In the night sky in clear weather you can see many small luminous lights - stars. In fact, their size can be enormous, hundreds or even thousands of times the size of the Earth. They can exist separately, but sometimes form a star cluster.
A star is a massive ball of gas. It is able to hold itself up due to the force of its own gravity. Stellar mass is usually greater than planetary mass. Thermonuclear reactions occur inside them, which contribute to the emission of light.
Stars are formed primarily from hydrogen and helium, as well as dust. Their internal temperature can reach millions of Kelvin, although the external temperature is much lower. The main characteristics for measuring these gas balls are: mass, radius and luminosity, that is, energy.
With the naked eye, a person can see approximately six thousand stars (three thousand in each hemisphere). We see the closest one to Earth only during the day - this is the Sun. It is located at a distance of 150 million kilometers. The closest star to our solar system is called Proxima Centauri.
Dust and gas, present in unlimited quantities, can be compressed under the action of the more densely they are compressed, the higher the temperature generated inside. As matter becomes denser, it gains mass, and if it is sufficient to carry out a nuclear reaction, a star will appear.
From a cloud of gas and dust, several stars are often formed at once, which capture each other and form star systems. Thus, there are double, triple and other systems. More than ten stars form a cluster.
A star cluster is a group of stars of common origin that are bound together by gravity and move as a single unit in the field of the galaxy. They are divided into spherical and scattered. In addition to stars, clusters may contain gas and dust. Groups of celestial bodies united by a common origin, but not connected by gravity, are called stellar associations.
People have been observing the night sky since ancient times. However, for a long time it was believed that the celestial bodies are evenly distributed throughout the vastness of the Universe. In the 18th century, astronomer William Herschel challenged science once again by saying that some areas clearly have more stars than others.
A little earlier, his colleague Charles Messier noted the existence of nebulae in the sky. Observing them through a telescope, Herschel discovered that this was not always the case. He saw that sometimes a stellar nebula is a collection of stars that appear as spots when viewed with the naked eye. He called what he discovered “heaps.” Later, another name was invented for these galactic phenomena - star clusters.
Herschel managed to describe about two thousand clusters. In the 19th century, astronomers determined that they differ in shape and size. Then globular and open clusters were identified. Detailed study of these phenomena began only in the 20th century.
Clusters differ among themselves in the number of stars and shape. An open star cluster can contain from ten to several thousand stars. They are quite young, their age may be only a few million years. Such a star cluster does not have clearly defined boundaries; it is usually found in spiral and irregular galaxies.
About 1,100 clusters have been discovered in our galaxy. They do not live long, since their gravitational connection is weak and can easily be broken due to passing near gas clouds or other accumulations. “Lost” stars become single.
Clusters are often found on spiral arms and near galactic planes, where the concentration of gas is greater. They have uneven, shapeless edges and a dense, clearly visible core. Open clusters are classified according to their density, differences in the brightness of their inner stars, and their distinctiveness compared to their surroundings.
Unlike open clusters, globular star clusters have a clear spherical shape. Their stars are bound by gravity much more closely, and rotate around the galactic center, acting as satellites. The age of these clusters is many times greater than the scattered ones, ranging from 10 billion years and above. But they are significantly inferior in number; about 160 globular clusters have been discovered so far in our galaxy.
The high density of stars in a cluster often leads to collisions. As a result, unusual classes of luminaries can be formed. For example, when the members of a binary merge, a blue straggler is created. It is much hotter than other blue stars and cluster members. Collisions can also produce other exotic space objects, such as low-mass X-ray binaries and millisecond pulsars.
Unlike clusters, associations of stars are not connected by a common gravitational field; sometimes it is present, but its strength is too weak. They appeared at the same time and have a small age, reaching tens of millions of years.
Stellar associations are larger than young open clusters. They are more rarefied in outer space, and include up to hundreds of stars in their composition. About a dozen of them are hot giants.
A weak gravitational field does not allow stars to remain in association for a long time. They need from several hundred thousand to a million years to decay - by astronomical standards this is negligible. Therefore, stellar associations are called temporary formations.
In total, several thousand star clusters have been discovered, some of them visible to the naked eye. The closest to Earth are the open clusters of the Pleiades (Stozhary) and Hyades, located in the former. The first contains about 500 stars; without special optics, only seven of them are distinguishable. The Hyades is located next to Aldebaran and contains about 130 bright and 300 low-burning members.
The open star cluster is also one of the closest. It is called the Nursery and contains over two hundred members. Many characteristics of the Manger and Hyades are the same, so there is a possibility that they were formed from the same gas and dust cloud.
A star cluster in the constellation Coma Berenices in the northern hemisphere is easily visible through binoculars. This is the globular cluster M 53, discovered back in 1775. It is located more than 60,000 light years away. The cluster is one of the most distant from Earth, although it is easily visible through binoculars. A huge number of globular clusters are located in
Star clusters are large groups of stars held together by gravitational forces. They number from ten to several million stars that have a common origin. Basically, globular and open clusters are distinguished, differing in shape, composition, size, number of members and age. In addition to them, there are temporary clusters called stellar associations. Their gravitational connection is too weak, which inevitably leads to the disintegration and formation of ordinary single stars.
Pleiades, open cluster
According to their morphology, star clusters are historically divided into two types - globular and open. In June 2011, it became known about the discovery of a new class of clusters, which combines the characteristics of both globular and open clusters.
Groups of gravitationally unbound stars or weakly bound young stars united by a common origin are called stellar associations.
On July 11, 2007, Richard Ellis (Caltech) using the 10-meter Keck II telescope discovered 6 star clusters that formed 13.2 billion years ago. Thus, they arose when it was only 500 million years old.
The globular cluster Messier 80 in the constellation Scorpius is located 28,000 light years from the Sun and contains hundreds of thousands of stars.
