In 1845, the English astronomer Lord Ross discovered a whole class of spiral-type nebulae. Their nature was established only at the beginning of the twentieth century. Scientists have proven that these nebulae are huge star systems similar to our Galaxy, but they are many millions of light-years away from it.
General information
Spiral galaxies (the photos in this article demonstrate the features of their structure) look like a pair of saucers stacked together or a biconvex lens. They can detect both a massive stellar disk and a halo. The central part, which visually resembles swelling, is commonly called the bulge. And the dark band (an opaque layer of the interstellar medium) that runs along the disk is called interstellar dust.
Spiral galaxies are usually denoted by the letter S. In addition, they are usually divided according to the degree of structure. To do this, the letters a, b or c are added to the main character. Thus, Sa corresponds to a galaxy with an underdevelopedspiral structure, but with a large core. The third class - Sc - refers to opposite objects, with a weak core and powerful spiral branches. Some star systems in the central part may have a jumper, which is commonly called a bar. In this case, the symbol B is added to the designation. Our Galaxy is of an intermediate type, without a jumper.
How did spiral disk structures form?
The flat disk-shaped forms are explained by the rotation of star clusters. There is a hypothesis that during the formation of a galaxy, the centrifugal force prevents the compression of the so-called protogalactic cloud in a direction perpendicular to the axis of rotation. You should also be aware that the nature of the movement of gases and stars inside nebulae is not the same: diffuse clusters rotate faster than old stars. For example, if the characteristic rotation velocity of the gas is 150-500 km/s, then the halo star will always move more slowly. And bulges consisting of such objects will have a speed three times lower than disks.
Star gas
Billions of star systems moving in their orbits inside galaxies can be considered as a collection of particles that form a kind of stellar gas. And what is most interesting, its properties are very close to ordinary gas. Such concepts as "concentration of particles", "density", "pressure", "temperature" can be applied to it. The analogue of the last parameter here is the averaged energy"chaotic" movement of stars. In rotating disks formed by stellar gas, waves of a spiral type of rarefaction-compression density close to sound waves can propagate. They are able to run around the galaxy at a constant angular velocity for several hundred million years. They are responsible for the formation of spiral branches. At the moment when gas compression occurs, the process of formation of cold clouds begins, which leads to active star formation.
This is interesting
In halo and elliptical systems, the gas is dynamic, that is, hot. Accordingly, the motion of stars in a galaxy of this type is chaotic. As a result, the average difference between their velocities for spatially close objects is several hundred kilometers per second (velocity dispersion). For stellar gases, the velocity dispersion is usually 10-50 km/s, respectively, their "degree" is noticeably cold. It is believed that the reason for this difference lies in those distant times (more than ten billion years ago), when the galaxies of the Universe were just beginning to form. Spherical components were the first to form.
Spiral waves are called density waves that run along a rotating disk. As a result, all the stars of a galaxy of this type are, as it were, forced out into their branches, then exit from there. The only place where the speeds of spiral arms and stars coincide is the so-called corotation circle. By the way, this is where the sun is located. For our planet, this circumstance is very favorable: the Earth exists in a relatively quiet place in the galaxy, as a result, for many billions of years it has not been particularly affected by cataclysms of a galactic scale.
Features of spiral galaxies
Unlike elliptical formations, each spiral galaxy (examples can be seen in the photos presented in the article) has its own unique flavor. If the first type is associated with calmness, stationarity, stability, then the second type is dynamics, whirlwinds, rotations. Maybe that's why astronomers say that the cosmos (the universe) is "furious". The structure of a spiral galaxy includes a central core, from which beautiful arms (branches) emerge. They are gradually losing their outlines outside their star cluster. Such an appearance cannot but be associated with a powerful, swift movement. Spiral galaxies are characterized by a variety of shapes as well as patterns of their branches.
How galaxies are classified
Despite this diversity, scientists were able to classify all known spiral galaxies. We decided to use the degree of development of the arms and the size of their core as the main parameter, and the level of compression faded into the background as unnecessary.
Sa
Edwin P. Hubble assigned to the Sa class those spiral galaxies that have underdeveloped branches. Such clusters always have large cores. Often the center of a galaxy of a given classis half the size of the entire cluster. These objects are characterized by the least expressiveness. They can even be compared to elliptical star clusters. Most often, the spiral galaxies of the Universe have two arms. They are located on opposite edges of the nucleus. The branches unwind in a symmetrical, similar way. With distance from the center, the brightness of the branches decreases, and at a certain distance they cease to be visible at all, being lost in the peripheral regions of the cluster. However, there are objects that have not two, but more sleeves. True, such a structure of the galaxy is quite rare. Even rarer are asymmetric nebulae, when one branch is more developed than the other.
