Spiral galaxies. Space, the Universe. Galaxies of the Universe

In 1845, an 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 proved that these nebulae are huge star systems similar to our Galaxy, but they are removed from it by many millions of light years.

general information

Spiral galaxies (the photos shown in this article demonstrate the features of their structure) resemble a pair of stacked plates or a biconvex lens. They can detect both a massive star disk and a halo. The central part, which visually resembles a swelling, is usually called a bulge. A dark band (an opaque interlayer of 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 by 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 a little spiral structure, but with a large nucleus. The third class, Sc, refers to opposing objects, with a weak core and powerful spiral arms. Some star systems in the central part may have a jumper, which is usually called a bar. In this case, the symbol B is added to the designation. Our Galaxy belongs to the intermediate type, without a jumper.

How did the spiral disk structures form?

Flat disc-shaped forms are explained by the rotation of star clusters. There is a hypothesis that in the process of galaxy formation, the centrifugal force prevents compression of the so-called protogalactic cloud in the perpendicular direction to the axis of rotation. We should also know that the nature of the motion of gases and stars inside nebulae is not the same: diffuse clusters rotate faster than old stars. For example, if the characteristic rotation speed of the gas is 150-500 km / s, then the halo star will always move more slowly. A bulge, consisting of such objects, will have a speed of three times lower than disks.

Star Gas

Billions of stellar systems moving in their orbits within 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. To it it is possible to apply such concepts, as "concentration of particles", "density", "pressure", "temperature". The analog of the last parameter here is the averaged energy of the "chaotic" motion of the stars. In rotating disks formed by a stellar gas, waves of spiral type of rarefaction-compression density, which are close to sound waves, can propagate. They are able to run around a galaxy with a constant angular velocity for several hundred million years. They are responsible for the formation of spiral arms. At the moment when the gas is compressed, the process of formation of cold clouds begins, which leads to active star formation.

It is interesting

In halo and in elliptical systems, the gas is dynamic, that is, hot. Accordingly, the motion of stars in a galaxy of this type has a chaotic character. 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. The first of them formed spherical components.

Spiral waves are density waves that run along a rotating disk. As a result, all the stars of a galaxy of this type are then forced into their branches, then they come out. The only place where the velocity of the spiral arms and stars coincides is the so-called corotation circle. By the way, it is in this place is the Sun. For our planet, this circumstance is very favorable: the Earth exists in a relatively quiet place of the galaxy, as a result, for many billions of years it does not experience a special effect of cataclysms of a galactic scale.

Features of spiral galaxies

Unlike elliptical formations, each spiral galaxy (examples can be seen in the photo presented in the article) has its own unique color. If the first type is associated with tranquility, stationarity, stability, then the second type is dynamics, vortices, and rotations. Maybe that's why astronomers say that the cosmos (the Universe) is "violent". The structure of a spiral-type galaxy includes a central core from which beautiful sleeves (branches) come out. They outside the star cluster gradually lose outlines. This appearance can not but be associated with a powerful, impetuous movement. Spiral galaxies are characterized by a variety of shapes, as well as drawings of their branches.

How do they classify galaxies

Despite such diversity, scientists were able to classify all known spiral galaxies. As the main parameter, they decided to use the degree of development of the sleeves and the size of their core, and the level of squeeziness, as unnecessary, receded into the background.

Sa

Edwin P. Hubble brought to the class Sa those spiral galaxies that have underdeveloped branches. Such clusters always have large nuclei. Often the center of a galaxy of this class is half the size of the entire cluster. These objects are characterized by the least expressiveness. They can even be compared with 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 are developed symmetrically, in a similar way. As you move away from the center, the brightness of the branches decreases, and at a certain distance they cease to be visible at all, are lost in the peripheral areas of the cluster. However, there are objects that do not have two, but more sleeves. True, such a structure of the galaxy is quite rare. Even more rare are asymmetrical nebulae, when one branch is more developed than the other.

Sb and Sc

The subclass of Sb, according to Edwin P. Hubble's classification, has much more developed sleeves, but they do not have rich branches. The nucleus is noticeably smaller than that 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 established that the structure of the spiral is the result of unstable motion of stars, which arises due to strong compression. In addition, it should be noted that in the sleeves, as a rule, hot giants are concentrated, and there also accumulate the main masses of diffuse matter-interstellar dust and interstellar gas. This phenomenon can also be considered on the other hand. There is no doubt that a very compressed star cluster in the course of its evolution can no longer lose its degree of contraction. Hence, the opposite transition is also impossible. As a result, we conclude that elliptical galaxies can not turn into a spiral galaxy, and vice versa, because the cosmos (the Universe) is arranged in this way. In other words, the star clusters of these two types are not two different stages of a single evolutionary development, but quite different systems. Each such type is an example of opposing evolutionary paths caused by a different compression ratio. And this characteristic, in turn, depends on the difference in the rotation of the galaxies. For example, if, during its formation, the star system receives a sufficient amount of rotation, it will be able to take a compressed form, and it will develop spiral arms. If the degree of rotation is insufficient, the galaxy will be less compressed, and the branches will not form - it will be a classical elliptical shape.

What are the differences

There are other differences between elliptical and spiral stellar systems. Thus, the first type of galaxy, having a low level of compression, is characterized by a small amount (or total absence) of diffuse matter. At the same time, spiral clusters having a high compression level contain both gas and dust particles. This difference scientists explain as follows. Dusts and gas particles periodically collide with their motion. This process is inelastic. After the collision, the particles lose some of their energy, and as a consequence, gradually settle in those places of the stellar system, where there is the lowest potential energy.

Strongly compressed systems

If the process described above occurs in a strongly compressed star system, the diffuse matter must settle on the main plane of the galaxy, because here 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. The particles move practically parallel in circular orbits. As a result of the collision here are quite rare. If they do occur, the energy losses are insignificant. It follows that matter does not move to the center of the galaxy, where the potential energy has an even lower level.

Weakly compressed systems

Now consider how the ellipsoidal 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 strongly 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 more than with plane scattering in a spiral system. The particles of dust and gas gathered in the center of the cluster under the action of the force of attraction will begin to contract, thereby forming a small zone of dense matter. Scientists suggest that new stars start to form from this matter. Important here is another - a small cloud of gas and dust in its dimensions, located in the nucleus of a weakly compressed galaxy, does not allow itself to be detected in the course of observation.

Intermediate stages

We have considered two main types of star clusters - with a weak and with a strong compression level. However, there are 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 gather along the entire basic plane of the cluster. At the same time, it is not sufficiently weak, so that the particles of gas and dust concentrate in the region of the nucleus. In such galaxies, diffuse matter is collected into a small plane, which collects around the core of the star cluster.

Galaxies with lintels

Another subtype of spiral galaxies is known - it is a star cluster with a bridge. Its peculiarity is as follows. If in the case of an ordinary spiral system, the sleeves exit directly from the disk-shaped nucleus, then in this type the center is located in the middle of the straight jumper. And the branches of such a cluster begin from the ends of a given segment. They are also called galaxies of crossed spirals. By the way, the physical nature of this jumper is still unknown.

In addition, scientists managed to discover another kind of star clusters. They are characterized by a nucleus, as in spiral galaxies, but they have no sleeves. The presence of the core indicates a strong compression, but all other parameters resemble ellipsoidal systems. Such clusters are called lenticular. Scientists suggest that these nebulae are formed as a result of the loss of its spiral galaxy of its diffuse matter.