The variety of galaxies

The massive stellar systems - galaxies, distant and mysterious, one of the most exciting and most studied objects in astronomy today. In this article we look at the types of galaxies, their main differences and peculiarities. Galaxies in the universe is not similar to each other. Some are round and elliptical shape, while others are observed in the form of swirling spirals. In the 20-xx years of the twentieth century, Edwin Habll in their work proposed to classify the galaxies into three basic types: elliptical, spiral and irregular, denoted, respectively, E, S and Irr.

The easiest way to look elliptical galaxies.  The structure of this type is characterized by an elliptical shape, uniform brightness, gradually decreasing from the center to krayu.Forma galaxies of this type range from almost perfectly round, to flattened ellipse.  Accordingly, to indicate the degree of oblateness of the galaxy to the letter E is added the number n, defined by the formula: n = 10 (a - b) / a, where a - b - are respectively the major and minor axis of the galaxy.  Thus, the elliptical galaxy circular will be classified as type E0, and strongly flattened can be classified as E6.Naibolee common type of galaxies are spiral.

The structure of the galaxies of this type is much more complicated than elliptic.  Spiral galaxy has a well-marked nucleus, as well as the spiral arms (arms) emanating directly from the nucleus, or from the so-called bridge (bar), which is located in the center of spiral galaxies yadro.V kernel is the most vibrant area that has the characteristics of elliptical galaxies.  Open Hubble's law of falling brightness of elliptical galaxies turned out to be true for the central nuclear regions of spiral systems.  The presence of a spiral galaxy bridge (bar) allows you to divide them into two basic types.  The first are normal spiral galaxies, denoted by the letter (S).  The second type includes the so-called crossed the galaxy, denoted (SB).  For a more precise characterization of a galaxy, dividing them into two types is not enough.  Therefore, spiral galaxies, Hubble classified into the following three criteria:

1) the relative size of the nucleus, as compared with the size of the entire galaxy;

2) according to how strongly or weakly twisted spiral arms;

3) the fragmentation of the spiral arms.

For type Sa or (SBa) are galaxies with very large nuclear-related and highly twisted spiral arms - continuous and smooth, not fragmentarnymi.Galaktiki type Sb and SBb have a relatively small nuclear region, and not so much twisted spiral arms, which are permitted to separate bright fragments.  Galaxy of type Sc, and are characterized by highly fragmented SBc fragmentary spiral arms.  In galaxies SBc even a bar is divided into separate fragments.  In addition, a separate type of isolated galaxies, intermediate between spirals and elliptical systems - galaxy type SO.  They have a very thick disk, a powerful bulge and spiral arms can be seen.  By the way, denoting the type of galaxies by the letter S, despite the lack of spirals, the Hubble thus stressed that the main difference in the spiral and elliptical systems is the star disk.K third type galaxies are all objects that can not be ranked nor elliptical galaxies, or to spiral - it's irregular galaxies (Irr). Irregular galaxies do not possess a characteristic structure.  They are extremely fragmented, and they can distinguish between the individual and the brightest stars of the hot emitting gas.  Hubble on the classification of irregular galaxies can be divided into two subtypes Irr and Irr 2.K a first sub-type are galaxies that have visible bar and many of them can distinguish fragments of structures resembling fragments of spiral arms.  They are like an extreme extension of the type of spiral galaxies.  However, the fragmented and patchy arms so high that they can no longer be called a spiral.  The second sub-type galaxies are irregular in shape, with a high content of gas, dust and other anomalies.  There are a number of systems that deserve to be treated separately.

Among the first to be called elliptical galaxies, which are denoted as the type of CD.  This is the largest galaxy of the universe, their stellar mass often exceeds the value of 1012 M sun.  Conventional, not CD - galaxies do not "hold out" to the value of 1012 M sun, and even the largest of them hardly go beyond the limits of the mass (1-3) * 1011 M sun.  Features and structure of a CD - systems.  Along with the central stellar component, is extremely compact and is not essentially different from the normal E - galaxies, they have a huge sparse (diffuse) stellar envelope, the size of which can be tens of kiloparsecs, and more.  Galaxy CD is extremely rare.  The closest and most studied system of this kind is the galaxy M 87.  A striking example of a CD - system - A galaxy of 1413.  Its diffuse envelope extends beyond 2 Mpc from the center.  CD - the galaxy - it is always a central system of clusters of galaxies, hence, no doubt, of their outstanding properties.  This type of galaxies is the so-called radio galaxies.  In 1943.  American astronomer K. Seifert found in the spectra of 12 galaxies, one feature that did not exist in other galaxies - namely, a very broad emission lines of hydrogen, helium, sometimes ionized iron Fe II, the width of which corresponded to the movement of the clouds emitting gas at a rate of several thousand kilometers per second.  They usually do not exceed 200 km / s.  Now there are cases where the line width "equal to" 30 000 km / s, ie, 10% the speed of light!  Systems, open Seyfert, were isolated in a separate class and became known by his name: Seyfert galaxies.

Typically, this is a giant spiral galaxies, and that, along with broad emission lines in the spectrum, they have a number of unusual properties.  Their nuclei are sources of strong radiation nonstellar nature, with a very wide range of electromagnetic waves, they emit radio waves, infrared, ultraviolet and X-rays.  But perhaps the most unusual of their property - the enormous power of the central source: the most energetic Seyfert total luminosity of the nucleus is much greater than the total luminosity of the hundreds of billions of stars throughout the galaxy.