Astronomers using the MUSE instrument at the Very Large Telescope in Chile have discovered a star in the cluster NGC 3201 that is behaving very strangely. One gets the feeling that it revolves around an invisible black hole, the mass of which is approximately four times the mass of the Sun. If it is true that scientists have discovered the first inactive stellar mass black hole, and in a globular star cluster. In addition, it will be the first to be discovered directly from 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 time of the beginning of the growth of the host galaxy and its evolution. To date, more than 150 star clusters are known that belong to the Milky Way.
One of these groups is called NGC 3201, it is located in the constellation Sail of the southern sky of the Earth. In this study, it was studied using the state-of-the-art MUSE instrument installed at the Very Large Telescope (VLT) of the European Southern Observatory in Chile. An international team of astronomers has found that one of the stars in the cluster is behaving very strangely - oscillating 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 life phase. This means that it has exhausted its hydrogen fuel and is now becoming a red giant.
Artist's rendering of the inactive black hole in NGC 3201. Source: ESO/L. Calçada/spaceengine.org
MUSE is currently surveying 25 globular clusters in the Milky Way. This work will allow astronomers to obtain spectra from 600 to 27,000 stars in each cluster. The study includes an analysis of the radial velocities of individual stars - the speed with which they move from the Earth or towards it, that is, along the line of sight of the observer. Thanks to the analysis of radial velocities, it is possible to measure the orbits of stars, as well as the properties of any large object around which they can rotate.
“This star is orbiting something that is completely invisible. It has a mass 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, moreover, by directly observing its gravitational influence, ”admires the lead author of the work Benjamin Giesers from the Georg-August University of Göttingen.
The relationship between black holes and star clusters looks very important to scientists, but mysterious. Due to their large masses and ages, these clusters are believed to have produced a large number 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 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 surrounding stars. Recently developed theories have led to the conclusion that black holes form a dense core in a group, which becomes, as it were, a separate part of the cluster. Movement in the center of the group should have expelled most of the black holes. This means that only some such objects could survive after a billion years.
The globular star cluster NGC 3201. The blue circle shows the proposed 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 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 provides astronomers with the unique ability to measure the motion of up to a thousand 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 does not currently absorb matter and is not surrounded by a hot disk of gas and dust. And the mass of the hole was estimated due to its huge gravitational influence on the star itself.
Since no radiation can escape a black hole, the main method of detecting them is to observe radio or X-ray emission from the hot material around them. But when a black hole does not interact with hot matter and does not accumulate mass, and does not emit radiation, in this case it is considered inactive or invisible. Therefore, it is required to use other methods for their detection.
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 counterpart lies within 4.36 solar masses, almost exactly a black hole. Since the dimmed object of this binary system cannot be observed directly, there is an alternative method, albeit less convincing, of what it might be. It is possible that scientists are observing a triple star system, composed of two densely connected neutron stars, around which the star that we observe revolves. This scenario requires that each tightly bound star 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 finding of gravitational wave signals created by the merger of two stellar-mass black holes, suggest that these relatively small black holes may be more widely distributed in clusters than previously thought.
“Until recently, we assumed that almost all black holes should disappear from globular star clusters after 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 in understanding 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.
From the most ancient times, man turned his gaze to the heavens, where countless star clusters shone, inaccessible, but alluring with their unique beauty.
The drawings of the stars that the ancient inhabitants of the Earth saw 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 make it possible to judge what associations the distant amazing luminaries sparkling on the dark canvas of the sky evoked. It was believed that the fate of people is inextricably linked with the relative position of the stars, which are able to bring wealth, happiness and good luck to those born under them, as well as bitterness, misfortune and disappointment.
Significance 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 heavenly vault have significantly changed and systematized, acquiring much more rational outlines, but the historical sonorous names have been preserved. It turned out that seemingly nearby stars can actually be far from each other and vice versa. Therefore, it became necessary to create a stellar hierarchy corresponding to modern ideas about the universe. So, in the astronomical classification, the term "star clusters" appeared, uniting a group of stars moving in their galaxy as one.
These formations are extremely interesting in that the luminaries included in them were formed approximately simultaneously and are located by space standards at the same distance from the earthly observer, which provides additional opportunities, making it possible to compare radiation from different sources of the same cluster without appropriate corrections. The signals coming from them are distorted in the same way, which greatly facilitates the work of astrophysicists who study the structure and evolution of stellar systems and the Universe as a whole, the principles of the formation of galaxies, the processes of star formation and their destruction, and much more.
Types of star clusters
Hubble on star clusters
Star clusters are usually divided into two large groups: globular and open. But from time to time they try to supplement this classification, since not all detected 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, having an age of over 10 billion years. 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 found in spiral or irregular galaxies, 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 ones, but when they are observed, each star can be seen separately, since they are located at a considerable distance from each other and do not merge in the general sky.
star associations
By analogy with the political and economic spheres of life, the celestial bodies are also capable of creating temporary associations, which have received the name "stellar associations" in astronomy.
