A spiral Galaxie is a whole including/understanding of the billion star S, generally flattened in the shape of disc, with a luminous and spherical convexity in the center (the galactic bulb). The disc contains several luminous arms typically, where are the youngest stars and most luminous. These arms are rolled up around the center in a spiral drawing, giving to the galaxies their name. The spiral galaxies belong to most beautiful the object S of the sky and it is rare that the aspect of the '' Cartwheel Galaxy '' leaves indifferent somebody who discovers it.
Our galaxy, the Milky Way, is a spiral galaxy.
DescriptionThe spiral galaxies are very dynamic entities. They are the bed of formation of stars, and contain many young stars in their discs. Their central bulge tends to being made older stars, and their diffuse halation is made older stars of the Univers. The stars are formed starting from the concentrations of gas which are present only in the galactic disc.
The modern Télescope S revealed that many spiral galaxies lodge black holes super solid masses in their center, with masses which can exceed a billion Suns! At the same time the spiral galaxies and elliptic S are known to contain these exotic objects; in fact, much of astronomers believe now that all the large galaxies contain a massive super black hole in their core. Our Galaxy is known to lodge a black hole in his center, of a mass of several million solar masses.
ClassificationThe galaxies are classified by using a “diagram of fork of adjustment”, called Séquence of Hubble. The end of the fork classifies the elliptic galaxies on a scale since roundest, which is with dimensions E0, until those which appear the most flattened, which is with dimensions E7. The “branches” of the fork of adjustment are there or the two types of spiral galaxies are classified: normal spirals and “barred” spirals. A barred spiral is one whose core is stretched in a line, in such a way that it resembles has a “star bar” in its center.
As much the shape a spiral galaxy seems to run of source when one imagines the work of the forces of gravitation (form familiar of the Tourbillon around an attractile exchange), as much that of the barred spiral intrigued the astronomers and the curious ones a long time. Simulation S on Ordinateur suggest that the spiral form of barred appears rather easily at the time of the crossing of two galaxies (which involves relatively little collisions, the average density of the galaxies being weaker than that of the cigarette smoke). A crossing of this type is envisaged between our galaxy is that of Andromède in four billion years (2004), without it being known today if it will give a result of the same kind or not.
The two types of spiral galaxies are subdivided according to the prominence of their central “bulge”, their brilliant surface and the tightening of their arms. These characteristics are dependant, in such a way that a galaxy Its have a large central bulge, a luminous large surface and tight arms rolled up in spiral. A galaxy Sb has a smaller bulge, a paler disc and more slackened arms, and so on for Sc and Sd. The barred galaxies use the same diagram of classification, indicated like SBa, SBb, SBc and SBd.
There is another named class of galaxies S0, which is morphologiquement a type of transition between the true spiral galaxies and the elliptic galaxies. Its spiral arms are rolled up so tight that they are not distinguishable; the S0 galaxies have a disc with a uniform luminosity. They have also a very important bulge.
---- Certain passages of this article, or a previous version of this article, come from the translation of an article of Mike Choatie, resulting from the AstroInfo project, under license GFDL.
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