Globular star cluster ( globular cluster) is a star cluster containing a large number of stars, tightly bound by gravity and orbiting the galactic center as a satellite. Unlike open star clusters, which are located in the galactic disk, globular clusters are located in the halo; they are much older, contain many more stars, have a symmetrical spherical shape and are characterized by an increase in the concentration of stars towards the center of the cluster. The spatial concentrations of stars in the central regions of globular clusters are 100-1000 stars per cubic parsec, the average distances between neighboring stars are 3-4.6 trillion km; for comparison, in the surrounding area the spatial concentration of stars is ≈0.13 pc −3, that is, our stellar density is 700-7000 times less. The number of stars in globular clusters is ≈10 4 -10 6 . The diameters of globular clusters are 20-60 pc, the masses are 10 4 -10 6 solar.
Globular clusters are quite common objects: at the beginning of 2011, 157 of them were discovered, about 10-20 more are candidates for globular clusters. In larger ones there may be more of them: for example, in the Andromeda Nebula their number can reach 500. In some giant ones, especially those located in the center, such as M 87, there can be up to 13,000 globular clusters. Such clusters orbit the galaxy in large orbits, with a radius of about 40 kpc (about 131,000 light years) or more.
Every galaxy of sufficient mass in the vicinity of the Milky Way is associated with a group of globular clusters; It also turned out that they exist in almost every major galaxy studied. in Sagittarius and the Canis Major dwarf galaxy appear to be in the process of "transferring" their globular clusters (eg Palomar 12) to the Milky Way. Many globular clusters in the past could have been acquired by our Galaxy in this way.
Globular clusters contain some of the earliest stars to appear in the galaxy, but the origin and role of these objects in galactic evolution is still unclear. It has been almost definitely established that globular clusters are significantly different from dwarf elliptical galaxies, that is, they are one of the products of star formation of the “native” galaxy, and were not formed from other attached galaxies. However, recently scientists have suggested that globular clusters and dwarf spheroidal galaxies may turn out to be not very clearly demarcated and different objects.
Globular cluster M 13 in the constellation Hercules. Contains several thousand stars.
The first globular star cluster, M 22, was discovered by German amateur astronomer Johann Abraham Ihle ( Johann Abraham Ihle) in 1665, however, due to the small aperture of the first telescopes, it was impossible to distinguish individual stars in the globular cluster. Charles Messier was the first to identify stars in a globular cluster during the observation of M 4. Later, Abbé Nicolas Lacaille added to his catalog from 1751-1752 the clusters later known as NGC 104, NGC 4833, M 55, M 69 and NGC 6397 (letter The M in front of the number refers to the Charles Messier catalog, and NGC refers to the New General Catalog of John Dreyer).
M 75 is a dense Class I globular cluster.
A program of exploration using large telescopes began in 1782 by William Herschel, which made it possible to distinguish stars in all 33 globular clusters known by that time. In addition, he discovered 37 more clusters. In Herschel's catalog of deep sky objects in 1789, he first used the name "globular cluster" ( globular cluster) to describe objects of this type. The number of globular clusters found continued to increase, reaching 83 by 1915, 93 by 1930, and 97 by 1947. By 2011, 157 clusters were discovered in the Milky Way, another 18 are candidates, and the total number is estimated at 180 ± 20. These undetected globular clusters are thought to be hidden behind galactic clouds of gas and dust.
Beginning in 1914, a series of studies of globular clusters was conducted by the American astronomer Harlow Shapley; their results were published in 40 scientific papers. He studied in clusters (which he assumed were Cepheids) and used the period-luminosity relationship to estimate the distance. RR Lyrae variables were later found to be less luminous than Cepheids, and Shapley actually overestimated the distance to the clusters.
The vast majority of globular clusters in the Milky Way are located in the region of the sky surrounding the galactic core; moreover, a significant amount is located in close proximity to the core. In 1918, Shapley took advantage of this significant asymmetric distribution of clusters to determine the size of our Galaxy. Assuming that the distribution of globular clusters around the galactic center was approximately spherical, he used their coordinates to estimate the position of the Sun relative to the galactic center. Although his estimate of the distance had a significant error, it showed that the size of the Galaxy was much larger than previously thought. The error was due to the presence of dust in the Milky Way, which partially absorbed the light from the globular cluster, making it dimmer and thus further away. Nevertheless, Shapley's estimate of the size of the Galaxy was of the same order as is now accepted.
Shapley's measurements also showed that the Sun was quite far from the center of the Galaxy, contrary to the existing ideas at that time, based on observations of the distribution of ordinary stars. In reality, stars are located in the galactic disk and are therefore often hidden behind gas and dust, while globular clusters are located outside the disk and can be seen from much greater distances.
Later, Henrietta Swope and Helen Sawyer (later Hogg) assisted in the study of the Shapley clusters. In 1927-1929 Shapley and Sawyer began classifying clusters according to the degree of concentration of stars. Clusters with the highest concentration were allocated to class I and were further ranked in decreasing concentration to class XII (sometimes classes are designated by Arabic numerals: 1-12). This classification is called the Shapley-Sawyer concentration classes.
NGC 2808 consists of three different generations of stars.
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, suggesting that they formed at approximately the same time. However, the history of star formation varies from cluster to cluster and in some cases there are different populations of stars within a cluster. An example of this is the globular clusters in the Large Magellanic Cloud, which exhibit bimodal populations. At an early age, these clusters may have collided with a giant molecular cloud, which triggered a new wave of star formation, but this period of star formation is relatively short compared to the age of globular clusters.
Observations of globular clusters indicate that they occur primarily in regions with efficient star formation, that is, where the interstellar medium is denser than normal star-forming regions. The formation of globular clusters predominates in regions with bursts of star formation and in interacting galaxies. Research also shows the existence of a correlation between the central mass and the size of globular clusters in elliptical and . The mass in such galaxies is often close to the total mass of the galaxy's globular clusters.
To date, no globular clusters are known to be actively forming stars, and this is consistent with the view that they are, as a rule, the oldest objects in the galaxy and consist of very old stars. The precursors of globular clusters may be very large star-forming regions known as giant star clusters (for example, Westerlund 1 in the Milky Way).
The stars in the Djorgovski 1 cluster contain only hydrogen and helium and are called "low metal".
