In 1845, English astronomer Lord Rosse was discovered a whole class of spiral nebulae type.Their nature is 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 on many millions of light years.
Spiral Galaxies (photo presented in this article demonstrate the features of their structure) in their appearance reminiscent of a pair of stacked plates or a lenticular lens.They can be found as a massive stellar disk and halo.The central part that visually resembles the swelling, called the bulge.A dark band (the opaque layer of the interstellar medium), extending along the disk, called interstellar dust.
Spiral galaxies usually denoted by the letter S. In addition, they can be divided according to the degree structure.To this is added to the main character letters a, b, or c.So, Sa galaxy correspond with underdeveloped spiral structure, but with a large nucleus.Third grade - Sc - refers to opposite objects, with a weak core and powerful spiral arms.Some star systems in the central part may be a jumper, which is called the bar.In this case, the designations have been the symbol V. Our galaxy belongs to the intermediate type, without jumpers.How
formed spiral disk structure?
flat disc-shaped star clusters account for the rotation.It is hypothesized that during the formation of galaxies centrifugal force prevents the so-called compression protogalactic clouds in a direction perpendicular to the axis of rotation.You should also know that the nature of the motion of gases and stars in nebulae is not the same: diffuse clusters rotate faster than the older stars.For example, if the characteristic velocity of rotation of the gas is 150-500 km / s, the halo stars will always move slowly.A bulge consisting of such facilities will have a speed three times lower than the discs.
Billions of stellar systems moving in their orbits within the galaxy can be seen as a set of particles that form a kind of stellar gas.And most interestingly, its properties are very similar to the normal gas.It can be used such terms as "the concentration of particles", "density", "Pressure", "Temperature".The analogue of the last parameter here is the average energy of the "chaotic" movement of the stars.In rotating disk formed stellar gas can spread density wave spiral type vacuum compression close to the sound.They can obegat galaxy with a constant angular velocity within a few hundred million years.It is they who are responsible for the formation of spiral arms.At the moment when the gas is compressed, the process of forming cold clouds, which leads to active star.
It is interesting
in the halo and the elliptic systems of gas is dynamic, that is hot.Accordingly, the motion of the stars in the galaxy of this type has a chaotic character.As a result, the average difference between their rates in a spatially close objects is several hundred kilometers per second (velocity dispersion).For gas stellar velocity dispersion is typically 10-50 km / s, according to their "degree" is much colder.It is believed that the reason for this difference lies in the distant times (over ten billion years ago), the universe when galaxies were just beginning to form.The first of them formed the spherical components.
spiral waves called density waves, which run on a rotating disk.As a result, all the stars of the galaxy of this type is, as it were forced out of their branches inside, then go from there.The only place where the speed of the spiral arms and star matches - the so-called corotation circle.Incidentally, in that place there is a sun.For our planet, this situation is very favorable: Earth exists in a relatively quiet location of the galaxy, as a result, for many billions of years, it does not feel much impact disasters galactic scale.
Features spiral galaxies
Unlike elliptical formations, each spiral galaxy (examples can be seen in the photo presented in this paper) has its own unique flavor.If the first type is associated with calmness, stationary, stable, the second type - the dynamics, vortices rotation.Maybe that's why astronomers say that the cosmos (universe) "berserk".The structure of the galaxy spiral type includes a central core from which overlook the beautiful sleeves (branch).They are outside of their star clusters are gradually losing shape.Such an appearance can not be associated with a powerful, swift movement.Spiral galaxies are characterized by a variety of forms, as well as pictures of their branches.
How classify galaxies
Despite this diversity, the researchers were able to classify all known spiral galaxies.As the main parameter decided to use the degree of development and size of their hoses core, and the level of compactness as unnecessary receded into the background.
Edwin P. Hubble took the class Sa those spiral galaxies, which have a less developed branches.Such clusters are always the core of large size.Often the center of the galaxy in this class is half the size of the entire cluster.These objects are characterized by the lowest expression.They can even compare with elliptical star clusters.More often spiral galaxies of the Universe have two arms.They are located on opposite sides of the nucleus.Unwinds symmetrical branches, in a similar manner.As the distance from the center of the brightness of the branches decreases, and at a certain distance and they did ceases to be visible, lost in the peripheral regions of the cluster.However, there are objects that do not have two or more of the sleeves.However, such a structure of the galaxy relatively rare.Even more rarely can meet asymmetric nebula when one branch is more developed than the other.