These formations are considered the youngest in the Universe and have an age of no more than tens of millions of years. 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 associations could not have arisen by the gravitational capture of passing stars, which means that the latter were born with her and are about 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. From a scientific point of view, the discovery of such neoplasms confirms the theory of the continuation of the processes of the birth of new stars in the Universe, and not one by one, but in whole groups.
New discoveries
Until recently, it was believed that globular clusters are the oldest stellar formations, which, due to age, should have lost the dynamics of internal rotational movements and can be considered as simple systems. However, in 2014, researchers from the Max Planck Institute for Extraterrestrial Physics, led by Maximilian Fabricius, as a result of long-term observations of 11 globular clusters in the Milky Way, found that their central part continues to rotate.
Most modern theories are not able to explain this fact, which means that if the information is confirmed, then changes are possible both in the theoretical aspects of knowledge and in 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? Alexei Rastorguev, Doctor of Physical and Mathematical Sciences, talks about this.
Apparently, almost all stars are born in groups, not individually. Therefore, there is nothing surprising in the fact that star clusters are a very common thing. Astronomers love to study star clusters because they know that all the stars in a cluster formed at about the same time and at about the same distance from us. Any noticeable differences in brightness between such stars are true differences. Whatever colossal changes these stars have undergone over time, they all began at the same time. It is especially useful to study star clusters from the point of view of the dependence of their properties on mass - after all, the age of these stars and their distance from the Earth are approximately the same, so that they differ from each other only in their mass.
Star clusters are interesting not only for scientific study - they are exceptionally beautiful as objects for photography and for observation by amateur astronomers. There are two types of star clusters: open and globular. These names are associated with their appearance. In an open cluster, each star is visible separately, they are distributed more or less evenly over some part of the sky. And globular clusters, on the contrary, are like a sphere so densely filled with stars that in its center individual stars are indistinguishable.
open star clusters
Perhaps the most famous open star cluster is the Pleiades, or Seven Sisters, in the constellation Taurus. Despite its name, most people can only see six stars without a telescope. The total number of stars in this cluster is somewhere between 300 and 500, and they are all in a patch 30 light-years across and 400 light-years away from us.
This cluster is only 50 million years old, which is very short by astronomical standards, and it contains very massive luminous stars that have not yet had time to turn into giants. The Pleiades is a typical open star cluster; Usually, such a cluster includes from several hundred to several thousand stars.
Among the open star clusters, there are many more young ones than old ones, and the oldest ones are hardly more than 100 million years old. It is believed that the rate at which they are formed does not change over time.
The fact is that in older clusters the stars gradually move away from each other until they mix with the main set of stars - the same ones, thousands of which appear before us in the night sky. Although gravity holds open clusters together to some extent, they are still rather fragile, and the gravity of another object, such as a large interstellar cloud, can tear them apart.
Some stellar groups are so weakly held together that they are not called clusters, but stellar associations. They do not last very long and usually consist of very young stars near the interstellar clouds from which they originated. A stellar association includes from 10 to 100 stars scattered over a region several hundred light years in size.
The clouds in which stars form are concentrated in the disk of our Galaxy, and it is there that open star clusters are found. Considering how many clouds there are in the Milky Way and how much dust is in interstellar space, it becomes obvious that the 1200 open star clusters that we know about should be only a tiny fraction of their total number in the Galaxy. Perhaps their total number reaches 100,000.
globular star clusters
In contrast to open ones, globular clusters are spheres densely filled with stars, of which there are hundreds of thousands and even millions. The stars in these clusters are so densely packed that if our Sun belonged to any globular cluster, we could see over a million individual stars in the night sky with the naked eye. The size of a typical globular cluster is from 20 to 400 light years.
In the densely packed centers of these clusters, the stars are in such close proximity to each other that mutual gravity binds them to each other, forming compact binary stars.
Sometimes there is even a complete merger of stars; in close approach, the outer layers of the star can collapse, exposing the central core to direct viewing. In globular clusters, double stars are 100 times more common than anywhere else. Some of these twins are X-ray sources.
Around our Galaxy, we know about 200 globular star clusters, which are distributed throughout the huge spherical halo that encloses the Galaxy. All these clusters are very old, and they appeared more or less at the same time as the Galaxy itself: from 10 to 15 billion years ago. The clusters appear to have formed when parts of the cloud from which the galaxy was created split into smaller fragments. Globular clusters do not diverge, because the stars in them sit very closely, and their powerful mutual gravitational forces bind the cluster into a dense single whole.
