THE GALAXIES
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Currently, the scientific community is inclined to think that the whole universe was concentrated in one area and than do to the strong underlying forces at one point there was a huge explosion called Big Bang . The universe began to expand rapidly, space and time began to make sense. Expanding the universe has cooled resulting after billions of years, the universe as we observe, consisting of billions of galaxies in the movement.

The Galaxy is a Macrosystem held together by gravitational forces, consisting of a concentration of gas, dust and stars with their planetary systems. All galaxies rotats around its central axis and generally moves away from each other, but they also have their own movement that can take them also to clash with another Galaxy. The galaxies give rise to clusters of dozens or even thousands of galaxies (Superclusters). Ours, called the Milky Way, is part of a cluster of a few dozen galaxies called the Local Group in which there is the Galaxy Andromeda which is one million light-years away and is slowly approaching us. Between the galaxies, there is deep space that is thought to have the dark matter, matter that does not emit or reflect radiation including light, sufficient to be detected. The amount of dark matter is crucial to predict the future of the universe, if it is in continuous expansion, or at some time will begin to contract because of the gravitational action.

 

A classification of galaxies based on their form is as follows:

Elliptical galaxies have spherical or ellipsoidal and a decreasing luminosity from Center. Do not locate concentrations in nuclei and arms. Elliptic Giants are very large and are usually centres of Superclusters.

Spiral galaxies have elongated forms ellipsoidal with spire consisting of arms that branch off from a nucleus. In the core it's  possible to see a concentration axial bar in shape from which leave the arms. In this case we speak of barred spiral galaxies. The arms may be only two or several but they are always crooked because of differential rotation that is generally greater in the middle and lower in the suburbs.

Lenticular Galaxies are spherical galaxies with mixed features of the previous two.

Irregular galaxies — Galaxies with no symmetries, small and not very bright.

 ESA

In all galaxies is marked off the Halo that surrounds and udergo the overall action of the Galaxy. In the Halo can be found of globular clusters, almost all spherical, made from millions of stars.The name Milky Way, our own galaxy,  descended from the ancient Greek kuklos (Milky circle) due to visible light trail in the sky in crisp nights consists of billion "small" star. The ancients thought milk was spread from the breast  of  ERA the mother's gods. This visible light trail was called by the Chinese the Celestial River where many small fish swam. The reality is different. Our Galaxy is a spiral with a differential rotation and consisting of a high core concentration and poor concentration of dust and gas with axial bar from which leave branch  curved backwards. All surrounded by disk that delimits the Galaxy. 

The nucleus rotates so stiff as all its stars revolving with the same angular velocity. The arms are a concentration of matter and rotate with a descending speed away from the Center. However, there are areas of disk where there has been an increase in orbital velocity. It is thought to be due to the presence of a significant amount of dark matter that holds an important gravitational action. Also the heavenly bodies, including the Sun, can be provided with a proper motions within the Galaxy.

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