Globular clusters typically consist of hundreds of thousands of old stars with low metallicity. The type of stars found in globular clusters is similar to stars in a bulge. They lack gas and dust, and are assumed to have long since turned into stars. Globular clusters have a high concentration of stars - on average about 0.4 stars per cubic parsec, and in the center of the cluster there are 100 or even 1000 stars per cubic parsec (for comparison, in the vicinity of the Sun the concentration is 0.12 stars per cubic parsec). It is believed that globular clusters are not favorable places for the existence of planetary systems, since the orbits in the cores of dense clusters are dynamically unstable due to disturbances caused by the passage of neighboring stars. A planet orbiting at a distance of 1 AU. e. from a star in the core of a dense cluster (for example, 47 Tucanae), theoretically could only last 100 million years. Nevertheless, scientists discovered a planetary system around PSR B1620-26 in the globular cluster M4, but these planets were probably formed after events that led to the formation of a pulsar.
Some globular clusters, such as Omega Centauri in the Milky Way and Mayall II in the Andromeda Galaxy, are extremely massive (several million solar masses) and contain stars from several stellar generations. These two clusters can be taken as evidence that supermassive globular clusters are the core of dwarf galaxies that have been absorbed by giant galaxies. About a quarter of the globular clusters in the Milky Way may have been part of dwarf galaxies.
Some globular clusters (e.g. M15) have very massive cores that may contain black holes, although simulations suggest that the existing observations are equally well explained by the presence of less massive black holes and by concentration (or massive ones).
The M 53 cluster surprised astronomers with the number of stars called blue stragglers.
Globular clusters typically consist of Population II stars, which have low abundances of heavy elements. Astronomers call heavy elements metals, and the relative concentration of these elements in a star metallicity. These elements are created through the process of stellar nucleosynthesis and then become part of a new generation of stars. Thus, the proportion of metals can indicate the age of a star, and older stars tend to have lower metallicities.
Dutch astronomer Pieter Oosterhoff noted that there are likely two populations of globular clusters, which are known as "Oosterhoff groups". Both groups have weak spectral lines of metallic elements, but the lines in type I (OoI) stars are not as weak as in type II (OoII) and the second group has a slightly longer period in RR Lyrae type variables. Thus, type I stars are called “metal-rich”, and type II stars are “metal-poor”. These two populations are observed in many galaxies, especially massive ellipticals. Both groups are almost the same age as the Universe itself, but differ from each other in metallicity. Various hypotheses have been put forward to explain this difference, including mergers with gas-rich galaxies, absorption of dwarf galaxies, and multiple phases of star formation in a single galaxy. In the Milky Way, metal-poor clusters are associated with a halo, and metal-rich clusters are associated with a bulge.
In the Milky Way, most low-metallicity clusters are aligned along a plane in the outer halo of the galaxy. This suggests that the Type II clusters were captured from a satellite galaxy and are not the oldest members of the Milky Way's globular cluster system, as previously thought. The difference between the two types of clusters in this case is explained by the delay between the time the two galaxies formed their cluster systems.
In globular clusters the density of stars is very high and therefore close passages and collisions often occur. A consequence of this is that certain exotic classes of stars (for example, blue stragglers, millisecond pulsars, and low-mass X-ray binaries) are more common in globular clusters. Blue stragglers are formed when two stars merge, possibly as a result of a collision with a binary system. Such a star is hotter than other stars in the cluster that have the same luminosity, and thus differs from the main sequence stars formed at the birth of the cluster.
Since the 1970s Astronomers are looking for black holes in globular clusters, but solving this problem requires high telescope resolution, so it was only with the advent that the first confirmed discovery was made. Based on observations, it was suggested that there is an intermediate mass black hole (4000 solar masses) in the globular cluster M 15 and a black hole (~ 2·10 4 M ⊙) in the Mayall II cluster in the Andromeda galaxy. X-ray and radio emissions from Mayall II are consistent with an intermediate mass black hole. They are of particular interest because they are the first black holes to have a mass intermediate between ordinary stellar-mass black holes and supermassive black holes in galactic nuclei. The mass of the intervening black hole is proportional to the mass of the cluster, which complements the previously discovered relationship between the masses of supermassive black holes and their surrounding galaxies.
Claims of intermediate-mass black holes have been met with some skepticism by the scientific community. This is because the densest objects in globular clusters are thought to gradually slow down and end up at the center of the cluster through a process called “mass segregation.” In globular clusters these are white dwarfs and neutron stars. Research by Holger Baumgardt and his colleagues noted that the mass-to-light ratio in M15 and Mayall II should increase sharply towards the center of the cluster, even without the presence of a black hole.
Color-apparent magnitude diagram of the M3 cluster. Around magnitude 19 there is a characteristic “knee” where the stars begin to enter the giant stage.
Hertzsprung-Russell diagram (H-R diagram) is a graph showing the relationship between absolute magnitude and color index. The B-V color index is the difference between a star's brightness in blue light, or B, and its brightness in visible light (yellow-green), or V. Large values of the B-V color index indicate a cool, red star, while negative values indicate a blue star with a hot surface. . When stars close to the Sun are plotted on a H-R diagram, it shows a distribution of stars of varying mass, age, and composition. Many of the stars in the diagram are relatively close to a sloping curve running from the upper left corner (high luminosities, early spectral types) to the lower right corner (low luminosities, late spectral types). These stars are called main sequence stars. However, the diagram also includes stars that are in later stages of stellar evolution and have left the main sequence.
Since all the stars in a globular cluster are at approximately the same distance from us, their absolute magnitude differs from their apparent magnitude by approximately the same amount. Main sequence stars in a globular cluster are comparable to similar stars in the solar vicinity and will align along the main sequence line. The accuracy of this assumption is confirmed by comparable results obtained by comparing the magnitudes of nearby short-period variable stars (such as RR Lyrae) and Cepheids with the same types of stars in the cluster.
By comparing the curves on the H-R diagram, one can determine the absolute magnitude of the main sequence stars in the cluster. This, in turn, makes it possible to estimate the distance to the cluster based on the apparent magnitude. The difference between the relative and absolute value, the distance modulus, gives an estimate of the distance.
When globular cluster stars are plotted on an H-R diagram, in many cases almost all the stars fall on a fairly defined curve, which differs from the H-R diagram of stars near the Sun, which combines stars of different ages and origins. The shape of the curve for globular clusters is a characteristic of groups of stars formed at approximately the same time from the same materials and differing only in their initial mass. Since the position of each star on the G-R diagram depends on its age, the shape of the curve for a globular cluster can be used to estimate the overall age of the stellar population.