Globular star clusters are observed not only around our Galaxy, but also around other galaxies of any kind. The brightest globular cluster, easily visible to the naked eye, is Omega Xntaurus in the southern constellation Centaur. It is located at a distance of 16,500 light years from the Sun and is the most extensive of all known clusters:
its diameter is 620 light years. The brightest globular cluster in the northern hemisphere is M13 in Hercules, barely visible to the naked eye.
In 1596 a Dutch amateur stargazer named David Fabricius (1564-1617) discovered a fairly bright star in the constellation Cetus; this star gradually began to fade, and after a few weeks it disappeared altogether from sight. Fabricius was the first to describe the observation of a variable star.
This star was named Mira - miraculous~. Over a period of 332 days, Mira changes its brightness from approximately 2nd magnitude (at the level of the North Star) to 10th magnitude, when it becomes much fainter than necessary for observation with the naked eye. Today, many thousands of variable stars are known, although most of them do not change their brightness as dramatically as Mira.
There are various reasons why stars change their brightness. Moreover, the brightness sometimes changes by many light magnitudes, and sometimes so insignificantly that this change can be detected only with the help of very sensitive instruments. Some stars change regularly.
Others - suddenly go out or suddenly flare up. Changes can occur cyclically, with a period of several years, or they can happen in a matter of seconds. To understand why a particular star is variable, it is first necessary to trace exactly how it changes. A graph of the magnitude of a variable star is called a light curve. In order to draw a light curve, light measurements must be taken regularly. To accurately measure stellar magnitudes, professional astronomers use an instrument called a photometer, but numerous observations of variable stars are made by amateur astronomers. With the help of a specially prepared map and after some practice, it is not so difficult to judge the magnitude of a changing star directly by eye, when compared with permanent stars located nearby.
Graphs of the brightness of variable stars show that some of the stars change in a regular (correct) way - a section of their graph over a period of time of a certain length (period) is repeated again and words. Other stars change completely unpredictably. Regular variable stars include pulsating stars and binary stars. The amount of light changes because the stars pulsate or throw out clouds of matter. But there is another group of variable stars that are double (binary).
When we see a change in the brightness of bitsars, this means that one of several possible phenomena has occurred. Both stars may be in our line of sight, since, moving in their orbits, opiums can pass directly in front of one another. Similar sysgems are grooved by eclipsing binary stars.
The most famous example of this kind is the star Algol in the constellation Perseus. In a closely spaced pair, material can rush from one star to another, often with dramatic consequences.
Getting acquainted with more and more objects to observe in a series of articles about us, we often come across space objects called. In appearance, the clusters are divided into 2 types: scattered(or open) and ball. Let's find out a little more about them.
open clusters
This type of cluster contains from 20 to several thousand stars. They are easy to observe and find in the starry sky with the naked eye, and already in a simple amateur telescope you can consider individual sections. Stars are bound together by gravitational attraction and are predominantly young and hot.
Such clusters are located near the band of the Milky Way. About 1000 open clusters are known, but, as astronomers suggest, their number may exceed several tens of thousands. They look like a group of stars located close to each other. The brightest cluster observed from Earth is Pleiades(or M45), with its magnitude equal to 1.6 m.
The photo above shows the cosmic dust between the stars - in fact, it is, which reflects the blue light of very hot and young stars.
Another good example of open clusters is the cluster Wild duck(or M11) in the constellation.
The youngest open star clusters surrounded by gas and dust nebulae are called star associations. Such associations are very difficult to distinguish against the background of other stars, but using spectral methods they can be divided into groups: O-association- contains hot stars O and B; T association- consists of young forming stars of classes F, G, K, M.
globular clusters
Globular clusters include from 10,000 to a million stars. With binoculars or an amateur telescope, it will be possible to consider only the shape and some outlines as a whole. For a more detailed study, you need a powerful tool.
Such clusters are located in close proximity to our Milky Way galaxy. They revolve in elongated elliptical orbits around the center of the galaxy.
All globular clusters have the appearance of a ball, very bright in the center, and weakening towards the edges, where the concentration of stars decreases. Due to the high brightness and strong luminosity, almost all clusters of this type can be observed. Their total number is a little over 100.
Globular star cluster M 12
Cluster M12 is in the constellation and in the first summer month you can hunt for it. Another prominent representative of the globular cluster, which is also located in this constellation, is M14:
Bright globular cluster M 14
Globular clusters are interesting for hunting even with binoculars. Despite the fact that it will not be possible to consider the details, the search itself is very exciting. I once wrote blog posts. Read.
In general, this is all you need to know about types of star clusters in order to be able to distinguish them in the starry sky and understand where they are located.
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 features 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.
