison of radiation from various sources one cluster without appropriate corrections. The signals coming from them are distorted in the same way, which greatly facilitates the work of astrophysicists studying structure and evolution star systems and the Universe as a whole, the principles of galaxy formation, the processes of star formation and their destruction, and much more.
Types of star clusters
Hubble on star clusters
Star clusters usually divided into two large groups: spherical and scattered. But from time to time they try to supplement this classification, since not all identified space formations strictly fit into one category or another.
Globular clusters
Globular clusters, and there are more than ten thousand of them in some galaxies, are old formations even by universal standards, over 10 billion years old. Being, most likely, the same age as the Universe, they can tell a lot to scientists who have managed to read the information they emit.
Gallery of globular clusters
These clusters have a shape close to a sphere or an ellipsoid, and consist of tens of thousands of stars of various sizes - from ancient red dwarfs to young blue giants, which are born in the cluster itself during collisions of the stars inhabiting it.
Open clusters
Open clusters are much younger than globular clusters - the age of such stellar conglomerates is usually estimated at hundreds of millions of years. They can only be detected in spiral or non-spiral galaxies. correct form, which tend to continue star formation processes, unlike, for example, elliptical ones.
Gallery of open clusters
Open clusters are much poorer in stars than globular clusters, but when observing them, you can see each star separately, since they are located at a considerable distance from each other and do not merge in the common sky.
Star associations
By analogy with political and economic spheres life celestial bodies They are also capable of creating temporary associations, which in astronomy are called “stellar associations.”
These formations are considered the youngest in the Universe and are no more than tens of millions of years old. The gravitational bonds in them are very weak and insufficient to maintain the stability of the system for a long time, and therefore they must inevitably disintegrate in a fairly short time.
It is believed that the association could not have arisen through the gravitational capture of passing stars, which means that the latter were born along with it and are approximately the same age. Compared to clusters, the number of “associated members” is not large and is measured in tens, and the distance between them is up to several hundred light years. WITH scientific point From our point of view, the discovery of such new formations confirms the theory of the continuation in the Universe of the processes of the birth of new stars, not individually, but in entire groups.
New discoveries
Until recently it was believed that globular clusters- the oldest stellar formations, which, due to age, should have lost the dynamics of their internal rotational movements and they can be considered as simple systems. However, in 2014, researchers from the Max Planck Institute for Extraterrestrial Physics, led by Maximilian Fabricius, long-term observations behind 11 globular clusters Milky Way established that they central part continues to rotate.
Majority modern theories is not able to explain this fact, which means that if the information is confirmed, then changes are possible as in theoretical aspects knowledge and applied mathematical models, describing the movement of spherical associations.
How are star clusters born? How do they differ, how are they located in the space of our Galaxy, and how is their age determined? Doctor of Physics and Mathematics Alexey Rastorguev talks about this.
Explore star clusters Universe: description, classification, scattered and spherical types with photos and videos, list of clusters, age, Messier catalog, destruction.
– a group of stars with common origin and gravitational connection on certain time. They are a useful tool for astronomers to study and model stellar evolution. There are two main types of star clusters: open clusters (open) and globular clusters. Learn more about the galaxy's star clusters in an interesting video.
Astronomer Alexey Rastorguev on the role of gravity in the Universe, open and globular clusters and the study of the history of galaxies:
Types of star clusters
So called because individual stars can be easily resolved. For example, the Pleiades and Hyades are so close that individual stars can be seen without any problems with the naked eye. Sometimes they are called galaxy clusters, as they are located in dusty spiral arms. Stars in an open cluster have a common origin (they were formed from the same initial molecular cloud). Typically, a cluster contains several hundred stars (can reach several thousand).
Stars are bound by gravity, but it is quite weak. The cluster rotates around the galaxy and in the final stage is dissipated due to gravitational contact with stronger objects. It is believed that the Sun appeared in an open cluster, which now no longer exists. Therefore, these are always young objects. Nebula is still visible in the Pleiades, hinting at recent formation.
Open clusters are filled with population I stars - young and with high level metallicity. The width ranges from 2 to 20 parsecs.