The most massive main sequence stars will have the highest absolute magnitude, and these stars will be the first to enter the giant stage. As a cluster ages, stars with lower masses will begin to enter the giant stage, so the age of a cluster with one type of stellar population can be measured by looking for stars that are just beginning to enter the giant stage. They form an "elbow" in the H-R diagram with a rotation toward the upper right corner relative to the main line of the sequence. The absolute magnitude near the turning point depends on the age of the globular cluster, so the age scale can be constructed on an axis parallel to the magnitude.
In addition, the age of a globular cluster can be determined by the temperature of the coolest white dwarfs. As a result of calculations, it was established that the typical age of globular clusters can reach up to 12.7 billion years. This makes them significantly different from open star clusters, which are only a few tens of millions of years old.
The age of globular clusters imposes a limit on the maximum age of the entire Universe. This lower limit was a significant obstacle in cosmology. In the early 1990s, astronomers were faced with estimating the ages of globular clusters that were older than what cosmological models had predicted. However, detailed measurements of cosmological parameters through deep sky surveys and the availability of satellites such as COBE have solved this problem.
Studies of the evolution of globular clusters can also be used to determine changes due to the combination of gas and dust that form the cluster. The data obtained from studying globular clusters is then used to study the evolution of the entire Milky Way.
Globular clusters contain some stars known as blue stragglers, which appear to continue moving along the main sequence towards brighter blue stars. The origin of these stars is still unclear, but most models suggest that the formation of these stars is the result of mass transfer between stars in binary and triple systems.
Globular clusters are collective members of our galaxy and are included in its spherical subsystem: they revolve around the center of mass of the galaxy in highly elongated orbits with speeds of ≈200 km/s and an orbital period of 10 8 -10 9 years. The age of the globular clusters of our Galaxy is approaching its age, which is confirmed by their Hertzsprung-Russell diagrams, which contain a characteristic break of the main sequence on the blue side, indicating the transformation of massive stars - members of the cluster into .
Unlike open clusters and stellar associations, the interstellar medium of globular clusters contains little gas: this fact is explained, on the one hand, by the low parabolic velocity of ≈10-30 km/s and, on the other hand, by their great age; An additional factor, apparently, is the periodic passage during revolution around the center of our Galaxy through its plane, in which gas clouds are concentrated, which contributes to the “sweeping out” of its own gas during such passages.
Cluster in the central region of the Tarantula Nebula, a cluster of young and hot stars
Relatively young globular clusters are also observed in other galaxies (for example, in the Magellanic clouds).
Most of the globular clusters in the LMC and MMC belong to young stars, unlike the globular clusters of our Galaxy, and are mainly immersed in interstellar gas and dust. For example, the Tarantula Nebula is surrounded by young globular clusters of blue-white stars. At the center of the nebula is a young, bright cluster.
Globular star clusters in the Andromeda Galaxy (M31):
To observe most of the globular clusters of M31, you need a telescope with a diameter of 10 inches, the brightest ones can be seen with a 5-inch telescope. The average magnification is 150-180 times, the optical design of the telescope does not matter.
Cluster G1 (Mayall II) is the brightest cluster of the Local Group, at a distance of 170,000 light years. years.
NGC 265, an open star cluster in the Small Magellanic Cloud.
Open star cluster ( open cluster) is a group of stars (up to several thousand in number) formed from one giant molecular cloud and having approximately the same age. More than 1,100 open clusters have been discovered in our Galaxy, but it is believed that there are many more. The stars in such clusters are bound together by relatively weak gravitational forces, so as they orbit the galactic center, the clusters can be destroyed by passing close to other clusters or clouds of gas, in which case the stars forming them become part of the normal population of the galaxy; individual stars can also be ejected as a result of complex gravitational interactions within the cluster. The typical age of clusters is several hundred million years. Open star clusters are found only in spiral and irregular galaxies, where active star formation processes occur.
Young open clusters can sit within the molecular cloud from which they were formed and “illuminate” it, resulting in a region of ionized hydrogen. Over time, radiation pressure from the cluster disperses the cloud. Typically, only about 10% of the mass of a gas cloud has time to form stars before the rest of the gas is dispersed by light pressure.
Open star clusters are key objects for studying stellar evolution. Because cluster members have similar ages and chemical compositions, the effects of other characteristics are easier to determine for clusters than for individual stars. Some open clusters, such as the Pleiades, Hyades or Alpha Persei Cluster, are visible to the naked eye. Some others, such as the Perseus Double Cluster, are barely visible without instruments, and even more clusters can only be seen with binoculars or a telescope, such as the Wild Duck Cluster (M 11).
A mosaic of 30 images of open clusters discovered by the VISTA telescope. From direct observation, these clusters are hidden by the dust of the Milky Way.
The bright open star cluster Pleiades has been known since antiquity, and the Hyades is part of the constellation Taurus, one of the most ancient constellations. Other clusters were described by early astronomers as inseparable, fuzzy patches of light. The Greek astronomer Claudius Ptolemy mentioned in his notes the Manger, the Perseus Double Cluster and the Ptolemy Cluster; and the Persian astronomer Al-Sufi described the Omicron Veli cluster. However, only the invention of the telescope made it possible to distinguish individual stars in these nebulous objects. Moreover, in 1603, Johann Bayer assigned these formations such designations as if they were separate stars.
The first person to use a telescope to observe the starry sky and record the results of these observations in 1609 was the Italian astronomer Galileo Galilei. While studying some of the nebulous objects described by Ptolemy, Galileo discovered that they were not individual stars, but groups of large numbers of stars. So, in Manger he distinguished more than 40 stars. While his predecessors identified 6-7 stars in the Pleiades, Galileo discovered almost 50. In his 1610 treatise Sidereus Nuncius, he writes: "...Galaxia is nothing more than a collection of numerous stars arranged in groups". Inspired by the work of Galileo, Sicilian astronomer Giovanni Godierna became perhaps the first astronomer to use a telescope to find previously unknown open clusters. In 1654, he discovered the objects now called Messier 41, Messier 47, NGC 2362 and NGC 2451.
