Galaxy

a separate article is devoted to our galaxy : The Milky Way. ----

In the Universe, the star S generally are not insulated but are not gathered within vast units called galaxies .

A galaxy includes also gases and dust of the interstellar Milieu and probably of great quantities of matter sinks. The whole of the matter constituting a galaxy is dependant Gravitation nellement and seems in orbit around a concentration of central mass. Many indices suggest that the center of many galaxies is occupied by a Black hole of important mass. The Milky Way, the galaxy in which the Solar system is, account approximately a few hundreds of billion stars. The majority of the typical galaxies comprise a similar number of stars, but there exist also dwarf galaxies cash a few million stars only, and galaxies giant cash several tens of thousands of billion stars. On the basis of these figure and size of the observable Universe, one estimates that this one counts a few hundreds of billion galaxies. The universe as a whole, whose real extension is not known, is him likely to count a number immensely larger of stars.

The galaxies are of three morphological types principal: elliptic, spiral, irregular. More extended description of the types of galaxies is given by the sequence of Hubble. Recent results seem to show that actually, the same galaxy can pass by various forms. More precisely, the presence of a bar in a Spiral galaxy would depend on its activity.

In the spiral galaxies, the arms have the approximate shape of spirals logarithmic curves. Like stars, the arms also turn around the center, but contrary to those, they do it with a constant angular velocity. That means that the stars pass successively in and out of the arms in spiral. It is thought that the arms in spiral are areas of strong density or rather of the “waves” of density: when the stars and the interstellar matter cross an arm, they slow down and of this fact create a higher density; it is a little like a “wave” of deceleration moving along a road filled with cars moving.

The arms are visible because the strong density which reign facilitates the star formation there: they thus lodge many massive stars (thus young people) which are very luminous.

Distribution of the galaxies

Like the star S, which are gathered in galaxies, the majority of the galaxies are gravitationally related to others. A structure containing to about fifty galaxies is a Groupe of galaxies. A structure containing several thousands of galaxies grouped in a sector of some mégaparsecs is a Galaxy cluster. The groups and galaxy clusters themselves are often gathered in Superamas, of the giant collections containing of tens of thousands of galaxies.

With very large scales, the distribution of the galaxy clusters uniform, but is not organized in plates or filaments.

Space between the galaxies is relatively empty, except the intergalactic clouds of gas.

Genesis of the concept

The true nature of the galaxies is known only since the beginning of the 20th century; previously, one called nebulous all Celestial object of diffuse aspect other that the Comet S (which could be distinguished thanks to their movement).

In 1610, Galileo used a glasses to study the Milky Way and discovered that it was made up of a great number of stars slightly luminous. In a treaty written in 1755, universal History of nature and theory of the sky , Emmanuel Kant, while being based on the first work of Thomas Wright, speculated with reason in the fact that our Galaxy could be a body in rotation made up of an enormous number of stars bound by the forces of the Gravitation, like the Planet S in the Solar system but on scales much larger. The disc resulting from stars would be seen, of our prospect, like a luminous band in the sky. Kant conjectured that some of the “Nébuleuse S” (within the meaning of diffuse object) visible in the night sky could be galaxies distinct from ours.

Towards the end of the 18th century, Charles Messier compiled a catalog containing a hundred Nébuleuse S (its goal was to index all the objects nebulous of the sphere of fixed in order not to confusing them with comets), which was followed later by the catalog of William Herschel including/understanding: 5,000 Nebulous S.

In 1845 William Parsons built a Télescope, much larger than those which existed at the time and could then distinguish elliptic nebulas from spiral nebulas. It was also able to distinguish (in astronomy one says to solve ) certain specific sources of light within these Nébuleuse S, thus confirming the conjecture of the Universe-islands of Kant. However, the majority of the astronomers of the time refuted the idea of the Universe-islands, asserting that these nebulas were inside our universe.

It is only in 1918 qu ' Harlow Shapley successful to determine the size of our galaxy, starting again the debate on the nature of the Nébuleuse S like their distance. It was intimate convinced that the universe was made up that of only one whole of stars, the Nébuleuse S being only elements of this universe-island.

April 16th, 1920 take place “the Great Debate” which opposed holding them of a monolithic universe represented by Harlow Shapley and those of a universe made up of universe-island defended by Heber D. Curtis. It is on this occasion that the term “galaxy” made its appearance. The debate did not make it possible however to decide between the two points of view.

Edwin Hubble announced on December 30th, 1924 that after the measurements made using new the telescope Hooker of 2,50 Mètre S, the “Nébuleuse S” were well beyond our clean “galaxy”.

It could solve the external parts of some Nébuleuse S spiral as star collections and identified some variable céphéides, which made it possible to consider the distance separating us from these Nébuleuse S: they were too distant to belong to the Milky Way. The first identified galaxies as such were NGC 6822 in 1925, M33 in 1926 and M31 in 1929. In 1936, Hubble conceived a system of classification of the galaxies which is still employed to date, the Séquence of Hubble.

The matter sinks

In the Années 1970, one realized that the visible mass total, in the galaxies, of stars and gas, could not correctly explain the number of revolutions of the galaxies, which brought to postulate the existence of the Matière sinks. As of the beginning of the Years 1990, the Space telescope Hubble made a great improvement in the remote observations. These new observations in particular made it possible to establish that the matter sinks of our Galaxy cannot be composed only of weak and small stars.

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