Open star clusters of the Messier catalog
Other famous open star clusters
Globular clusters galaxies contain from a couple of thousand to a million stars located in a spherical gravitational system. They are in a halo and represent the most ancient stars - population II (developed, but low metallicity). The clusters are so old that any star (above class G or F) has already crossed the main sequence. There is little dust and gas in a globular cluster because new stars do not form there. The density in the interior regions is much higher than in areas near the Sun.
In globular clusters, stars also share a common origin. But this type holds objects firmly by gravity (stars do not scatter). There are approximately 200 globular clusters in the Milky Way. Among them we can recall 47 Tucanas, M4 and Omega Centauri. Although there are suggestions about the latter that it could be a dwarf spheroidal galaxy.
Globular clusters
Astronomer Vladimir Surdin about the types of star clusters, the galactic nucleus and life on the planets of globular clusters:
Globular star clusters of the Messier catalog
Other famous globular star clusters
Age of star clusters
Star clusters are incredibly valuable to astronomers because they can be used to determine the age of a star and track its evolution.
Stars in open clusters have a single origin, so their metallicity level converges, which means that all members will pass through the same path. evolutionary stages. In addition, they are located at the same distance, which also allows you to display the absolute value. If you see bright stars that stand out, they are much brighter than their fainter neighbors.
With this information, scientists create digital diagrams for the clusters. They display the apparent value of V on vertical axis relative to the digital index B – V horizontally. Using spectrographic parallax, the diagram can be calibrated to derive an absolute value.
If we build diagrams for them, we get the bottom graph. Since they are at different distances, it is calibrated to absolute values.
A new scale is visible on the right vertical axis. “Years” is the age of the cluster. The couple in Perseus is so young that most of stars are at the stage main sequence. The Pleiades are slightly older and do not have stars above color index 0 ( spectral class A0). More massive objects have already stepped over to the giant branches. M67 does not have a star hotter than color index 0.4. Highest value has a turning point in the diagram where the cluster switches off the main sequence. The lower the main sequence, the older the cluster.
Globulars are usually much older than open ones, so the colored magnitude in the diagram shows more developed stars. They are also deprived of objects with large mass. This point is illustrated below using the M55 example.
A group of hot main sequence stars above the shutdown point is visible here. They are called blue stragglers. Scientists believe that due to the high stellar densities in globular clusters, some are capable of merging. The combined mass makes the star hotter and brighter than the main stellar mass. Star clusters are not eternal structures and they are destroyed. Learn this process in the video. Also use an online star map to find the clusters yourself. If you can’t buy a telescope, then visit our page with a virtual model of the Milky Way galaxy or look at photos from the list of clusters.
Collapse of star clusters
Astronomer Alexey Rastorguev about stellar dynamics, the lifetime of star clusters and gravitational potential of our Galaxy
IN outer space the stars are unevenly distributed. In some places they form groups. These groups are called star clusters and associations, depending on the size and concentration of stars.
Star clusters
star cluster - gravitationally bound group of stars, having a common origin and moving in the gravitational field of the galaxy as a single whole. Some star clusters also contain, in addition to stars, clouds of gas and/or dust.
Star clusters are divided into two types - spherical and scattered. It was opened in June 2011 new class clusters, which combines the characteristics of both globular and open clusters.
Globular star clusters
This is the name given to star clusters that differ from open clusters in having a larger number of stars, clearly defined symmetrical shape, close to spherical, and an increase in the concentration of stars towards the center of the cluster. The picture in the preamble shows the globular cluster M13 in the constellation Hercules. It contains several thousand stars. The concentration of stars in the central regions of globular clusters is 700-7000 times greater than in the vicinity of the Sun.
The first globular star cluster M22 was discovered by a German amateur astronomer Johann Ihl back in 1665. But due to the imperfections of the first telescopes, it was impossible to distinguish individual stars in a globular cluster. The first person to identify stars in a cluster was Charles Messier while observing the globular cluster M4. Then globular star clusters began to be actively studied, and to date, 152
clusters out of an estimated total of 180 ± 20. These undetected globular clusters are thought to be hidden behind clouds of gas and dust. Most globular star clusters in the Milky Way are located in close proximity from the galactic core and a larger number of them are located on the side of the astronomical sky in the center of the core.