In 1767, the English naturalist Reverend John Michell calculated that even for one such group as the Pleiades, the probability that its constituent stars coincidentally lined up for an earthly observer was 1 in 496,000; it became clear that the stars in clusters are physically connected. In 1774-1781, French astronomer Charles Messier published a catalog of celestial objects that have a comet-like, nebulous appearance. This catalog includes 26 open clusters. In the 1790s, English astronomer William Herschel began a comprehensive study of nebulous celestial objects. He discovered that many of these formations could be broken down into groups of individual stars. Herschel suggested that stars were initially scattered in space, and then, as a result of gravitational forces, formed star systems. He divided nebulae into 8 categories, and classes VI to VIII were reserved for the classification of star clusters.
Thanks to the efforts of astronomers, the number of known clusters began to increase. Hundreds of open clusters were listed in the New General Catalog (NGC), first published in 1888 by the Danish-Irish astronomer J. L. E. Dreyer, and in two additional index catalogs published in 1896 and 1905. Telescopic observations made it possible identify two different types of clusters. The former consisted of thousands of stars arranged in a regular spherical distribution; they were found throughout the sky, but most densely towards the center of the Milky Way. The stellar population of the latter was more sparse and more irregular in shape. Such clusters were usually located in or near the galactic plane. Astronomers dubbed the first globular star clusters, and the second - open star clusters. Because of their location, open clusters are sometimes called galaxy clusters, the term was coined in 1925 by Swiss-American astronomer Robert Julius Trumpler.
Micrometric measurements of the positions of stars in clusters were made first in 1877 by the German astronomer E. Schönfeld, and then by the American astronomer E. E. Barnard in 1898-1921. These attempts did not reveal any signs of stellar movement. However, in 1918, the Dutch-American astronomer Adrian van Maanen, by comparing photographic plates taken at different times, was able to measure the proper motion of stars for part of the Pleiades cluster. As astrometry became more and more accurate, it became clear that clusters of stars shared the same proper motion in space. By comparing photographic plates of the Pleiades taken in 1918 with those from 1943, van Maanen was able to identify stars whose proper motion was similar to the cluster average, and thus identify probable members of the cluster. Spectroscopic observations revealed common radial velocities, showing that the clusters consist of stars interconnected in a group.
The first color-luminosity diagrams for open clusters were published by Einar Hertzsprung in 1911, along with diagrams of the Pleiades and Hyades. Over the next 20 years, he continued his work on open clusters. From spectroscopic data, he was able to determine the upper limit of internal motion for open clusters and estimate that the total mass of these objects does not exceed several hundred solar masses. He demonstrated the relationship between the colors of stars and their luminosity, and in 1929 noted that the stellar population of the Hyades and Manger was different from the Pleiades. This was subsequently explained by the difference in age of the three clusters.
Infrared radiation reveals a dense cluster emerging at the heart of the Orion Nebula.
The formation of an open cluster begins with the collapse of part of a giant molecular cloud, a cold, dense cloud of gas and dust many thousands of times the mass of the Sun. Such clouds have a density of 10 2 to 10 6 neutral hydrogen molecules per cm 3, while star formation begins in parts with a density greater than 10 4 molecules/cm 3. Typically, only 1-10% of a cloud's volume exceeds this density. Before collapse, such clouds can maintain mechanical equilibrium due to magnetic fields, turbulence and rotation.
There are many factors that can upset the equilibrium of a giant molecular cloud, which will lead to collapse and the beginning of the process of active star formation, which can result in the formation of an open cluster. These include: shock waves from nearby clouds, collisions with other clouds, gravitational interactions. But even in the absence of external factors, some parts of the cloud may reach conditions where they become unstable and susceptible to collapse. The collapsing region of the cloud experiences hierarchical fragmentation into smaller regions (including relatively dense regions known as infrared dark clouds), which ultimately leads to the birth of large numbers (up to several thousand) of stars. This star formation process begins in a shell of a collapsing cloud, which hides from view, although it allows infrared observations. In the Milky Way galaxy, it is believed that one new open cluster is formed every few thousand years.
The "Pillars of Creation" is a region of the Eagle Nebula where a molecular cloud is blown away by the stellar wind from young, massive stars.
The hottest and most massive of the newly formed stars (known as OB stars) emit intense ultraviolet light, which continually ionizes the surrounding molecular cloud gas and forms an H II region. Stellar winds and radiation pressure from massive stars begin to accelerate the hot ionized gas at speeds comparable to the speed of sound in the gas. After several million years, the cluster experiences its first supernova explosion ( core-collapse supernovae), which also pushes gas out of its surroundings. In most cases, these processes accelerate all the gas within 10 million years, and star formation ceases. But about half of the resulting protostars will be surrounded by circumstellar disks, many of which will be accretion disks.
Since only 30 to 40% of the gas from the center of the cloud forms stars, the dispersion of gas greatly impedes the process of star formation. Consequently, all clusters experience a strong loss of mass at the initial stage, and a fairly large part at this stage disintegrates completely. From this point of view, the formation of an open cluster depends on whether the born stars are gravitationally bound; if this is not the case, then an unrelated stellar association will arise instead of a cluster. If a cluster like the Pleiades does form, it will only be able to hold on to 1/3 of the original number of stars, and the rest will cease to be connected once the gas dissipates. Young stars that no longer belong to their native cluster will become part of the general population of the Milky Way.
Due to the fact that almost all stars are formed in clusters, the latter are considered the main building blocks of galaxies. Intense processes of gas dispersion, which both form and destroy many star clusters at birth, leave their mark on the morphological and kinematic structures of galaxies. Most newly formed open clusters have a population of 100 stars or more and a mass of 50 solar masses. The largest clusters can have a mass of up to 10 4 solar masses (the mass of the Westerlund 1 cluster is estimated at 5 × 10 4 solar masses), which is very close to the masses of globular clusters. While open and globular star clusters are completely different entities, the appearance of the rarest globular clusters and the richest open clusters may not be that different. Some astronomers believe that the formation of these two types of clusters is based on the same mechanism, with the difference that the conditions necessary for the formation of very rich globular clusters - hundreds of thousands of stars in number - no longer exist in our Galaxy.
