Galaxies are huge star systems that contain from several millions to hundreds of billions of stars. In addition to stars in the composition of galaxies are the interstellar gas of interstellar dust, cosmic rays. The amount of gas in galaxies mass substantially less than the stars and different from galaxies of various types. Number of other types of substances still substantially less than gas.

Most galaxies can be divided into four main types.

Among relatively close to us galaxies (brighter 13th apparent magnitude) around 13 % belongs to the class of elliptic. Their denoted by the letter E. They are spherical or ellipsoidal shape. The study of the spectra of these galaxies shows that the stars in them are moving with almost the same rate in all directions, and they rotate slowly. The density of stars per unit volume increases to the center and gradually decreases from the center to the edge. In most elliptical galaxies are very few gas - less than 0.1 % of the total weight.

Other types of galaxies are spiral galaxies, they are denoted by the letter S. Among relatives of spiral galaxies are a few more than 60 %. They are distinguished by the presence of two (and sometimes more) spiral arms, forming a flat system-"drive". In addition to the disk in the S-Galactica is the so-called spherical component. It is formed by objects that are approximately spherical-symmetrical around the galactic centre. In spiral branches are concentrated many young bright stars and heated them glowing gas clouds. There are also cold gas-dust cloud.

Unlike spherical component of stars and gas disk turn around the galactic centre, and with a different angular velocity PA different distances from the center.

The amount of gas and spiral galaxies varies from one to fifteen percent of the total mass.

Gas in galaxies (not only in the spiral, and other types) is composed by weight 70 % of hydrogen and 30 % helium. The mixture of heavier elements is extremely small.

The bulk of gas in galaxies is in the form of neutral atoms. The temperature of such gas fields depends on the density of the gas and other conditions. Here the gas is heated soft cosmic rays, and ultraviolet and x-ray radiation. Temperatures range from 10 To in dense clouds to several thousand degrees Kelvin in rarefied bezoblachnoe environment. Recently it was found that in such a relatively cold areas essential part of molecular hydrogen in the state. In the vicinity of hot stars gas Insulza their ultraviolet radiation. This so-called area of ionized hydrogen N (recall that hydrogen is the main element by mass). The mass of ionized hydrogen around individual stars can reach 104 Solar masses (the mass of the Sun is equal to 2·1033 grams and is indicated by Mc). The temperature of these areas about 104 K.

The next type of lenticular galaxies are denoted S0. Among nearby galaxies 22 %. In these galaxies bright main flattened body, "lens", surrounded by a faint halo. Sometimes the lens has around him the ring.

About 4 % of nearby galaxies are wrong galaxy. These are referred to Ir. To this class belong all not included in the above classes. A class of irregular galaxies are extremely heterogeneous. The amount of gas in the wrong galaxies can reach up to 50 % of the total mass, in other cases, it may be only a few percent.

Masses of various galaxies differ markedly from each other. Also very different and the luminosity of the galaxies. Masses of galaxies are determined by the movement of stars and gas clouds. In spiral galaxies on the shift of spectral lines are determined by the speed at different distances from the center. The law of universal gravitation allow these speeds to determine the mass. In the case of elliptical galaxies, which have no noticeable rotation, the mass is determined by the dispersion (variance) of the velocities of stars. Dispersion of velocities leads to the broadening of spectral lines. The greater the dispersion of velocities and therefore the greater the width of spectral lines, the more weight. The greatest diversity is found among elliptical galaxies. Among them are supergiants, which emit several tens of times more powerful than our Galaxy and have a mass of about 1013 MS (mass of our Galaxy is about 1011 MS). But in the class of elliptical galaxies are found and very dwarf, the so-called pygmies, the radiation power of tens of thousands of times smaller than our Galaxy, and the weight is only 106 MS. Supergiants in the class of spiral galaxies are rare. Irregular galaxies are usually relatively small luminosity (0,1-0,01 from the luminosity of our Galaxy) and relatively small weight within 1010 - 108 Mc.

Some galaxies are strong sources of radio emission; radio frequency energy their radiation is much stronger than in the field of optical lengths will. Such galaxies are called radio galaxies.

Most powerful radio galaxies main part of radio emission coming from extensive areas (hundreds of thousands of Persikov), located symmetrically on either side of visible in the optical rays in the galaxy.

In the centers of many bright galaxies there is thickening, called the kernel, and inside the nuclei of some galaxies are bright nucleolus - cores. The nature of nuclei dramatically different from the nature of the other parts of galaxies. In them there are active processes associated with the release of energy. The importance of these phenomena indicated in 1958, academician C. A. Ambartsumian.