To date, the formation of globular clusters is not fully understood and it is still unclear whether a globular cluster consists of stars of one generation, or whether it consists of stars that have gone through multiple cycles over several hundred million years. In many globular clusters, most stars are at approximately the same stage of stellar evolution, indicating that they formed at approximately the same time. But some clusters contain different populations of stars, such as the globular clusters in the Large Magellanic Cloud. These clusters arise mainly in regions with efficient star formation, that is, where the interstellar medium has more high density compared to normal star-forming regions. To date, no globular clusters with active star formation are known. This is consistent with the view of them as the oldest objects in the galaxy, which are composed of very old stars.
Globular clusters typically consist of hundreds of thousands of old stars with low metallicity. They lack gas and dust, and are assumed to have long since turned into stars. Globular clusters have a high concentration of stars. Some globular clusters, such as Omega Centauri in the Milky Way, are extremely massive (several million solar masses) and contain stars from several stellar generations. This is evidence that supermassive globular clusters are the core dwarf galaxies engulfed by giant galaxies. About a quarter of the globular clusters in the Milky Way may have been part of dwarf galaxies.
The age of the globular clusters of our Galaxy is approaching its age. Relatively young globular clusters are also observed in other galaxies (for example, the Magellanic Clouds).
Open star clusters
These are clusters that, unlike globular ones, contain relatively few stars, and the cluster can often have irregular shape. In our galaxy and similar galaxies, open clusters are collective members of the galaxy. Some such clusters have been known since ancient times, for example. Others were known as fuzzy nebulae, and it was only with the invention of the telescope that they could be separated into their constituent stars.
open star cluster Pleiades (M45) has other names: Seven Sisters, Stozhary(Russian), Hima(in the Bible and Torah). This cluster is located in the constellation Taurus, is one of the closest star clusters to Earth and one of the most visible to the naked eye. The Pleiades are clearly visible in winter in the northern hemisphere and in summer in southern hemisphere(except Antarctica and its environs). It has long been known that the Pleiades are a physically connected group of stars, and not stars at different distances from the Earth that accidentally happened to be nearby on celestial sphere. Priest John Michell in 1767 calculated the probability of a random combination of such a number bright stars in such a small area of the sky. This probability turned out to be 1:500,000, and he suggested that the Pleiades, like other star clusters, should be physically connected. This was confirmed when the first measurements were taken relative speed their stars: it turned out that their own motions are very close, which indicates their gravitational connection. The Pleiades star cluster is about 12 light-years in diameter and contains about 1,000 stars. Many of them are multiples, i.e. total number There are about 3000 stars in the cluster. Hot blue stars predominate there. Up to 14 of them can be seen with the naked eye.
There are many brown dwarfs in the Pleiades, and there are several white dwarfs.
Due to the fact that the Pleiades are clearly visible naked eye, they are often mentioned in many cultures - both ancient and modern.
Star associations
Star associations- these are groups of gravitationally unbound stars or weakly bound young (up to several tens of millions of years old) stars united by a common origin. It is the lack of gravity that distinguishes them from star clusters.
Star associations discovered V. A. Ambartsumyan in 1948 and predicted their collapse. He also coined the term “star associations.” Subsequently, other astronomers confirmed the fact of the expansion of stellar associations. Stellar associations are larger in size than young open star clusters and lower in density: the number of stars in an association is from tens to hundreds (in open star clusters - from hundreds to thousands). Stellar associations occur in the star-forming regions of molecular cloud complexes, and star formation in them also occurs in modern era.
There are star associations following types:
OB associations containing mainly massive stars of spectral types O and B.
T-associations containing mainly low-mass variable stars of the T Tauri type.
R-associations (from R - reflection), in which stars of spectral classes O - A2 are surrounded by reflection gas and dust nebulae.
Stellar associations are also observed in other galaxies, including in our Galaxy signs of star formation in stellar associations are observed. Study of star associations – important stage in studies of the evolution of stars and their systems, it is an important center of star formation.