July 11, 2007 Richard Ellis (California Institute of Technology) on the 10-meter Keck II telescope discovered 6 star clusters that formed 13.2 billion years ago. Thus, they originated when there were only 500 million years.
globular star cluster
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 ( global cluster) is a star cluster containing a large number of stars, tightly bound by gravity and revolving around 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 vicinity, 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 were discovered in them, and about 10-20 more are candidates for globular clusters. In larger ones, there may be more: 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 circulate near the galaxy in large orbits with a radius of the order of 40 kpc (approximately 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 are in almost every studied large galaxy. in Sagittarius and the dwarf galaxy in Canis Major are apparently 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 that appeared in the galaxy, but the origin and role of these objects in galactic evolution is still not clear. It is almost certain that globular clusters are significantly different from dwarf elliptical galaxies, that is, they are one of the star formation products of the "native" galaxy, and were not formed from other acceding galaxies. However, recently scientists have suggested that globular clusters and dwarf spheroidal galaxies may not be quite clearly demarcated and different objects.
Observation history
Globular cluster M 13 in the constellation Hercules. Contains several thousand stars.
The first globular star cluster M 22 was discovered by the 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 a globular cluster. It was Charles Messier who first distinguished stars in a globular cluster during his observation of M 4. Later, Abbot 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 Charles Messier's catalog and NGC to John Dreyer's New General Catalog).
M 75 is a dense class I globular cluster.
A program of research 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 1789 catalog of deep sky objects, he first used the name "globular cluster" ( global cluster) to describe objects of this type. The number of globular clusters found continued to grow, reaching 83 by 1915, 93 by 1930, and 97 by 1947. By 2011, 157 clusters have been discovered in the Milky Way, 18 more are candidates, and the total number is estimated at 180 ± 20. These undetected globular clusters are believed 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 a period-luminosity relationship to estimate distance. It was later found that the luminosity of RR Lyrae variables was less than that of 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 nucleus; moreover, a significant amount is located in the immediate vicinity of the nucleus. In 1918, Shapley took advantage of this large skewed distribution of clusters to determine the size of our galaxy. Assuming that the distribution of globular clusters around the center of the galaxy is approximately spherical, he used their coordinates to estimate the position of the Sun relative to the center of the galaxy. Despite the fact that his estimate of the distance had a significant error, it showed that the dimensions of the Galaxy were 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 accepted now.
Shapley's measurements also showed that the Sun was quite far from the center of the Galaxy, contrary to what was then believed based on observations of the distribution of ordinary stars. In fact, the stars are in the disk of the Galaxy and therefore are often hidden behind gas and dust, while globular clusters are outside the disk and can be seen from a much greater distance.
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. Accumulations with the highest concentration were assigned to class I and further ranked as the concentration decreased to class XII (sometimes classes are denoted by Arabic numerals: 1-12). This classification is called the Shapley-Sawyer concentration classes.
Formation
NGC 2808 is made up of three distinct generations of stars.
To date, the formation of globular clusters has not been fully understood and it is still unclear whether a globular cluster consists of stars of the same generation, or whether it consists of stars that have gone through multiple cycles over several hundred million years. In many globular clusters, most of the stars are in roughly the same stage of stellar evolution, suggesting that they formed around the same time. However, the history of star formation varies from cluster to cluster, and in some cases a cluster contains different populations of stars. An example of this would be globular clusters in the Large Magellanic Cloud, which show a bimodal population. At an early age, these clusters could have collided with a giant molecular cloud that 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 show that they occur mainly in regions with effective star formation, that is, where the interstellar medium has a higher density than normal star formation regions. The formation of globular clusters dominates in regions with bursts of star formation and in interacting galaxies. Studies also show 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.
No actively star-forming globular clusters are currently known, and this is consistent with the view that they tend to be 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).
Compound
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, low-metallicity stars. The type of stars found in globular clusters is similar to those in the bulge. They lack gas and dust, and it is assumed that they have long since turned into stars. Globular clusters have a high concentration of stars - an average of 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). Globular clusters are not considered to be a favorable place 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 exist 100 million years. the event that led to the formation of the 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. Both of these clusters can be considered evidence that supermassive globular clusters are the core of dwarf galaxies that have been swallowed up by giant galaxies. About a quarter of the globular clusters in the Milky Way may have been part of dwarf galaxies.
Some globular clusters (for example, M15) have very massive cores that may contain black holes, although modeling shows that the available observations are equally well explained by the presence of less massive black holes, as well as by the concentration (or massive ).
The cluster M 53 surprised astronomers with a number of stars called blue stragglers.
Globular clusters are usually composed of population II stars that have a low abundance of heavy elements. Astronomers call heavy elements metals, and the relative concentration of these elements in a star, metallicity. These elements are created in 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 usually have lower metallicities.