The known galaxy with an unusually active processes in nuclei. This so-called Seyfert galaxies, N-radio galaxies and others. In the cores of these galaxies happen powerful gas movement with speeds of thousands of kilometers per second, there are releases of substances. The brightness of the nuclei often variable.

A very special class of objects is quasars, discovered by Dutch astronomer M. Schmidt (USA) in 1963, These objects emit in the optical range hundreds of times more powerful than the galaxies, and the basic part of radiation comes from the core of a size not more than 0.1 parsec or even less! The core of the galaxy is surrounded by a gas shell which stretches for hundreds of Persikov. Quasars are also powerful radiation and, in addition, some of them - infrared and x-ray radiation. In the optical range quasars Shine peremenin like peremenin Shine active galactic nuclei.

Two years after the discovery of quasars A. Sendig opened the so-called kasagi that such quasars, except that they do not have visible radiation.

Currently, most researchers believe that the quasars are the nuclei of galaxies that are being extremely strong activity. Galaxy of stars surrounding the quasar, usually not visible, since quasars are located at large distances and glitter of the quasar can't see the faint light of the stars.

In recent years around several dozens of relatives of quasars discovered the weak long-distance optical nebula. Their average size is about 90 thousand parsecs, and the luminosity is several times less than the luminosity of the brightest galaxies. It turned out that these nebulae are a lot of ionized gas, mostly hydrogen. The question of the presence in them of stars is not yet entirely clear.

Most galaxies is part of the clusters. Published directories containing thousands of clusters of galaxies. Clusters are divided PA right and wrong. In addition, an important division of clusters into two groups, there are classification of clusters on different parameters, for example, the richness (the number of members with enough powerful radiation), in the presence of bright galaxies in the center, in the presence of peculiar galaxies, etc.,

Correct clusters consist of a large number of galaxies (sometimes more than 104 members), possess spherical symmetry, a large concentration to the center. Striking members of these clusters are likely only to types E and S0. In the center of mass is often one or two brightest elliptical galaxies, surrounded by a halo. These galaxies are called cD galaxies. A typical representative of the correct clusters - cluster in the constellation coma (secondary School). Wrong (diffuse) clusters have an irregular shape, often adopt a separate thickening. Are these clusters of galaxies of all types. They can be rich (more than a thousand members) and very poor. According to the Soviet astrophysics B. A. Vorontsov-Velyaminov the General field galaxies consists of a weak external parts and numerous overlapping open clusters and small groups

The most well-studied correct clusters. The correct size of accumulations in the constellation Hair Veronica about 4 IPC. The total number of galaxies in the cluster (including weak) is estimated in the tens of thousands. The variance of the radial velocities of galaxies is about dv= 1000 km/S.

An example of improper accumulation is the accumulation of the virgin. It contains thousands of members; size of about 3 IPC.

In some clusters detected large mass of hot, ionized gas, at a temperature of about 108 K. This gas emits x-ray radiation. The total mass of the hot gas in the cluster can comprise a significant fraction of the total mass of all the galaxies clusters.

Interesting observation effects occur when the scattering of the cosmic background radiation on the electrons of this hot gas. Y. B. Zeldovich and R. A. Sunyaev has shown that if you watch the relic radiation in the centimeter range forovan, its intensity in the direction of accumulation is less than in neighbouring areas. Comparison of observations of the cosmic background radiation and x-ray observations can calculate the linear dimensions of the cloud of gas in the cluster that provides yet another way of estimates of the Hubble constant.

Long debated whether there are clusters of clusters of galaxies. Apparently, a significant discontinuity of the density at the scales at hundreds of times the size of large clusters, does not exist. The strongest arguments in favor of this give indirect considerations based PA observation of isotropy of the cosmic background radiation, as we have already mentioned above. If there were significant irregularities in the distribution of matter on a scale approaching thousands megaparsec, it is noticeably affect the isotropy of the intensity of the background radiation in the sky, which is not observed. In the scope of more than 200 IPC universe is homogeneous.

In addition to large accumulations there is a huge number of small clusters, groups and multiples of galaxies.

Recent studies of the Estonian astrophysicists Ya, Einasto, A. Saar, M. Eyewear and others, American specialists P. of Peebles, O. Gregory, L. Thompson et al. have shown that the most large-scale irregularities in the distribution of galaxies are "cellular" character. In "the cells walls" many galaxies and their clusters, and inside the void. Cell sizes about 100 IPC, wall thickness 3-4 IPC. Large clusters of galaxies are the nodes of this cellular structure.

Separate fragments of cellular structure is sometimes referred to as corstophine. The supercluster often have highly elongated shape like a thread.