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 adopted according to appearance, but as we studied further, it became clear that globular and open clusters are dissimilar in literally everything - in age, stellar composition, nature of motion, etc.
Globular star clusters contain from tens of thousands to millions of stars. This type of cluster is characterized by a regular spherical or somewhat oblate shape (which, apparently, is a sign of axial rotation of the cluster). But star-poor clusters are also known, indistinguishable in appearance from open ones (for example, NGC 5053), and classified as globular according to characteristic features spectrum-luminosity diagrams. The two brightest of the globular clusters are designated Omega Centauri (NGC 5139) and 47 Tucanae (NGC 104), as ordinary stars, because due to their significant apparent brightness (+3. 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 a visible angular size is about 1".
There are currently about 150 known globular clusters in the Galaxy, but it is obvious that this is only a small part of those that actually exist (their total number is estimated at about 400-600). Their distribution across the celestial sphere is uneven - they are strongly concentrated towards the galactic center, forming an extended halo around it. About half of them are located no further than 30 degrees from the visible center of the Galaxy (in Sagittarius), i.e. in an area whose area is only 6% of the entire area of the celestial sphere. This distribution is a consequence of the peculiarities of the rotation of globular clusters around the center of the Galaxy, characteristic of objects of the spherical subsystem - in highly elongated orbits. Once per period (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 most bright stars are red giants. In addition, variable stars are observed in them (especially often of the RR Lyrae type), as well as the end products of the evolution of massive stars (involving in close binary systems with normal star white dwarfs, neutron stars and black holes), manifesting themselves as X-ray sources 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 various types- flaring, Cepheids, etc. Chemical composition stars included in open clusters are quite homogeneous, and on average the content of heavy elements is close to the solar one, which is typical for objects in the galactic disk.
Another feature of open clusters is that they are often visible together with a gas-dust nebula - a remnant of the cloud from which the stars of this cluster once formed. Stars can heat up or illuminate “their” nebula, making it visible. The well-known Pleiades (see photo) are also immersed in a blue, cold nebula. In a galaxy, open clusters can only exist where there are many gas clouds. In spiral galaxies such as ours, such places are found in abundance in the flat component of the galaxy, and young clusters serve as good indicators of 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 a few stars to several hundreds, thereby differing from young star clusters large size and lower density of stars. The attraction between stars in associations is usually too weak to keep them together, and therefore associations do not last long (by cosmic standards) - in just 10-20 million years they expand so much that their stars no longer stand out from other stars. The existence 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 gravitational interaction. Over millions of years, due to the tidal action of galaxies, clusters gradually disintegrate - the stars included in them move further and further away from each other and gradually lose their gravitational connections. Sometimes, by 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 the sky large area. This stream consists of about 100 stars, among which are Gemma (alpha of the Northern Crown), and even Sirius!
In the topic of star clusters, it would be useful to finally mention asterisms- characteristic configurations (often - correct form, 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.
Explore star clusters Universe: description, classification, scattered and spherical types with photos and videos, list of clusters, age, Messier catalog, destruction.
– a group of stars with a common origin and gravitational connection for a certain time. This useful tool for astronomers, as they help study and model stellar evolution. There are two main types of star clusters: open clusters (open) and globular clusters. Learn more about the galaxy's star clusters in an interesting video.
Astronomer Alexey Rastorguev on the role of gravity in the Universe, open and globular clusters and the study of the history of galaxies:
So called because individual stars can be easily resolved. For example, the Pleiades and Hyades are so close that individual stars can be seen without any problems with the naked eye. They are sometimes called galaxy clusters because they are located in dusty spiral arms. Stars in an open cluster have a common origin (they were formed from the same initial molecular cloud). Typically, a cluster contains several hundred stars (can reach several thousand).
Stars are bound by gravity, but it is quite weak. The cluster rotates around the galaxy and in the final stage is dissipated due to gravitational contact with stronger objects. It is believed that the Sun appeared in an open cluster, which now no longer exists. Therefore, these are always young objects. Nebula is still visible in the Pleiades, hinting at recent formation.
Open clusters are filled with population I stars - young and with high level metallicity. The width ranges from 2 to 20 parsecs.
Globular clusters galaxies contain from a couple of thousand to a million stars located in a spherical gravitational system. They are in a halo and represent the most ancient stars - population II (developed, but low metallicity). The clusters are so old that any star (above class G or F) has already crossed the main sequence. There is little dust and gas in a globular cluster because new stars do not form there. The density in the interior regions is much higher than in areas near the Sun.
In globular clusters, stars also share a common origin. But this type holds objects firmly by gravity (stars do not scatter). There are approximately 200 globular clusters in the Milky Way. Among them we can recall 47 Tucanas, M4 and Omega Centauri. Although there are suggestions about the latter that it could be a dwarf spheroidal galaxy.