The Dutch astronomer Peter Oosterhof observed that there are probably two populations of globular clusters known as the "Oosterhof 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 variables. Thus, type I stars are called "rich in metals", and type II stars - "low metal". These two populations are observed in many galaxies, especially in massive ellipticals. Both age groups are almost the same 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 several phases of star formation in a single galaxy. In the Milky Way, low-metal clusters are associated with the halo, while metal-rich clusters are associated with the bulge.
In the Milky Way, most low-metal clusters are aligned along a plane in the outer part of the galaxy's halo. 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 when the two galaxies formed their cluster systems.
Exotic Components
In globular clusters, the density of stars is very high, and therefore close passages and collisions often occur. A consequence of this is the greater abundance of certain exotic classes of stars in globular clusters (for example, blue stragglers, millisecond pulsars, and low-mass X-ray binaries). Blue stragglers form when two stars collide, possibly as a result of a collision with a binary system. Such a star is hotter than the other stars in the cluster, which have the same luminosity, and thus differs from the main sequence stars that formed when the cluster was born.
Since the 1970s astronomers are looking for black holes in globular clusters, but this task requires a high resolution of the telescope, so only with the advent was the first confirmed discovery made. Based on observations, an assumption was made about the presence of an intermediate-mass black hole (4,000 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 emission from Mayall II corresponds to an intermediate-mass black hole. They are of particular interest because they are the first black holes to have an intermediate mass between ordinary stellar-mass black holes and supermassive black holes in the cores of galaxies. The mass of the intermediate 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. The fact is that the densest objects in globular clusters are supposed to gradually slow down their movement and end up in the center of the cluster as a result of a process called “mass segregation”. In globular clusters, these are white dwarfs and neutron stars. Research by Holger Baumgardt and 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.
Hertzsprung-Russell diagram
A color-magnitude diagram of the M3 cluster. Around magnitude 19 is a characteristic "knee" where stars begin to enter the giant stage.
The 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 the star's blue-light brightness, or B, and the visible-light (yellow-green), or V, color index values. Large B-V color index values indicate a cool red star, while negative values correspond to a blue star with a hot surface. . When stars close to the Sun are plotted on an H-R diagram, it shows the distribution of stars of different masses, ages, and compositions. Many stars in the diagram are relatively close to the sloping curve from the upper left (high luminosities, early spectral types) to the lower right (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 descended from the main sequence.
Because all the stars in a globular cluster are about the same distance from us, their absolute magnitude differs from their apparent magnitude by about the same amount. Main sequence stars in a globular cluster are comparable to similar stars in the vicinity of the Sun and will line up 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.
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 value of the apparent stellar magnitude. The difference between the relative and absolute values, the distance modulus, gives an estimate of the distance.
When the stars of a globular cluster are plotted on a G-R diagram, in many cases almost all the stars fall on a fairly definite curve, which differs from the G-R diagram of stars near the Sun, which combines stars of different ages and origins into one whole. The shape of the curve for globular clusters is a characteristic of groups of stars that formed at about the same time from the same materials and differ only in their initial mass. Since the position of each star in the H-R diagram depends on age, the shape of the curve for a globular cluster can be used to estimate the total 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, lower-mass stars will begin to transition to 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 transition to the giant stage. They form a “knee” in the H-R diagram with a rotation to the upper right corner with respect to the main sequence line. The absolute magnitude in the region of the turning point depends on the age of the globular cluster, so the age scale can be plotted on an axis parallel to the magnitude.
In addition, the age of a globular cluster can be determined from the temperature of the coldest white dwarfs. As a result of calculations, it was found that the typical age of globular clusters can reach up to 12.7 billion years. In this they differ significantly 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 age limit of the entire Universe. This lower limit has been a significant hurdle in cosmology. In the early 1990s, astronomers were faced with estimates of the age of globular clusters that were older than what cosmological models suggested. However, detailed measurements of cosmological parameters through deep sky surveys and the presence 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 the study of globular clusters is then used to study the evolution of the entire Milky Way.
In globular clusters, there are some stars known as blue stragglers that appear to continue moving down 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 a mass transfer between stars in binary and triple systems.
Globular star clusters in the Milky Way galaxy
Globular clusters are collective members of our galaxy and are part of its spherical subsystem: they revolve around the center of mass of the galaxy in highly elongated orbits with velocities of ≈200 km/s and an orbital period of 10 8 -10 9 years. The age of globular clusters in our Galaxy is approaching its age, which is confirmed by their Hertzsprung-Russell diagrams, which contain a characteristic break in 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 a 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 in the course of revolution around the center of our Galaxy through its plane, in which gas clouds are concentrated, which contributes to the "sweeping out" of one's own gas during such passages.
Globular star clusters in other galaxies
A cluster in the central region of the Tarantula Nebula, a cluster of young and hot stars
In other galaxies (for example, in the Magellanic Clouds), relatively young globular clusters are also observed.
