Space is a complex system, the elements of which are closely interconnected: planets unite around one star, stars form galaxies, and they form even larger associations, such as the Local Group of galaxies. Multiplicity is a very common phenomenon in the Universe associated with high gravity. Thanks to it, a center of mass is formed, around which both relatively small objects like stars, and galaxies and their associations revolve.
Composition of the group
It is believed that the basis of the Local Group are three large objects: the Milky Way, the Andromeda Nebula and the Triangulum Galaxy. Their satellites, as well as a number of dwarf galaxies, whose belonging to one of the three systems cannot yet be established, are associated with them by gravitational attraction. In total, the Local Group of galaxies includes at least fifty large celestial objects, and with the improvement of the quality of technology for astronomical observations, this number is growing.
Virgo Supercluster
As already mentioned, the multiplicity inthe scale of the universe - a common occurrence. The Local Group of galaxies is not the largest of these associations, although its size is impressive: in diameter, it occupies a distance of about one megaparsec (3.8 × 1019 km). Along with other similar associations, the Local Group is included in the Virgo supercluster. Its dimensions are hard to imagine, but the mass is relatively accurately measured: 2 × 1045 kg. In total, this association includes about a hundred galactic systems.
It should be noted that the multiplicity does not end there. The Virgo Supercluster, like several others, form the so-called Laniakea. The study of such giant systems has allowed astrophysicists to create a theory of the large-scale structure of the universe.
Types of galaxies that form the Local Group
Scientists have found that the age of all members of the Local Group is approximately 13 billion years. In addition, the matter that forms them has the same composition, which allows us to speak about the common origin of the galaxies of the Local Group. They are not arranged in random order: most of them are built around an imaginary line that runs between the Milky Way and the Andromeda Nebula.
The largest member of the Local Group of Galaxies in terms of size is the Andromeda Nebula: its diameter is 260 thousand light-years (2.5 × 1018 km). In terms of mass, the Milky Way clearly stands out - approximately 6 × 1042 kg. Along with such large objects, there are also dwarf objects like the SagDEG galaxy, located in the constellation Sagittarius.
MostThe Local Group galaxies are classified as irregular, but there are also spiral galaxies like the Andromeda Nebula and elliptical ones, like the already mentioned SagDEG.
Milky Way subgroup
The accuracy of astronomical observations of the Local Group depends on which galaxy we are in. That is why the Milky Way is, on the one hand, the most studied object, and on the other hand, it raises the most questions. To date, it has been established that the satellites of our galaxy are at least 14 objects, among which are the galaxies Ursa Major, Sagittarius, Sculptor and Leo.
Special note is the SagDEG galaxy in Sagittarius. It is the most distant from the gravitational center of the Local Group. According to calculations, the Earth is separated from this galaxy by 3.2 × 1019 km.
Milky Way and Magellanic Clouds
Among the discussions is the question of the connection of the Milky Way with the Magellanic Clouds - two galaxies so close to us that they can be observed with the naked eye from the Southern Hemisphere. For a long time it was believed that they are satellites of our galaxy. In 2006, using the latest technology, it was found that they are moving much faster than other satellites of the Milky Way. Based on this, it was suggested that they have no gravitational connection with our galaxy.
But the fate of the Magellanic Clouds is indisputable. Their movement is directed towardsMilky Way, so their absorption by a larger galaxy is inevitable. Scientists estimate that this will happen after 4 billion years.
Andromeda Nebula and its satellites
After 5 billion years, a similar fate threatens our galaxy, only Andromeda, the largest galaxy in the Local Group, poses a threat to it. The distance to the Andromeda galaxy is 2.5 × 106 light years. It has 18 satellites, of which M23 and M110 (catalogue numbers from the 18th century French astronomer Charles Messier) are the most famous due to their brightness.
Although the Andromeda Nebula is the closest galaxy to the Milky Way, it is difficult to observe due to its structure. It is one of the spiral galaxies: it has a pronounced center, from which two large spiral arms emerge. However, the Andromeda Nebula is turned edge to Earth.
Triangle Galaxy
Its significant remoteness from the Earth significantly complicates the study of both the galaxy itself and its satellites. The number of satellites of the Triangulum galaxy is debatable. For example, the dwarf Andromeda II lies exactly in the middle between the Triangulum and the Nebula. The state of modern observation devices does not allow us to determine which gravitational field of the two largest members of the Local Group of galaxies this space object belongs to. Most still assume that Andromeda II is associated with the Triangulum. But there are also representatives of the opposite point of view, who even propose to rename it toAndromeda XXII.
The Triangulum galaxy also has one of the exotic objects in the universe - the black hole M33 X-7, whose mass is 16 times that of the sun, making it one of the largest black holes known to modern science, excluding supermassive ones.
The problem of globular clusters
The number of members of the Local Group is constantly changing, not only because of the discovery of other galaxies orbiting the same center of mass. Improving the quality of astronomical technology has made it possible to establish that objects previously considered to be galaxies, in fact, are not.
To a greater extent this applies to globular star clusters. They contain a large number of stars tied to one gravitational center, and their shape resembles spherical galaxies. Quantitative relationships help to distinguish between them: the density of stars in globular clusters is much higher, and the diameter is correspondingly higher. For comparison: in the vicinity of the Sun, there is one star per 10 cubic parsecs, while in globular clusters this figure can be 700 or even 7000 times higher.
Dwarf galaxies have long been considered Palomar 12 in the constellation Capricorn and Palomar 4 in Ursa Major. Recent studies have shown that they are in fact quite large globular clusters.
History and difficulties of studying the Local Group of galaxies
Until the second quarter of the 20th century, it was believed that the Milky Way and the Universe were identical concepts. All matter is supposedly within ourgalaxies. However, in 1924, Edwin Hubble, using his telescope, recorded several Cepheids - variable stars with a pronounced luminosity period - the distance to which clearly exceeded the size of the Milky Way. This proved the existence of extragalactic objects. Scientists have thought about the fact that the Universe is more complicated than it seemed before.
Hubble's discovery also proved that the Universe is expanding all the time, and objects are moving away from each other. Improvements in technology brought new discoveries. So it was discovered that the Milky Way has its own satellites, the distances between them were calculated and the prospects for existence were determined. Such discoveries were enough to formulate for the first time the idea of the existence of the Local Group as an impressive association of closely related galaxies, and even to suggest that associations of a higher rank may exist, since satellites were also found in the nearest galaxy to the Milky Way - the Andromeda Nebula. The very term "Local Group" was first used by the same Hubble. He mentions it in his work on measuring distances to other galaxies.
It can be argued that the study of the Cosmos has just begun. This also applies to the Local Group. The SagDEG galaxy was discovered relatively recently, but the reason for this is not only its low luminosity, which has not been recorded by telescopes for a long time, but also the presence in the Universe of a substance that does not have visible radiation - the so-called "dark matter".
Additionally, observations are complicated by diffuse interstellar gas (usually hydrogen) and cosmic dust. However, observational technology does not stand still, which allows us to count on new amazing discoveries in the future, as well as on the refinement of existing information.
