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 proven that these nebulae are huge stellar systems similar to our Galaxy, but they are many millions of light years distant from it.
general information
Spiral galaxies (the photos in this article demonstrate the features of their structure) resemble a couple of plates or a biconvex lens in their appearance. In them you can find both a massive star disk and a halo. The central part, which visually resembles bloating, 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. So, Sa corresponds to a galaxy with an underdeveloped spiral structure, but with a large core. The third class - Sc - refers to opposing objects, with a weak core and powerful spiral branches. 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 spiral disk structures form?
Flat disk-shaped forms are explained by the rotation of star clusters. There is a hypothesis that in the process of galaxy formation, centrifugal force prevents the compression of the so-called protogalactic cloud in the perpendicular direction to the axis of rotation. You should also know 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 speed of gas rotation 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 stellar systems moving in their orbits inside galaxies can be considered as a set of particles that form a kind of stellar gas. And most interestingly, its properties are very close to ordinary gas. Such concepts as “particle concentration”, “density”, “pressure”, “temperature” can be applied to it. An analogue of the last parameter here is the averaged energy of the "chaotic" motion of stars. In rotating disks formed by stellar gas, spiral-type waves of rarefaction-compression density close to sound can propagate. They are capable of circling 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.
It is interesting
In the halo and in 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 in 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 either forced out inside their branches, or emerge from there. The only place where the speed of the spiral arms and stars coincides is the so-called corotation circle. By the way, the Sun is in such a place. This circumstance is very favorable for our planet: the Earth exists in a relatively quiet place of 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 calm, stationarity, stability, then the second type is dynamics, vortices, rotations. Maybe that’s why astronomers say that the cosmos (the Universe) is "frantic." The structure of a spiral-type galaxy includes a central core from which beautiful arms (branches) emerge. Outside their star cluster, they are gradually losing shape. Such an appearance cannot but be associated with a powerful, rapid 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. They decided to use the degree of development of the sleeves and the size of their core as the main parameter, and the level of compression was relegated to the background as unnecessary.
Sa
Edwin P. Hubble took to the Sa class 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, spiral galaxies of the Universe have two arms. They are located on opposite edges of the core. Branches are unwound in a symmetrical, similar manner. As you move away from the center, the brightness of the branches decreases, and at a certain distance they completely cease to be visible, are lost in the peripheral areas of the cluster. However, there are objects in which not two, but a greater number of sleeves. True, such a structure of the galaxy is quite rare. More rarely, asymmetric nebulae can be found when one branch is more developed than the other.
Sb and sc
The subclass Sb according to the classification of Edwin P. Hubble has noticeably more developed sleeves, but they do not have rich branches. The nuclei are noticeably smaller than 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 structure of the spiral is the result of 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 be considered on the other hand. There is no doubt that a very compressed star cluster in the process of its evolution will no longer be able to lose its degree of compression. Hence, the opposite transition is also impossible. As a result, we conclude that elliptical galaxies will not be able to turn into a spiral, and vice versa, because the cosmos (the Universe) is so 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 opposing evolutionary paths due to different compression ratios. And this characteristic, in turn, depends on the difference in the rotation of galaxies. For example, if during the course of its formation the star system receives a sufficient amount of rotation, then it will be able to take a compressed form, and it will 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 classical elliptical shape.
What else are the differences
There are other differences between elliptical and spiral stellar systems. So, the first type of galaxy, having a low level of compression, is characterized by a small amount (or complete absence) of diffuse matter. At the same time, spiral clusters having a high level of compression contain both gas and dust particles. Scientists explain this difference as follows. Dust particles and gas particles during their movement periodically collide. This process is inelastic. After the collision, the particles lose part of their energy, and as a result, gradually settle in those places of the stellar system where there is the least potential energy.
Heavily compressed systems
If the process described above occurs in a highly compressed stellar 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. Gaseous and dust particles are also collected here. Further, diffuse matter begins its motion in the main plane of the star cluster. Particles move almost parallel in circular orbits. Collisions here are quite rare. If they do occur, then the energy loss is negligible. From this it follows 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 ellipsoidal galaxy behaves. A star system of this type is characterized 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 result, the density of diffuse matter here will be very high, much higher than with plane scattering in a spiral system. Dust and gas particles collected in the center of the cluster under the action of the attractive force will begin to compress, thereby forming a small-sized zone of dense matter. Scientists suggest that new stars begin to form from this matter in the future. What is important here is something else - a small cloud of gas and dust, located in the core of a weakly compressed galaxy, does not allow itself to be detected during observation.
Intermediate stages
We examined two main types of star clusters - with a weak and a strong level of compression. 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 collect along the entire main plane of the cluster. And at the same time, it is not weak enough for gas and dust particles to concentrate in the core region. In such galaxies, diffuse matter gathers in a small plane, which gathers around the core of a star cluster.
Jumper galaxies
Another subtype of spiral galaxies is known - this is a star cluster with a jumper. Its feature is as follows. If in a conventional spiral system the arms exit directly from the disk-shaped core, then in this type the center is located in the middle of a direct bridge. And the branches of such a cluster begin from the ends of this segment. They are also called galaxies of crossed spirals. Incidentally, the physical nature of this jumper is still unknown.
In addition, scientists were able to detect another type of star cluster. They are characterized by a nucleus, as in spiral galaxies, but they do not have sleeves. The presence of a nucleus indicates 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 by a spiral galaxy of its diffuse matter.