Most of the globular clusters in the LMC and MMO belong to young stars, in contrast to the globular clusters of our Galaxy, and are mostly 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 M31 globular clusters, you need a telescope with a diameter of 10 inches, the brightest can be seen in a 5-inch telescope. The average magnification is 150-180 times, the optical scheme of the telescope does not matter.
The G1 (Mayall II) cluster is the brightest cluster in the Local Group, at a distance of 170,000 ly. years.
open star cluster
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 1100 open clusters have been discovered in our Galaxy, but it is assumed that there are many more. The stars in such clusters are connected to each other by relatively weak gravitational forces, therefore, as they revolve around the galactic center, clusters can be destroyed due to close passage near other clusters or clouds of gas, in which case the stars that form 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 take place.
Young open clusters can be inside the molecular cloud from which they were formed, and "illuminate" it, resulting in a region of ionized hydrogen. Over time, the radiation pressure from the cluster disperses the cloud. As a rule, only about 10% of the mass of a gas cloud has time to form stars before the rest of the gas is dispersed by the pressure of light.
Open star clusters are key objects for studying stellar evolution. Due to the fact that cluster members have the same age and chemical composition, the effects of other characteristics are easier to determine for clusters than for individual stars. Some open clusters, such as the Pleiades, Hyades, or the Alpha Perseus Cluster, are visible to the naked eye. Some others, such as the Perseus Double Cluster, are barely visible without instruments, and many more can only be seen with binoculars or a telescope, such as the Wild Duck Cluster (M 11).
Historical observations
Mosaic of 30 images of open clusters discovered by the VISTA telescope. From direct observation, these clusters are obscured by the dust of the Milky Way.
The bright open star cluster Pleiades has been known since antiquity, and the Hyades are 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 Double Cluster at Perseus, and the Cluster of Ptolemy; and the Persian astronomer As-Sufi described the Omicron Sails 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 in 1609 to observe the starry sky and record the results of these observations was the Italian astronomer Galileo Galilei. When studying some of the nebulous objects described by Ptolemy, Galileo discovered that they were not individual stars, but groups of a large number of stars. So, in the Manger, he distinguished more than 40 stars. While his predecessors distinguished 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 located in groups". Inspired by the work of Galileo, the Sicilian astronomer Giovanni Hodierna was perhaps the first astronomer to find previously unknown open clusters with a telescope. In 1654, he discovered the objects now called Messier 41, Messier 47, NGC 2362, and NGC 2451.
In 1767, the English naturalist Rev. John Michell calculated that even for a single group such as the Pleiades, the probability that its constituent stars were randomly 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, the French astronomer Charles Messier published a catalog of celestial objects that had a comet-like hazy appearance. This catalog includes 26 open clusters. In the 1790s, the English astronomer William Herschel began a comprehensive study of nebulous celestial objects. He found that many of these formations could be broken down into groups of individual stars. Herschel suggested that initially the stars were scattered in space, and then, as a result of gravitational forces, formed star systems. He divided nebulae into 8 categories, and assigned classes VI to VIII to classify star clusters.
Through 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, as well as in two additional index catalogs published in 1896 and 1905. identify two different types of clusters. The first consisted of thousands of stars arranged according to a regular spherical distribution; they met throughout the sky, but most densely - in the direction of the center of the Milky Way. The stellar population of the latter was more rarefied, and the shape was more irregular. Such clusters were usually located inside 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 referred to as galaxy clusters, the term was proposed in 1925 by the 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. Schoenfeld, and then by the American astronomer E. E. Barnard in 1898-1921. These attempts have not revealed any signs of stellar motion. However, in 1918, the Dutch-American astronomer Adrian van Maanen, by comparing photographic plates taken at different points in time, was able to measure the proper motion of stars for part of the Pleiades cluster. As astrometry became more and more precise, it became clear that clusters of stars share the same proper motion in space. By comparing photographic plates of the Pleiades obtained in 1918 with those of 1943, van Maanen was able to isolate stars whose proper motion was similar to the average for the cluster, and thus identify likely members of the cluster. Spectroscopic observations have revealed common radial velocities, thus showing that clusters are composed of stars linked together into 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. In the next 20 years, he continued his work on the study of 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 Mangers differed from those of the Pleiades. Subsequently, this was explained by the difference in the age of these three clusters.
Education
Infrared shows a dense cluster being born in 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 with a mass many thousands of times greater than 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 . As a rule, only 1-10% of the cloud volume exceeds this density. Before collapse, such clouds can maintain mechanical equilibrium due to magnetic fields, turbulences, and rotation.