STAR CLUSTERS, groups of stars connected by mutual forces gravitational attraction and having a common origin, similar age and chemical composition. The number of stars in one cluster can range from 20-30 to several million. Typically, star clusters have a dense central clump (core) surrounded by a less dense coronal region (corona). The diameters of star clusters range from several pc to 280 pc. With regard to the star clusters of our Galaxy, they have historically been divided into open and globular. The difference between them is mainly determined by the mass and age of these formations. Open star clusters are relatively young and typically contain tens to thousands of stars, while much older globular star clusters contain tens of thousands to several million stars. Because globular star clusters are rich in stars, they appear more regular, spherical, while open star clusters have a more lumpy appearance (Figures 1 and 2). Examples of open clusters are the Pleiades and Hyades; examples of globular clusters are M3 in the constellation Canes Venatici and M13 in the constellation Hercules.
The designations and names of star clusters do not have a specific system. Some of the star clusters were discovered even before the invention of the telescope and therefore have their own historical names, such as the Pleiades and Hyades, as well as a nebulous speck in the constellation Cancer known as the Manger. In Manger, 160 pc distant from us, the brightest stars have a magnitude of about 6.5 magnitude: only an extremely keen eye can distinguish them, and only in a very dark sky; but through binoculars this cluster is visible very well. Several more clusters can be seen with the naked eye as faint “foggy stars,” but it was impossible to guess their true nature before the invention of the telescope. There are also counter examples: in the constellation Coma Berenices, a scattering of faint stars has long been known, giving it its name. Although these stars are clearly visible to the naked eye, the fact that the scattering of stars in Coma Berenices is not a random grouping in the sky, but is a single cluster, was proven only in 1915.
Most star clusters are designated by numbers according to some catalogue; Often one star cluster has several designations. For example, a bright globular cluster in the constellation Hercules is designated as M13 according to the Messier catalog, and according to the New general catalog nebulae and star clusters (New General Catalog of Nebulae and Clusters of Stars, NGC), published by J. Dreyer in 1888, this cluster is designated NGC 6205. The Pleiades have their own number in the Messier catalog (M45), but they are not in the NGC catalog. Some star clusters were first mapped as stars and given appropriate designations, only later being assigned numbers in catalogs of non-stellar objects, for example the globular clusters 47 Tucanae (NGC 104) and Omega Centaur (ω Cen, NGC 5139).
Open star clusters in our Galaxy are concentrated in the plane of symmetry of the Milky Way (galactic plane) and have low velocities relative to the Sun (on average 20 km/s). Among them, we can distinguish star clusters concentrating towards the spiral arms of the Galaxy, which arose relatively recently (less than 100 million years ago), and star clusters of intermediate age (disk clusters), which do not show connections with the spiral arms and are weaker concentrated towards the Galactic plane. So far, a little more than 1,500 open star clusters have been discovered and studied, but many thousands more are probably hidden in remote regions of the Galaxy, hidden from us by clouds interstellar dust. All open star clusters have normal metal contents characteristic of stars in the flat component of the Galaxy.
Globular star clusters in the Galaxy are distributed in a spheroidal volume, the center of which coincides with the center of the Galaxy, are highly concentrated towards this center and are characterized by high velocities relative to the Sun (on average 170 km/s). By the beginning of the 21st century, about 160 globular clusters had been discovered. Only those hiding behind the dust clouds of the galactic disk could remain unnoticed; but since globular star clusters are distributed throughout the Galaxy, and not just in its disk, there should be few such undetected clusters. Calculations show that there are no more than 200 globular star clusters in the Galaxy. They are usually poor in metals, but objects observed in the central region of the Galaxy are richer in metals than those observed on the periphery of our star system.
Important information about the evolution of star clusters is obtained from the study of Hertzsprung-Russell diagrams. For typical open and globular star clusters in the Galaxy, these diagrams are significantly different. Open clusters at the main sequence stage have significantly more massive stars than globular ones. In some open star clusters there are stars with a mass of up to 15-20 Μ Θ (Μ Θ is the mass of the Sun). These brightest stars in open clusters have short lifespans, indicating that the clusters themselves are young.
In globular star clusters, the luminosities of the vast majority of stars are small. These stars are at the main sequence stage, their masses are less than 0.7-0.8 Μ Θ. The brightest stars in globular star clusters are relatively few red giants that are in late stages of evolution (after leaving the main sequence on the Hertzsprung-Russell diagram, when the cores of stars have already run out of thermonuclear reactions with the participation of hydrogen); their masses are about 0.8 Μ Θ. Interpretation of Hertzsprung-Russell diagrams from the point of view of the theory of stellar evolution allows us to conclude that the stars of globular star clusters are 12-14 billion years old, i.e. they are much older than the stars open clusters.