The formation of more than one open cluster from a single molecular cloud is a typical phenomenon. Thus, in the Large Magellanic Cloud, the clusters Hodge 301 and R136 were formed from the gas of the Tarantula Nebula; Tracing the trajectories of the Hyades and Manger, two prominent and nearby clusters of the Milky Way, suggests that they also formed from the same cloud about 600 million years ago. Sometimes clusters born at the same time form a double cluster. A striking example of this in our Galaxy is the Perseus Double Cluster, consisting of NGC 869 and NGC 884 (sometimes erroneously called "χ and h Perseus" ( "hi and ash of Perseus"), Although h refers to a neighboring star, and χ - to both clusters), however, besides it, at least 10 similar clusters are known. Even more of them have been discovered in the Small and Large Magellanic Clouds: these objects are easier to detect in external systems than in our Galaxy, since due to the projection effect, distant friends from each other, the clusters may appear connected to each other.
Open clusters can range from sparse groups of a few stars to large agglomerations containing thousands of members. They typically consist of a clearly visible dense core surrounded by a more diffuse “corona” of stars. The core diameter is usually 3-4 St. g., and crowns - 40 St. l. The standard stellar density at the center of the cluster is 1.5 stars/light. g. 3 (for comparison: in the vicinity of the Sun this number is ~0.003 mag./light g. 3).
Open star clusters are often classified according to a scheme developed by Robert Trumpler in 1930. The name of the class according to this scheme consists of 3 parts. The first part is designated by the Roman numeral I-IV and indicates the concentration of the cluster and its distinguishability from the surrounding star field (from strong to weak). The second part is an Arabic numeral from 1 to 3, indicating the variation in the brightness of the members (from small to large variation). The third part is a letter p, m or r, indicating, respectively, a low, medium, or high number of stars in a cluster. If the cluster is located inside a nebula, then the letter is added at the end n.
For example, according to Trumpler's scheme, the Pleiades are classified as I3rn (highly concentrated, rich in stars, nebula present), and the closer Hyades - as II3m (more dispersed and with fewer numbers).
NGC 346, an open cluster in the Small Magellanic Cloud.
More than 1000 open clusters have been discovered in our Galaxy, but their total number can be up to 10 times greater. In spiral galaxies, open clusters are mainly located along the spiral arms, where the gas density is highest and, as a result, star formation processes are most active; such aggregations usually disperse before they can leave the arm. Open clusters have a strong tendency to be near the galactic plane.
In irregular galaxies, open clusters can be found anywhere, although their concentration is higher where the gas density is greater. Open clusters are not observed in elliptical galaxies, since star formation processes in the latter ceased many millions of years ago, and the last of the formed clusters have since dispersed long ago.
The distribution of open clusters in our Galaxy depends on age: older clusters are mainly located at greater distances from the Galactic center and at a considerable distance from the Galactic plane. This is explained by the fact that the tidal forces that contribute to the destruction of clusters are higher near the center of the galaxy; on the other hand, giant molecular clouds, which also cause destruction, are concentrated in the inner regions of the galactic disk; therefore, clusters from the inner regions are destroyed at an earlier age than their “colleagues” from the outer regions.
A multimillion-year-old cluster of stars (lower right) illuminates the Tarantula Nebula in the Large Magellanic Cloud.
Because open star clusters typically disintegrate before most of their stars complete their life cycles, most of the radiation from the clusters is light from young, hot, blue stars. Such stars have the greatest mass and the shortest lifetime - on the order of several tens of millions of years. Older star clusters contain more yellow stars.
Some star clusters contain hot, blue stars that appear much younger than the rest of the cluster. These blue scattered stars are also observed in globular clusters; It is believed that in the densest cores of globular clusters they are formed when stars collide and form hotter and more massive stars. However, the stellar density in open clusters is much lower than in globular clusters, and the number of observed young stars cannot be explained by such collisions. Most are thought to form when a binary star system, due to dynamic interactions with other members, merges into a single star.
Once low- and medium-mass stars use up their supply of hydrogen through nuclear fusion, they shed their outer layers and form a planetary nebula to form a white dwarf. Even though most open clusters disintegrate before most of their members reach the white dwarf stage, the number of white dwarfs in clusters is usually still much smaller than would be expected based on the age of the cluster and the estimated initial mass distribution of the stars. . One possible explanation for the dearth of white dwarfs is that when a red giant sheds its envelope and forms a planetary nebula, some slight asymmetry in the mass of the ejected material can impart a speed of several kilometers per second to the star - enough for it to leave the cluster.
Due to the high stellar density, close passages of stars in open clusters are not uncommon. For a typical cluster of 1000 stars and a half-mass radius of 0.5 pc, on average each star will approach another every 10 million years. This time is even shorter in denser clusters. Such passages can greatly affect the expanded circumstellar disks of material around many young stars. Tidal disturbances for large disks can cause the formation of massive planets and, which will be located at distances of 100 AU. e. or more from the main star.
NGC 604 in the Triangulum Galaxy is an extremely massive open cluster surrounded by a region of ionized hydrogen.
Many open clusters are essentially unstable: due to their small mass, the speed of escape from the system is less than the average speed of its constituent stars. Such clusters disintegrate very quickly within a few million years. In many cases, the ejection of the gas from which the entire system was formed by radiation from young stars reduces the mass of the cluster so much that it disintegrates very quickly.
Clusters that, after the surrounding nebula has dispersed, have sufficient mass to be gravitationally bound can maintain their shape for many tens of millions of years, but over time, internal and external processes also lead to their disintegration. A close passage of one star next to another can increase the speed of one of the stars so much that it exceeds the escape velocity from the cluster. Such processes lead to the gradual “evaporation” of cluster members.
On average, every half a billion years, star clusters experience the influence of external factors, such as passing near or through a molecular cloud. Gravitational tidal forces from such close proximity tend to destroy a star cluster. Eventually it becomes by a stream of stars: due to the large distances between the stars, such a group cannot be called a cluster, although its constituent stars are connected to each other and move in the same direction at the same speeds. The period of time after which a cluster disintegrates depends on the initial stellar density of the latter: closer ones live longer. The estimated half-life of the cluster (after which half of the original stars will be lost) varies from 150 to 800 million years, depending on the initial density.