There are many factors that can upset the balance of a giant molecular cloud, which will lead to collapse and the beginning of the process of active star formation, which may result in an open cluster. These include: shock waves from close ones, collisions with other clouds, gravitational interactions. But even in the absence of external factors, some parts of the cloud can reach conditions where they become unstable and prone to collapse. The collapsing region of the cloud experiences hierarchical fragmentation into smaller regions (including relatively dense regions known as infrared dark clouds), which eventually leads to the birth of a large number (up to several thousand) of stars. This process of star formation begins in a shell of a collapsing cloud that hides from view, although it allows infrared observations. It is believed that in the Milky Way galaxy, one new open cluster forms once every several thousand years.
"Pillars of Creation" - a region of the Eagle Nebula, where a molecular cloud is blown away by a stellar wind from young massive stars.
The hottest and most massive of the newly formed stars (known as OB stars) radiate intensely in the ultraviolet, which constantly ionizes the surrounding molecular cloud gas and forms the H II region. The stellar wind and radiation pressure from massive stars begin to accelerate the hot ionized gas at speeds comparable to the speed of sound in the gas. A few million years later, the first supernova explosion occurs in the cluster ( core-collapse supernovae), which also pushes gas out of its vicinity. In most cases, these processes accelerate all the gas within 10 million years, and star formation stops. But about half of the formed 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 the 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 breaks up completely. From this point of view, the formation of an open cluster depends on whether the gravitationally born stars are 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, however, it can only hold 1/3 of its original number of stars, and the rest will no longer be bound once the gas dissipates. Young stars that no longer belong to the home cluster will become part of the general population of the Milky Way.
Due to the fact that almost all stars form in clusters, the latter are considered the basic building blocks of galaxies. Intense processes of gas scattering, which both form and destroy many star clusters at birth, leave their imprint on the morphological and kinematic structures of galaxies. Most newly formed open clusters have a population of 100 or more stars and a mass of 50 or more solar. The largest clusters can have masses 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 clusters are completely different formations, the appearance of the rarest globular clusters and the richest open clusters may not be so 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 - no longer exist in our Galaxy.
The formation of more than one open cluster from one molecular cloud is a typical phenomenon. Thus, in the Large Magellanic Cloud, the Hodge 301 and R136 clusters formed from the gas of the Tarantula Nebula; tracking the trajectories of the Hyades and the Manger, two prominent and nearby clusters of the Milky Way, leads to the conclusion 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 prime example of this in our galaxy is the Perseus Double Cluster, consisting of NGC 869 and NGC 884 (sometimes erroneously called "χ and h Persei" ( "hi and ash Perseus"), although h refers to the neighboring star, and χ - to both clusters), however, besides it, at least 10 similar clusters are known. Even more of them are discovered in the Small and Large Magellanic Clouds: these objects are easier to detect in external systems than in our Galaxy, because due to the projection effect, distant friends clusters can look related to each other from one another.
Morphology and classification
Open clusters can represent both sparse groups of several stars, and large agglomerations, including thousands of members. They tend to consist of a well-defined, dense core surrounded by a more diffuse "crown" of stars. The core diameter is usually 3-4 St. g., and the crown - 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 sv./St. g. 3).
Open star clusters are often classified according to the scheme developed by Robert Trumpler in 1930. The class name according to this scheme consists of 3 parts. The first part is denoted by the Roman numerals I-IV and means 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, meaning the spread in the brightness of the members (from small to large spread). The third part is a letter p, m or r, denoting, respectively, a low, medium, or large number of stars in a cluster. If the cluster is inside a nebula, then a letter is added at the end n.
For example, according to the Trumpler scheme, the Pleiades are classified as I3rn (highly concentrated, rich in stars, there is a nebula), and the closer Hyades - as II3m (more fragmented and with less abundance).
Number and distribution
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 higher. In spiral galaxies, open clusters are mainly located along spiral arms, where the gas density is highest and, as a result, star formation processes are most active; such clusters usually disperse before they have time to leave the arm. Open clusters have a strong tendency to be near the galactic plane.
In irregular galaxies, open clusters can be anywhere, although their concentration is higher where the gas density is greater. Open clusters are not observed in elliptical galaxies, since the processes of star formation in the latter ceased many millions of years ago, and the last of the formed clusters have long since dispersed.
The distribution of open clusters in our Galaxy depends on age: older clusters are located mainly at greater distances from the galactic center and at a considerable distance from the galactic plane. This is due to 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, the giant molecular clouds, which are also the cause of destruction, are concentrated in the inner regions of the disk of the galaxy; therefore, clusters from the inner regions are destroyed at an earlier age than their "colleagues" from the outer regions.
Star cast
A multi-million-year-old cluster of stars (lower right corner) illuminates the Tarantula Nebula in the Large Magellanic Cloud.
Due to the fact that open star clusters usually decay before most of their stars have completed their life cycles, most of the radiation from clusters is light from young hot blue stars. Such stars have the largest 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 during the collision of stars and the formation of 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. It is believed that most of them are formed when a binary star system merges into one star due to dynamic interactions with other members.