The kinematic characteristics and spatial distribution of globular clusters of the Galaxy reflect the features of the distribution of matter from which early stage existence of the Galaxy, these formations arose. In the modern era in the Galaxy, star clusters appear only near the galactic plane, in the regions of gas-dust spiral branches. In this case, relatively low-mass star clusters are formed. In some neighboring galaxies, very massive young star clusters similar to globular ones are observed (for example, NGC 1866 in the Large Magellanic Cloud).
Star clusters form in the depths of giant clouds interstellar matter due to its gravitational instability. As a rule, this occurs in the densest and coldest part of the cloud - in its core. Once massive stars appear in a forming star cluster, they heat up the surrounding cloud and destroy it. Together with the remnants of gas, the fastest-moving stars also leave young star clusters, forming a stellar association. The remaining stars, which have retained their gravitational connection with each other, form a relatively long-lived star cluster.
Under the influence of external and internal forces dynamic evolution of star clusters occurs. Rendezvous between stars in cluster cores leads to mutual exchange the energy of their movement. As a result, some stars receive excess energy and immediately leave the cluster or move to the corona region, from where they later “evaporate” under the influence of gravitational disturbances from the Galaxy. The process of destruction of star clusters is intensified under the influence of gravitational “pushes” from massive clouds of interstellar matter flying past them. The gravitational “pushes” from giant molecular clouds, whose masses reach 10 6 Μ Θ, are especially strong. Star clusters with a small number of members, i.e., open clusters, are destroyed most quickly. Therefore, of the old star clusters in our Galaxy, only the most massive - globular ones - have survived.
Among the dim members of young open star clusters, Orion and flare variable stars are commonly observed. Cepheids are sometimes found among the bright stars in open star clusters. Some globular star clusters contain variable stars such as RR Lyrae and W Virgo. X-ray sources have been discovered in the cores of two dozen of the densest globular star clusters. They are associated with close binary systems that have a neutron star or black hole, surrounded by an accretion disk.
The star clusters closest to the Sun (for example, the Hyades), in the proper movements of whose members, perspective phenomena (directions) are observed own movements when they continue on the celestial sphere, they intersect at one point) are called moving. Moving star clusters play a special role in the problem of determining stellar distances, since the distances to them can be reliably determined by a simple geometric method.
See also Stellar astronomy,Stellar dynamics.
Lit.: Kholopov P. N. Star clusters. M., 1981; Spitzer L. Dynamic evolution of globular clusters. M., 1990.
P. N. Kholopov, V. G. Surdin.
According to modern data, at least 70% of the stars in the Galaxy are part of binary and multiple systems, and single stars (such as our Sun) are rather an exception to the rule. But often stars gather into more numerous “collectives” - star clusters. A star cluster is a group of stars located in space close to each other, connected by a common origin and mutual gravity. All stars included in the cluster are at the same distance from us (up to the size of the cluster) and have approximately the same age and chemical composition, but at the same time they are at different stages of evolution (determined by the initial mass of each star), which makes them convenient an object for testing theories of the origin and evolution of stars. There are two types of star clusters: globular and open. Initially, this division was adopted according to appearance, but as we studied further, it became clear that globular and open clusters are dissimilar in literally everything - in age, stellar composition, nature of motion, etc.
Globular star clusters contain from tens of thousands to millions of stars. This type of cluster is characterized by a regular spherical or somewhat oblate shape (which, apparently, is a sign axial rotation clusters). But star-poor clusters are also known, indistinguishable in appearance from open ones (for example, NGC 5053), and classified as globular according to characteristic features spectrum-luminosity diagrams. The two brightest of the globular clusters are designated Omega Centauri (NGC 5139) and 47 Tucanae (NGC 104), as ordinary stars, because due to their significant apparent brightness (+3.m6 and +4.m1, respectively) they are clearly visible to the naked eye, but only in southern countries. And in the middle latitudes northern hemisphere accessible to the naked eye, although with difficulty (even for a dark, unexposed sky), only two are in the constellations Sagittarius (M22) and Hercules (M13).