After the cluster is no longer bound by gravity, many of its constituent stars will still retain their speed and direction of motion in space; the so-called star association(or moving group of stars). Thus, several bright stars of the “bucket” of Ursa Major are former members of an open cluster, which turned into such an association called the “moving group of Ursa Major stars.” Eventually, due to slight differences in their speeds, they will disperse throughout the Galaxy. Larger clusters become streams, provided that the sameness of their speeds and ages is established; otherwise the stars will be considered unconnected.
Hertzsprung-Russell diagrams for two open clusters. The cluster NGC 188 is older and shows less deviation from the main sequence than M 67.
In a Hertzsprung-Russell diagram for an open cluster, most stars will be on the main sequence (MS). At some point, called the turning point, the most massive stars leave the MS and become red giants; The “distance” of such stars from the MS makes it possible to determine the age of the cluster.
Due to the fact that the stars in the cluster are located at almost the same distance from and were formed at approximately the same time from the same cloud, all differences in the apparent brightness of the stars in the cluster are due to their different masses. This makes open star clusters very useful objects for studying stellar evolution, since when comparing stars, many variable characteristics can be assumed to be fixed for the cluster.
For example, studying the content of lithium and beryllium in stars from open clusters can seriously help in unraveling the mysteries of the evolution of stars and their internal structure. Hydrogen atoms cannot form helium atoms below 10 million K, but lithium and beryllium nuclei are destroyed at temperatures of 2.5 million and 3.5 million K, respectively. This means that their abundances directly depend on how much matter is mixed in the interior of the star. When studying their abundance in cluster stars, variables such as age and chemical composition are fixed.
Research has shown that the abundance of these light elements is much lower than predicted by stellar evolution models. The reasons for this are not entirely clear; One explanation is that in the interior of the star there are ejections of matter from the convective zone into a stable zone of radiative transfer ( convection overshoot).
Wild Duck (M 11) is a very rich cluster located towards the center of the Milky Way.
Determining distances to astronomical objects is key to understanding them, but the vast majority of such objects are too far away for distances to be measured directly. The graduation of the astronomical distance scale depends on a sequence of indirect and sometimes indefinite measurements in relation to first the nearest objects, the distances to which can be measured directly, and then more and more distant ones. Open star clusters are the most important step on this ladder.
The distances to the clusters closest to us can be measured directly in one of two ways. First, for stars in nearby clusters, parallax can be determined (a slight shift in the apparent position of an object during the year due to the Earth’s motion along the Sun’s orbit), as is usually done for individual stars. The Pleiades, Hyades and some other clusters in the vicinity of 500 St. years are close enough for this method to give reliable results for them, and data from the Hipparchus satellite made it possible to establish exact distances for a number of clusters.
Another direct method is the so-called moving cluster method. It is based on the fact that stars in a cluster share common parameters of motion in space. Measuring the proper motions of the cluster members and plotting their apparent movements across the sky will reveal that they are converging on one point. The radial velocities of cluster stars can be determined from measurements of Doppler shifts in their spectra; Once all three parameters—radial velocity, proper motion, and angular distance from the cluster to its vanishing point—are known, simple trigonometric calculations will allow one to calculate the distance to the cluster. The most famous case of application of this method concerned the Hyades and made it possible to determine the distance to them at 46.3 parsecs.
Once distances to nearby clusters are established, other methods can extend the distance scale to more distant clusters. By comparing the main sequence stars on the Hertzsprung-Russell diagram for a cluster whose distance is known with the corresponding stars of a more distant cluster, the distance to the latter can be determined. The closest known cluster is the Hyades: although the Ursa Major group of stars is about twice as close, it is still a stellar association, not a cluster, since the stars in it are not gravitationally bound to each other. The most distant known open cluster in our Galaxy is Berkeley 29, at a distance of approximately 15,000 parsecs. In addition, open clusters can be easily detected in many galaxies of the Local Group.
Accurate knowledge of the distances to open clusters is vital for calibrating the period-luminosity relationship that exists for variable stars such as Cepheids and RR Lyrae stars, which will allow them to be used as “standard candles”. These powerful stars can be seen at great distances and with their help extend the scale further - to the nearest galaxies of the Local Group.
Stellar associations 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.
Stellar associations were discovered by V. A. Ambartsumyan in 1948 and predicted their collapse. Subsequent measurements by A. Blaauw, W. Morgan, V. E. Markaryan, I. M. Kopylov and others confirmed the fact of expansion of stellar associations.
Unlike young open star clusters, stellar associations have a larger size (tens of parsecs, for the cores of open star clusters - units of parsecs) and lower density: the number of stars in the association is from tens to hundreds (in open star clusters - from hundreds to thousands) . The origin of stellar associations is due to the star formation regions of molecular cloud complexes.
The following types of star associations are distinguished:
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General astronomy. Star clusters and associations
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 accepted based on appearance, but with further study 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 of axial rotation of the cluster). But star-poor clusters are also known, indistinguishable in appearance from scattered ones (for example, NGC 5053), and classified as globular based on the characteristic features of the spectrum-luminosity diagram. The two brightest of the globular clusters are given the designations Omega Centauri (NGC 5139) and 47 Tucanae (NGC 104), as ordinary stars, because due to their significant apparent brightness (+3. m 6 and +4. m 1, respectively) they are clearly visible to the naked eye , but only in southern countries. And in the middle latitudes of the northern hemisphere, only two are accessible to the naked eye, albeit with difficulty (even for a dark, unexposed sky), in the constellations Sagittarius (M22) and Hercules (M13).
