Galaxy: what is it?



A galaxy is a collection of stars, dust, and interstellar gas held together by gravitation. Galaxies present a great diversity in size (between 2,000 and 500,000 light years in diameter) and shape. The radiation coming from galaxies makes it possible to divide them into normal galaxies and active galaxies, among which are quasars. The groupings of galaxies that we observe in the universe are called clusters and superclusters.


The galaxy to which the solar system is here is the Milky Way. The closest large galaxy to the Milky Way, Andromeda, is located 2.3 million light years away. Our galaxy, however, has two small satellite galaxies (the Large and Small Magellanic Clouds ) located respectively about 150,000 and 200,000 light years away.


It has been less than a century since we were sure that objects like the Andromeda galaxyOrM87 are indeed concentrations of stars similar to the Milky Way, but not part of it. Confirming Thomas' intuitions Wright and Immanuel Kant, this discovery was made by Edwin Hubble in the early 1920s. Later, astronomers and cosmologists even classified galaxies, supposed to represent an evolutionary pattern. The Hubble sequence is no longer considered as such today but we continue to present the morphological classification of the galaxies it constitutes.


It has three main types. There are the spirals, containing large quantities of gas and dust, with a disk where we find young stars and a more or less large bulge containing old stars. Then come the ellipticals, mainly made up of old stars and poor in gas and dust, which have a spheroidal structure. And finally the irregular ones, smaller in size and rich in young stars.


Galaxy collisions are essential phenomena for the appearance of life. Indeed, these collisions can lead to the formation of a type of star that makes oxygen. This video is taken from the French-speaking multiplatform project on contemporary cosmology www.dubigbangauvivant.com © ECP-YouTube Group


From gas clouds to stars in galaxies

The spiral galaxies like the Milky Way are the most numerous. They have the shape of a disc rotating around its center, the core (or bulb), is very dense, surrounded by a thin halo. The disk is frequently extended by one or two spiral arms composed of stars, dust, and gas.


According to theories of star formation, stars, of which there are hundreds of billions within galaxies, are shaped by the condensation of cloud molecules. This phenomenon tends to rarefy this gas in the interstellar medium during the evolution of galaxies. Gas is almost absent in ancient galaxies. The positions and trajectories of the stars there depend on the class to which they belong.


Ellipticals are thought to result from the merger of several spiral galaxies following collisions between galaxies. It would therefore be an advanced state of the evolution of galaxies, an evolution which began with the formation of dwarf galaxies. During the first billion years of the observable universe, gravitational interactions and collisions between these dwarf galaxies were significant, giving rise to the formation of irregular galaxies and large galaxies by merger.


During merger collisions, star flares occurred, consuming the interstellar gas present. In this logic, we understand that the most evolved galaxies are also the poorest in gas and dust, containing the most old stars. So it would be the ellipticals. They form around 20% of observed galaxies compared to 77% of spirals.


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