Omega Centauri is one of the brightest in absolute magnitude, for it it is -10.m2, while for one of the weakest (NGC 6366) it is only -5.m. The linear diameters of globular clusters generally range from 15 to 200 pc, while the concentration of stars in their central regions reaches thousands and tens of thousands per 1 pc3 (in the vicinity of the Sun - only 0.13 stars per 1 pc3). The apparent angular dimensions depend on both the linear diameter and the distance to the cluster, and therefore differ more strongly. The largest is again Omega Centauri (54" - more than one and a half times the apparent diameter of the Moon!), and of those visible in the middle latitudes of the northern hemisphere - M4 in Scorpius (34", and besides, it is one of the closest, up to it is 2 kpc) and the already mentioned M22 in Sagittarius (32"). The smallest ones have a visible angular size is about 1".
There are currently about 150 known globular clusters in the Galaxy, but it is obvious that this is only a small part of those that actually exist (their total number is estimated at about 400-600). Their distribution across the celestial sphere is uneven - they are strongly concentrated towards the galactic center, forming an extended halo around it. About half of them are located no further than 30 degrees from the visible center of the Galaxy (in Sagittarius), i.e. in an area whose area is only 6% of the entire area of the celestial sphere. This distribution is a consequence of the peculiarities of the rotation of globular clusters around the center of the Galaxy, characteristic of objects of the spherical subsystem - in highly elongated orbits. Once per period (108-109 years), a globular cluster passes through the dense central regions of the Galaxy and its disk, which contributes to the “sweeping out” of interstellar gas from the cluster (observations confirm that there is very little gas in these clusters). Some globular clusters are so far from the center of the Galaxy (NGC 2419 - 100 kpc) that they can be classified as intergalactic.
The spectrum-luminosity diagram for globular clusters has characteristic shape due to lack massive stars on the main sequence branch. This indicates a significant age of globular clusters (10-12 billion years, i.e. they were formed simultaneously with the formation of the Galaxy itself) - during this time, the reserves of hydrogen are exhausted in stars with a mass close to the Sun, and they leave the main sequence (and the greater the initial mass of the star, the faster), forming a branch of subgiants and giants. Therefore, in globular clusters the most bright stars are red giants. In addition, they observe variable stars(especially often - the RR Lyrae type), as well as the end products of the evolution of massive stars (those in close dual systems With normal star white dwarfs, neutron stars and black holes), manifesting themselves as X-ray sources different types. But in general in globular clusters double stars are rare. It should be noted that in other galaxies (for example, in the Magellanic Clouds) globular clusters that are typical in appearance have been found, but with a stellar composition of small age, and therefore such objects are considered young globular clusters. Another feature of globular clusters is the reduced content of heavy (heavier than helium) elements in the atmospheres of their constituent stars. Compared to their content in the Sun, the stars of globular clusters are depleted in these elements by 5-10 times, and in some clusters - up to 200 times. This feature is characteristic of objects in the spherical component of the Galaxy and is also associated with the great age of the clusters - their stars were formed from primordial gas, while the Sun was formed much later and contains heavy elements formed by previously evolved stars.
Open star clusters contain relatively few stars - from several tens to several thousand, and, as a rule, there is no question of any regular shape here. The most famous open cluster is the Pleiades, visible in the constellation Taurus. In the same constellation is another cluster - the Hyades - a group of faint stars around bright Aldebaran.
There are about 1,200 known open star clusters, but it is believed that there are many more of them in the Galaxy (about 20 thousand). They are also distributed unevenly across the celestial sphere, but, unlike globular clusters, they are strongly concentrated towards the plane of the Galaxy, therefore almost all clusters of this type are visible near the Milky Way, and are generally no more than 2 kpc from the Sun. This fact explains why such a small proportion of the total number of clusters is observed - many of them are too distant and are lost against the background of high stellar density Milky Way, or hidden by light-absorbing gas-dust clouds, also concentrated in the galactic plane. Like other objects in the galactic disk, open clusters orbit the galactic center in nearly circular orbits. The diameters of open clusters range from 1.5 pc to 15-20 pc, and the concentration of stars ranges from 1 to 80 per 1 pc3. As a rule, clusters consist of a relatively dense core and a more sparse crown. Among open clusters, double ones are known (such as Chi and Al Perseus) and multiples, i.e. groups characterized by their spatial proximity and similar proper motions and radial velocities.
