What are the galaxies?

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What are the galaxies?
What are the galaxies?
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The sequence of evolution of stellar systems was proposed by Edwin Hubble. The spiral galaxy is the most common type, and our galaxy, the Milky Way, belongs to this type. What other types of galaxies are there?

Andromeda Galaxy

According to this classification, there are four main types of galaxies. Sometimes dwarf galaxies are referred to a separate species, however, they do not stand out in anything other than their relatively small size and they themselves belong to one type or another in the classical categorization.

Elliptical galaxy

From the side it looks like a giant star - a luminous ball with the strongest brightness in the center and dimming towards the edges. Elliptical, or spheroidal, galaxies are almost entirely composed of old stars, so they always have a yellow or reddish tint. New stars are practically not formed in them, since the amount of interstellar gas and dust in them is negligible (although there are exceptions). Elliptical star systems differ from each other only in size and compression ratio. It is by compression that they are classified, from E0 to E7. They make up about a quarter of the visible galaxies. According to the Hubble classification, this is the initial stage of galactic evolution.

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Spiral galaxy

The most common type, and probably the most beautiful, accounts for more than half of all known galaxies. It looks like a disk with a bright yellow ball in the center, around which dimmer branches-arms of a bluish hue are twisted in the form of spirals (due to the presence of special stars - white and blue supergiants).

It differs from elliptical star systems in a number of structural features. First, spiral galaxies have arms where active star formation takes place. Secondly, there is a stellar disk - a relatively thin layer of matter along the plane of the galaxy, where the bulk of the system's objects are located, and in which the stars rotate around the center of the disk. Third, the presence of interstellar gas and dust is widely observed - the medium necessary for the birth of stars. Many spiral galaxies have at their center a kind of bar (bar), from the ends of which the arms diverge. They are classified with the letter S and differ in the density of the sleeves (Sa-Sd, with a jumper - SBa-SBd).

The number of sleeves is on average a pair, but there are more; in some cases, the sleeves vary in size. All of them (if they do not survive a galactic collision) are twisted in one direction around the center, where the bulk of matter is concentrated in the form of a supermassive black hole and a dense spherical cluster of old stars - the bulge.

Both our galaxy, the Milky Way, and the Andromeda Nebula, which we will inevitably encounter in 4 billion years, are both spiral galaxies. The sun is located between the arms and far from the galactic center, and the speed of its movement is approximately equal to the speed of rotation of the arms; Thus, the solar system avoids areas of active star formation, dangerous for terrestrial life, where supernovae often break out.

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Lenticular galaxy

According to the Hubble classification, this is an intermediate type between elliptical and spiral galaxies (S0). Lenticular stellar systems have a stellar disk around a central globular cluster-bulge, but the arms are relatively small and not very pronounced, and the amount of interstellar gas and dust matter is insufficient for the active birth of new stars.The main inhabitants are old big stars, red or yellow.

They differ in the amount of interstellar dust and the density of the bar in the galactic center. They make up about 20% of the number of galaxies.

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Wrong galaxy

Neither ellipse nor spiral - irregular galaxies do not have any of the common shapes. As a rule, these are star clusters chaotically bound by gravity, sometimes without a clear shape or even a pronounced center. They make up about 5% of galaxies.

Why are they so different from their galactic counterparts? It is very likely that each such star system was once elliptical or spiral, but it was disfigured by a collision with another galaxy, or close proximity to it.

They are divided into two main types: those who have at least some semblance of structure, allowing them to be attributed to the Hubble sequence (Irr I), and those who do not even have a similarity (Irr II).

Sometimes a third type is distinguished - dwarf irregular galaxies (dl or dIrr). They contain a low amount of heavy elements and a large amount of interstellar gas, which makes them similar to the protogalaxies of the early Universe. Therefore, the study of this kind of irregular galaxies is essential for understanding the process of galactic evolution.

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