Sb and Sc
The Edwin P. Hubble subclass Sb has noticeably more developed arms, but they do not have rich ramifications. The nuclei are noticeably smaller than those of the first species. The third subclass (Sc) of spiral star clusters includes objects with highly developed branches, but their center is relatively small.
Is rebirth possible?
Scientists have found that the spiral structure is the result of the unstable motion of stars, resulting from strong compression. In addition, it should be noted that, as a rule, hot giants are concentrated in the arms and the main masses of diffuse matter - interstellar dust and interstellar gas - accumulate there. This phenomenon can also be viewed from another angle. There is no doubt that a very compressed star cluster in the course of its evolutioncan no longer lose its degree of compression. Hence, the opposite transition is also impossible. As a result, we conclude that elliptical galaxies cannot turn into a spiral one, and vice versa, because this is how the cosmos (the Universe) is arranged. In other words, star clusters of these two types are not two different stages of a single evolutionary development, but completely different systems. Each such type is an example of opposite evolutionary paths due to a different compression ratio. And this characteristic, in turn, depends on the difference in the rotation of galaxies. For example, if a star system receives enough rotation during its formation, it can contract and develop spiral arms. If the degree of rotation is insufficient, then the galaxy will be less compressed, and its branches will not form - it will be a classic elliptical shape.
What else are the differences
There are other differences between elliptical and spiral star systems. Thus, the first type of galaxy, which has a low level of compression, is characterized by a small amount (or complete absence) of diffuse matter. At the same time, spiral clusters with a high level of compression contain both gas and dust particles. Scientists explain this difference in the following way. Dust particles and gas particles periodically collide during their movement. This process is inelastic. After the collision, the particles lose some of their energy, and as a result, they gradually settle into thoseplaces in the star system where there is the least potential energy.
Highly compressed systems
If the process described above takes place in a highly compressed star system, then diffuse matter should settle on the main plane of the galaxy, because it is here that the level of potential energy is the lowest. This is where gas and dust particles are collected. Further, diffuse matter begins its movement in the main plane of the star cluster. Particles move almost parallel in circular orbits. As a result, collisions here are quite rare. If they do occur, then the energy losses are negligible. It follows from this that matter does not move further to the center of the galaxy, where the potential energy has an even lower level.
Weakly compressed systems
Now consider how an ellipsoid galaxy behaves. A star system of this type is distinguished by a completely different development of this process. Here, the main plane is not at all a pronounced region with a low level of potential energy. A strong decrease in this parameter occurs only in the central direction of the star cluster. And this means that interstellar dust and gas will be attracted to the center of the galaxy. As a consequence, the density of diffuse matter here will be very high, much higher than with flat scattering in a spiral system. The particles of dust and gas gathered in the center of the accumulation under the action of the force of attraction will begin to shrink, thereby forming a small zone of dense matter. Scientists suggest that from this matter in the futurenew stars begin to form. Something else is important here - a small cloud of gas and dust, located in the core of a weakly compressed galaxy, does not allow itself to be detected in the process of observation.
Intermediate stages
We have considered two main types of star clusters - with a weak and with a strong level of compression. However, there are also intermediate stages when the compression of the system is between these parameters. In such galaxies, this characteristic is not strong enough for diffuse matter to accumulate along the entire main plane of the cluster. And at the same time, it is not weak enough for particles of gas and dust to concentrate in the region of the core. In such galaxies, diffuse matter gathers into a small plane that gathers around the core of the star cluster.
Barred galaxies
Another subtype of spiral galaxies is known - this is a star cluster with a bar. Its feature is as follows. If in a conventional spiral system the arms come out directly from the disk-shaped core, then in this type the center is located in the middle of the straight bridge. And the branches of such a cluster start from the ends of this segment. They are also called galaxies of crossed spirals. By the way, the physical nature of this jumper is still unknown.
In addition, scientists have discovered another type of star clusters. They are characterized by a core, like spiral galaxies, but they do not have arms. The presence of a core indicates strong compression, butall other parameters resemble ellipsoidal systems. Such clusters are called lenticular. Scientists suggest that these nebulae are formed as a result of the loss of diffuse matter by a spiral galaxy.