The main difference between open clusters and globular clusters is the large variety of spectrum-luminosity diagrams in the former, caused by differences in their ages. The youngest clusters are about 1 million years old, the oldest are 5-10 billion years old. Therefore, the stellar composition of open clusters is diverse - they contain blue and red supergiants, giants, variables various types- flaring, Cepheids, etc. Chemical composition stars included in open clusters are quite homogeneous, and on average the content of heavy elements is close to the solar one, which is typical for objects in the galactic disk.
Another feature of open clusters is that they are often visible together with a gas-dust nebula - a remnant of the cloud from which the stars of this cluster once formed. Stars can heat up or illuminate “their” nebula, making it visible. The well-known Pleiades (see photo) are also immersed in a blue, cold nebula. In a galaxy, open clusters can only exist where there are many gas clouds. IN spiral galaxies, such as ours, such places are found in abundance in the flat component of the galaxy, and young clusters serve as good indicators of the spiral structure, since during the time that has passed since their formation, they do not have time to move away from the spiral branches in which this formation occurs.
A special type of open cluster is moving clusters, for which it is possible to accurately measure the proper motions of the stars included in it. Examples of such clusters are the Hyades, Pleiades, Manger and some others. The extensions of the directions of these movements (either backwards or forwards) intersect at a point called the radiant - this is convergence parallel lines due to perspective. The study of such clusters is of fundamental importance due to the fact that knowledge of the proper motions of stars, their radial velocities and angular distances to the radiant makes it possible to calculate the full spatial velocity of these stars, and therefore the exact distance to them (more precisely than by the trigonometric parallax method). And knowing the distance makes it possible for at least one cluster to “calibrate” the spectrum-luminosity diagram, i.e. tie it to absolute stellar magnitudes. Such a reference is very important for determining the distances to other clusters from the “spectrum-visible brightness” diagrams obtained directly from observations, since combining the main sequence of such a diagram and the “calibrated” one immediately gives the difference between the visible and absolute values, depending only on distance. It is most convenient to use the Hyades as a “reference” cluster, as it is the closest (40 pc), and it can be said without exaggeration that until recently (before the launch of the HIPPARCOS mission) the entire scale of interstellar distances was maintained on the Hyades.
Star associations- rarefied groups of stars whose age does not exceed several tens of millions of years (with the youngest of them being no more than a million years old). Typically, a stellar association has a size of 50-100 pc and contains from a few stars to several hundreds, thereby differing from young star clusters large size and lower density of stars. The attraction between stars in associations is usually too weak to keep them together, and therefore associations do not last long (by cosmic standards) - in just 10-20 million years they expand so much that their stars no longer stand out from other stars. The existence of star clusters and associations in the Galaxy itself of various ages irrefutably indicates that stars are formed not alone, but in groups, and the process of star formation itself continues to this day. An example of a star association is a group of young blue stars in the constellation Orion, the core of which is the "trapezium of Orion".
Not only the stars included in the clusters, but also the clusters themselves are not eternal. The distances between stars in open clusters are relatively large, which means that the forces gravitational interaction. Over millions of years, due to the tidal action of galaxies, clusters gradually disintegrate - the stars included in them move further and further away from each other and gradually lose their gravitational connections. Sometimes by general movement and the distance to a group of stars, one can guess the former open cluster in it. Such groups are called star streams. Few people know that 5 stars of the Bucket Ursa Major are part of one of these groups (see photo on the left), located especially close to the Sun (about 28 pc), and therefore will occupy the sky large area. This stream consists of about 100 stars, among which are Gemma (alpha of the Northern Crown), and even Sirius!
In the topic of star clusters, it would be useful to finally mention asterisms- characteristic configurations (often of a regular shape, or resembling the outline of some object), formed by random stars that are in no way connected with each other. Large formations are also considered asterisms, such as constellation figures (for example, the main stars of the Orion figure are called the “Butterfly” asterism), and even several constellations at once (for example, Vega, Deneb and Altair form the well-known “spring-summer triangle”), and very small, visible through binoculars or a telescope (for example, the “Hanger” asterism in Lisichka). None scientific interest asterisms do not represent anything, but from an aesthetic point of view they are quite impressive.