As soon as low- and medium-mass stars use up their supply of hydrogen in the process of nuclear fusion, they shed their outer layers and form a planetary nebula with the formation of a white dwarf. Even though most open clusters decay 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 from the age of the cluster and the estimated initial stellar mass distribution. . One possible explanation for the lack of white dwarfs is that when a red giant sheds its shell and forms a planetary nebula, some slight asymmetry in the mass of the ejected material can give the star a speed of several kilometers per second - 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 1,000 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 matter 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.
Fate
NGC 604 in the Triangulum Galaxy is an extremely massive open cluster surrounded by a region of ionized hydrogen.
Many open clusters are inherently unstable: due to their small mass, the escape velocity from the system is less than the average velocity of its component stars. Such clusters break up very quickly over several million years. In many cases, the pushing out 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 decays very quickly.
Clusters that, after dispersal of the surrounding nebula, have enough mass to be gravitationally bound, can retain their shape for many tens of millions of years, but over time, internal and external processes also lead to their decay. The close passage of one star next to another can increase the speed of one of the stars so much that it exceeds the speed of escape from the cluster. Such processes lead to the gradual "evaporation" of cluster members.
On average, every half a million years, star clusters experience the influence of external factors, for example, passing next to or through a molecular cloud. Gravitational tidal forces from such close proximity tend to destroy star clusters. Eventually it becomes star stream: 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 with the same speeds. The period of time after which the cluster breaks up 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 movement in space; the so-called star association(or moving group of stars). So, several bright stars of the "bucket" of the Big Dipper are former members of the open cluster, which has turned into such an association called the "moving group of stars of the Big Dipper". Eventually, due to small differences in their speeds, they will disperse throughout the galaxy. Larger accumulations become streams, provided that the sameness of their speeds and ages can be established; otherwise, the stars will be considered unconnected.
Stellar Evolution Research
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 the Hertzsprung-Russell diagram for an open cluster, most of the stars will belong to the main sequence (MS). At some point, called the turning point, the most massive stars leave the MS and become red giants; The “remoteness” 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 at almost the same distance from and were formed at about 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 a cluster.
For example, the study of 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 at temperatures 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 strongly the 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.
Studies have shown that the abundance of these light elements is much lower than models of stellar evolution predict. The reasons for this are not entirely clear; one of the explanations is that in the interior of the star there are ejections of matter from the convective zone to the stable zone of radiative transfer ( convection overshoot).
Astronomical distance scale
The Wild Duck (M 11) is a very rich cluster located towards the center of the Milky Way.
Determining the distances to astronomical objects is key to understanding them, but the vast majority of such objects are too far away to be measured directly. The graduation of the astronomical scale of distances depends on a succession of indirect and sometimes indeterminate measurements in relation first to the nearest objects, the distances to which can be measured directly, and then to more and more distant ones. Open star clusters are the most important rung on this ladder.
Distances to clusters closest to us can be measured directly in one of two ways. First, for the stars of the nearest clusters, one can determine the parallax (a slight shift in the apparent position of the object during the year due to the movement of the Earth in the orbit of the Sun), as is usually done for individual stars. Pleiades, Hyades and some other clusters in the vicinity of 500 St. years are close enough for such a 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 the stars in the cluster share the same parameters of movement in space. Measuring the proper motions of the members of the cluster and plotting their apparent movement across the sky on a map will make it possible to establish that they converge at one point. The radial velocities of cluster stars can be determined from measurements of Doppler shifts in their spectra; when all three parameters - radial velocity, proper motion, and angular distance from the cluster to its vanishing point - are known, simple trigonometric calculations will allow the distance to the cluster to be calculated. The most famous case of using this method concerned the Hyades and made it possible to determine the distance to them at 46.3 parsecs.
Once distances to nearby clusters have been established, other methods can extend the distance scale for more distant clusters. By comparing the main sequence stars in the Hertzsprung-Russell diagram for a cluster whose distance is known with the corresponding stars in a more distant cluster, one can determine the distance to the latter. 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 about 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" dependence 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 can be used to extend the scale further - to the nearest galaxies of the Local Group.
star association
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.
Star associations were discovered by V. A. Ambartsumyan in 1948 and predicted their disintegration. Later measurements by A. Blaauw, W. Morgan, V. E. Markaryan, I. M. Kopylov, and others confirmed the 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 - a few parsecs) and a lower density: the number of stars in an association is from tens to hundreds (in open star clusters - from hundreds to thousands) . The origin of stellar associations is due to the regions of star formation of molecular cloud complexes.
There are the following types of star associations:
- OB associations containing mainly massive stars of spectral types O and B
- T-associations containing mostly low-mass variables
- R-associations (from R - reflection), in which the stars of spectral types O - A2 surrounded by reflective gas and dust nebulae.