Omega Centauri is one of the brightest in absolute magnitude, for it it is -10. m 2, while one of the weakest (NGC 6366) has 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 pc 3 (in the vicinity of the Sun - only 0.13 stars per 1 pc 3). 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 an apparent angular size of 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 (10 8 -10 9 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 a characteristic shape due to the absence of 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 brightest stars are red giants. In addition, variable stars are observed in them (especially often of the RR Lyrae type), as well as the final products of the evolution of massive stars (white dwarfs, neutron stars and black holes included in close binary systems with a normal star), manifesting themselves in the form of X-ray sources of different types. But in general, double stars are rare in globular clusters. 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 the high stellar density of the Milky Way, or are hidden by light-absorbing gas and 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 pc 3. 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 of various types - flares, Cepheids, etc. The chemical composition of the stars included in open clusters is quite homogeneous, and on average the content of heavy elements is close to that of the Sun, 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 spiral structure, since in the time that has passed since their formation, they do not have time to move away from the spiral arms 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 continuation of the directions of these movements (either backward or forward) intersect at a point called the radiant - this is the convergence of 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 total 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 distances to other clusters from “spectrum-visible light” diagrams obtained directly from observations, since combining the main sequence of such a diagram with the “calibrated” one immediately gives a difference between the visible and absolute magnitudes, depending only on the 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 several stars to several hundreds, thereby differing from young star clusters in its larger 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 in the Galaxy of star clusters and associations of very different ages irrefutably indicates that stars are formed not alone, but in groups, and the process of star formation itself continues at the present time. An example of a stellar 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 of gravitational interaction are also small. 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 the general movement and distance to a group of stars, you can guess the former open cluster in it. Such groups are called star streams. Few people know that the 5 stars of the Ursa Major Dipper 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 a large area in the sky. 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). Asterisms are not of any scientific interest, but from an aesthetic point of view they can be quite spectacular.
Astronomers using the MUSE instrument on the Very Large Telescope in Chile have discovered a star in the cluster NGC 3201 that is behaving very strangely. It appears to be orbiting an invisible black hole whose mass is approximately four times that of the Sun. If this is true, then scientists have discovered the first inactive black hole of stellar mass, and in a globular star cluster. In addition, it will be the first to be detected directly by its gravity. This is a very important discovery that is sure to have an impact on our understanding of the formation of such star clusters, black holes and the origin of gravitational wave release events.
Globular star clusters are so named because they are huge spheres containing several tens of thousands of stars. They are located in most galaxies, are among the oldest known stellar associations in the universe, and their appearance is attributed to the beginning of the growth of the host galaxy and its evolution. Today, more than 150 star clusters belonging to the Milky Way are known.
One of these groups is called NGC 3201, it is located in the constellation Velus of the southern sky of the Earth. In this study, it was studied using the state-of-the-art MUSE instrument on the European Southern Observatory's Very Large Telescope (VLT) in Chile. An international team of astronomers has found that one of the stars in the cluster behaves very strangely - it oscillates back and forth at speeds of several hundred thousand kilometers per hour with a certain periodicity of 167 days. The discovered star is a main sequence star at the end of its main phase of life. This means that it has exhausted its hydrogen fuel and is now becoming a red giant.
Artist's impression of an inactive black hole in the cluster NGC 3201. Source: ESO/L. Calçada/spaceengine.org
MUSE is currently surveying 25 globular star clusters in the Milky Way. This work will allow astronomers to obtain spectra from 600 to 27,000 stars in each cluster. The study involves analyzing the radial velocities of individual stars - the speed at which they move away from or towards the Earth, that is, along the observer's line of sight. By analyzing radial velocities, the orbits of stars can be measured, as well as the properties of any large object they may orbit.
“This star is orbiting something that is completely invisible. It has a mass that is four times that of the Sun, and it can only be a black hole. It turns out that for the first time we have found such an object in a star cluster, and directly observing its gravitational influence,” says lead author Benjamin Giessers from the Georg-August University of Göttingen.
The relationship between black holes and star clusters seems very important but mysterious to scientists. Because of their large masses and ages, these clusters are believed to have produced large numbers of stellar-mass black holes—objects formed by the explosion of large stars and collapsing under the force of the entire cluster.
In the absence of the continuous formation of new stars, which is exactly what happens in globular star clusters, stellar-mass black holes soon become the largest objects in existence. Typically, such holes in globular clusters are about four times larger than the stars around them. Recently developed theories have led to the conclusion that black holes form a dense core in a group, which becomes like a separate part of the cluster. Movements in the center of the group should have expelled most of the black holes. This means that only a few such objects could survive beyond a billion years.
Globular star cluster NGC 3201. The blue circle shows the estimated location of the inactive black hole. Source: ESA/NASA
Stellar-mass black holes themselves, or simply collapsars, are formed when large stars die, collapsing under the influence of their own gravity, and explode as powerful hypernovae. The remaining black hole contains most of the mass of the former star, which is several times the mass of the Sun, and their size is several tens of times larger than our star.
The MUSE instrument gives astronomers a unique opportunity to measure the motion of up to thousands of distant stars simultaneously. With this new discovery, the team was for the first time able to detect an inactive black hole at the center of a globular cluster. It is unique in that it is not currently absorbing matter and is not surrounded by a hot disk of gas and dust. And the mass of the hole was estimated thanks to its enormous gravitational influence on the star itself.
Since no radiation can escape from a black hole, the main method of detecting them is to observe radio or X-ray emissions coming from the hot material around them. But when a black hole does not interact with hot matter and does not accumulate mass or emit radiation, then it is considered inactive or invisible. Therefore, it is necessary to use other methods for detecting them.
Astronomers were able to determine the following parameters of the star: its mass is approximately 0.8 solar masses, and the mass of its mysterious colleague lies within 4.36 solar masses, almost certainly a black hole. Since the faint object of this binary system cannot be observed directly, there is an alternative, albeit less convincing, method for explaining what it might be. Scientists may be observing a triple star system, made up of two tightly bound neutron stars around which the star we are observing revolves. This scenario requires each tightly bound star to be at least twice as massive as the Sun, and such a binary system has never been observed before.
Recent detections of radio and X-ray sources in globular star clusters, as well as the 2016 discovery of gravitational wave signals created by the merger of two stellar-mass black holes, suggest that these relatively small black holes may be more widespread in clusters than previously thought.
“Until recently, we assumed that almost all black holes should disappear from globular clusters within a short time, and that systems like this should not even exist! But in reality this is not the case. Our discovery is the first direct observation of the gravitational effects of a stellar-mass black hole in a globular cluster. This discovery will help us understand the formation of such groups, the development of black holes and binary star systems - vital in the context of understanding the sources of gravitational waves."
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