Sb and Sc
Subclass Sb classification Edwin P. Hubble has much more developed sleeves, but they do not have the rich branches.Kernels noticeably smaller than the first kind.The third subclass (Sc) spiral star clusters are objects with highly developed branches, but the center of their relatively small.
Scientists have found that the spiral structure is a result of the unstable motion of the stars, which occurs as a result of strong contraction.In addition, it should be noted that the focus in the arms, usually hot giants and there accumulate the main mass of diffuse matter - the interstellar dust and interstellar gas.This phenomenon can be considered from another side.There is no doubt that very succinct star cluster in the course of their evolution will not be able to lose a degree of compactness.Hence, the opposite transition is also impossible.As a result, we conclude that elliptical galaxies can not turn into a spiral, and on the contrary, it is so arranged space (Universe).In other words, star clusters of these two types are not two different stages of the evolutionary development of a single, but a completely different system.Each type is an example of the opposite evolutionary paths due to different compression ratio.A characteristic of this in turn depends on the difference in rotation of galaxies.For example, if in the course of its formation, the star system is getting enough rotation, it can take a compressed form, and it will develop the spiral arms.If the degree of rotation is insufficient, the galaxy will be less compressed, and her branches are formed - this is a classic elliptical shape.
What else are the differences
between elliptical and spiral star systems, there are other differences.Thus, the first type of galaxy that has a low level of compression, characterized by low (or no) of diffuse matter.At the same time, spiral clusters having a high level of compression, and contain a gas, and dust particles.This difference is explained by scientists as follows.Specks of dust and gas particles in their motion periodically faced.This process is inelastic.After the collision, the particles lose some of its energy, and as a result, gradually settle in those places of the star system, where there is the lowest potential energy.
strong compression system
If the process described above takes place in a highly compressed star system, the diffuse matter should settle on the main plane of the galaxy, because it is here that the level of the potential energy is the smallest.This also and collected gas and dust particles.Next diffuse matter begins to move in the main plane of the star cluster.Particles move almost parallel circular orbits.As a result of the collision are quite rare.If they occur, the energy losses are negligible.From this it follows that the matter further to the center of the galaxy is not moving, where the potential energy is still lower level.
Low compression system
Now consider the behavior of the elliptical galaxy.The star system of this type is different completely different development of the process.Here the main plane is not a clearly defined area with low potential energy.The strong decrease in this parameter occurs only in the direction of the central star cluster.This means that the interstellar gas and dust will be attracted to the center of the galaxy.As a result, the density of diffuse matter here is very high, much higher than the flat dispersion in a spiral system.Gathered in the center of the particle accumulation of dust and gas under the force of gravity will be compressed, thus formed a small-size area of dense material.Scientists theorize that because of this matter in the future begin to form new stars.Important here is different - small in size cloud of gas and dust located in the nucleus of the galaxy slightly compressed, does not allow himself to discover in the process of observation.
We considered two basic types of star clusters - with a weak and a strong level of compression.However, there are intermediate stages, when the compression system is located between these parameters.In these galaxies, this characteristic is not strong enough to diffuse matter gathered along the main plane of the cluster.And at the same time being sufficiently weak and in order that the particles of dust and gas are concentrated in the nucleus.In these galaxies, diffuse matter is collected in a small plane that was going around the core of the star cluster.
Yet another sub-type spiral galaxies - a star cluster with a jumper.Its feature is as follows.If a conventional spiral hose system output directly from the disc-shaped core, then this type of center is located in the middle of the line jumper.A branch of the cluster starts from the ends of the segment.Yet they are called crossed spiral galaxies.Incidentally, the physical nature of this bridge is still unknown.
In addition, scientists have discovered another type of star clusters.They are characterized by a core, as in spirals, but they do not have sleeves.The presence of the nucleus shows strong compression, but all other parameters are reminiscent ellipsoidal system.Such clusters are called lenticular.Scientists suggest that these nebulae are formed as a result of the loss of a spiral galaxy of its diffuse matter.