Galaxy, definition and study. A-Z index of Cognitio.

A galaxy is a great set of stars, systems, clusters, and star associations, gases, and powders, linked by the reciprocal force of gravity. The name comes from the Greek γαλαξίας (galaxìas), meaning “milk”; is a clear allusion to the Milky Way, the galaxy, which is part of the solar system.

Solar system, definition and study. A-Z Index of Cognitio.

Solar system

Galaxies are objects of enormous size; ranging from the smallest dwarf galaxies, containing tens of millions of stars, giant galaxies, which also have a thousand billion stars, orbiting around a common mass center. The galaxies have been categorized according to their apparent form, that is, based on their visual morphology. A very widespread type is the elliptical one, which, as one may well argue with the name, has an ellipse profile. The spiral galaxies, on the other hand, have a discoid shape with spiral-shaped structures that depart from the nucleus. Galaxies with irregular or unusual shape are called peculiar galaxies; Their strange form is usually the result of the effects of tidal interactions with nearby galaxies. If such interactions are particularly intense, due to the great proximity of galactic structures, the fusion of the two galaxies may result, resulting in the formation of an irregular galaxy. The collision between two galaxies often results in intense stellar formation phenomena (in starburst jargon). The stars within the galaxies are in constant motion; in elliptical galaxies, due to the balance between speed and gravity, the movements are relatively low, the stars move in random directions and the rotational movements around the nucleus are minimal; this gives the galaxy the typical spherical shape. In spiral galaxies, the dynamics are considerably more complex. The nucleus, of spheroidal shape, possesses a high density of matter, which implies that this behaves in a manner similar to a rigid body. The distances between the galaxies belonging to the same cluster average are slightly greater than the order of magnitude of the larger galaxy diameter; For this reason, interactions between galaxies are relatively frequent, and play a decisive role in their evolution.  Material exchanges between galaxies are rather frequent and are caused by distortions due to tidal forces, often also due to gas and dust exchanges between the two galactic systems. Collisions occur when two galaxies pass directly through one another at a sufficient speed to prevent them from coming to a fusion. The stars of these galaxies are slightly affected by the interaction: their trajectories remain imperturbable and the occurrence of direct interaction phenomena is rare. However, the gases and dust of the two galaxies necessarily come to an interaction: the forces exerted on colliding clouds can trigger a violent phenomenon of star formation and the interstellar medium dissipates and compresses.

A collision can distort enormously the shape of one or both galaxies, forming bars, rings or flat structures. If the interaction is particularly strong, the galaxies melt together; in this case, the speed at which the two systems strike is not enough to allow a “quiet” passage in the other. Instead, they tend to join gradually to form a single large galaxy, often elliptical. If one of the galaxies is much larger than the other, the result is known as galactic cannibalism; in this case, the largest galaxy does not undergo major deformations by fusion, while the smallest galaxy is destroyed and its stars are part of the larger galaxy. The nucleus of some of these smaller galaxies can be arranged separately in the galactic alien, assuming characteristics similar to those of globular clusters.

The Milky Way is currently being blended with the Sagittarius elliptical dwarf galaxy and with the elliptical Canis Major Galaxy. Studies on formation and galactic evolution seek to provide answers about how galaxies have been formed and what their evolutionary course has been during the history of the Universe. Some theories about this are now fully accepted, but in the astrophysical field this study front is still open.

Astrophysics, definition and study. A-Z index of Cognitio.


Current cosmological models of the origin and early stages of the Universe are based on the Big Bang theory.

Big Bang, definition and study. A-Z index of Cognitio.

Big Bang

About 300,000 years after this event, the cosmos temperature dropped to allow the formation of hydrogen and helium atoms in an event called recombination. Almost all the hydrogen was neutral and absorbed light, but no star had ever been formed; for this reason this phase is called “dark age of the Universe”. It was from the density fluctuations of this primordial matter that began to appear in the first large-scale structures; As a result, baryonic matter began to condense with the dark matter alone.

Dark matter, definition and study. A-Z index of Cognitio.

Dark matter

These primordial structures would then become the galaxies that we observe today.

Galaxy, definition and study. A-Z index of Cognitio.



To open the video  click on the image, good view from your Alessandro Brizzi.

Galaxy, definition and study. A-Z index of Cognitio.