Globular clusters
Astronomer Vladimir Surdin about the types of star clusters, the galactic nucleus and life on the planets of globular clusters:
Star clusters are incredibly valuable to astronomers because they can be used to determine the age of a star and track its evolution.
Stars in open clusters have a single origin, so their metallicity levels converge, which means that all members will go through the same evolutionary stages. In addition, they are located at the same distance, which also allows you to display the absolute value. If you see bright stars that stand out, they are much brighter than their fainter neighbors.
With this information, scientists create digital diagrams for the clusters. They display the apparent value of V on the vertical axis relative to the digital index B – V horizontally. Using spectrographic parallax, the diagram can be calibrated to derive an absolute value.
If we build diagrams for them, we get the bottom graph. Since they are at different distances, it is calibrated to absolute values.
A new scale is visible on the right vertical axis. “Years” is the age of the cluster. The pair in Perseus is so young that most of the stars are at the main sequence stage. The Pleiades are slightly older and do not have stars above color index 0 (spectral class A0). More massive objects have already stepped over to the giant branches. M67 does not have a star hotter than color index 0.4. Highest value has a turning point in the diagram where the cluster switches off the main sequence. The lower the main sequence, the older the cluster.
Globulars are usually much older than open ones, so the colored magnitude in the diagram shows more developed stars. They are also devoid of objects with large mass. This point is illustrated below using the M55 example.
A group of hot main sequence stars above the shutdown point is visible here. They are called blue stragglers. Scientists believe that due to the high stellar densities in globular clusters, some are capable of merging. The combined mass makes the star hotter and brighter than the main stellar mass. Star clusters are not timeless designs and they are destroyed. Learn this process in the video. Also use an online star map to find the clusters yourself. If you can’t buy a telescope, then visit our page with a virtual model of the galaxy Milky Way or look at the photo from the list of clusters.
Collapse of star clusters
Astronomer Alexey Rastorguev about stellar dynamics, the lifetime of star clusters and the gravitational potential of our Galaxy
Astronomers call star clusters groups of dynamically interconnected groups containing a large number of stars and differing in type and stellar composition. Based on their appearance, there are two groups of star clusters: open clusters, which include tens and hundreds of stars, and globular clusters, which can contain tens and hundreds of thousands.
Open star clusters
photo: Globular star cluster m55
Globular star clusters
The color-apparent magnitude diagram for stars in globular clusters usually clearly shows a characteristic horizontal branch, or giant branch, connected to the main sequence, as well as the main sequence itself, which begins in a region with lower luminosities than in the usual Hertzsprung-Russell diagram. Sometimes in globular clusters you can see a significant amount variable stars, most often of the RR Lyrae type, allowing one to determine the distances to these objects.
In 1947, Soviet astrophysicist Viktor Ambartsumyan and his colleagues discovered special groups of stars, which were called stellar associations. These are groups of stars of a certain type, the stellar density of which is much greater than the average stellar density of stars of this type in the Galaxy. Scientists have identified two types. The first - O-associations - includes stars of early spectral types from O to B2, the magnitude of which is tens and hundreds of parsecs, i.e. many times larger than open star clusters. Associations of the second type include τ Tauri stars and are therefore called τ associations.
rice. Color diagram - apparent magnitude of the globular cluster M3
Getting to know more and more a large number objects for observation in a series of articles about us we often come across space objects called . Based on their appearance, clusters are divided into 2 types: scattered(or open) and ball. Let's find out a little more about them.
This type of cluster contains from 20 to several thousand stars. They are easy to observe and find on starry sky with the naked eye, but with a simple amateur telescope you can see individual areas. The stars are connected to each other gravitational attraction and are predominantly young and hot.
Such clusters are located near the strip of the Milky Way. About 1000 open clusters are known, but astronomers suggest that 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 M 45), with its magnitude equal to 1.6 m.
The photo above shows cosmic dust between the stars - in fact, it reflects the blue light of very hot and young stars.
One more good example open clusters is a cluster Wild duck(or M 11) 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 emerging stars of classes F, G, K, M.
Globular clusters include from 10 thousand to a million stars. Through binoculars or an amateur telescope, you can only see the shape and some of the overall outlines. For a more detailed study, you will need a powerful tool.
Such clusters are located in close proximity to our Milky Way galaxy. They rotate 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 their high brightness and strong luminosity, almost all clusters of this type can be observed. Their total number is slightly more than 100.
Globular star cluster M 12
Cluster M 12 is located in the constellation and in the first month of summer you can hunt for it. Another bright representative of the globular cluster, which is also located in this constellation, is M 14:
Bright globular cluster M 14
Globular clusters are interesting to hunt 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 notes on a blog. Read it.
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.
