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The structure of our Galaxy?

What substance in the Universe is not scattered, but concentrated in the giant star clusters, scientists thought in the eighteenth century (I. Kant, U. Herschel), but finally they have seen this only in the beginning of XX century.

Star system associated gravity, called galaxies.

Our Sun is part of the milky Way galaxy (otherwise our galaxy indicate the word with a capital letter - Galaxy). The thickness of our galaxy is no more than 1% of its diameter, that is, it is shaped like a disk, and if more precisely, the two folded edges of the plates. This component of the Galaxy is called the stellar disk. The diameter of the disc 30 KPS (100,000 light years), the thickness is 1000 light years and a mass greater than the mass of the Sun 150 billion times. Along the drive goes dark band, which is a layer of opaque matter - of interstellar dust and gas.

The stellar disk and the band in the middle of the disk
The stellar disk of the Galaxy and the band in the middle of the disk
(side view)

The disk of the Galaxy has no clearly defined boundaries - just as there is no clear upper bound in the Earth's atmosphere. However, in the plane of the disk, the density of stars is much higher than outside of it.

The galactic disk is rotated around its center. The rotation of the Galaxy is clockwise when looking at the Galaxy from its North pole, located in the constellation coma. The disk of the Galaxy has a spiral structure, which gave the name to this type of star clusters, spiral galaxies. Spirals represent waves propagating in the direction of rotation of the disk of the Galaxy, with a constant angular velocity. Stars inside the disk move in circular paths around the center of the Galaxy with a constant linear velocity. Therefore, the angular velocity of the rotation depends on the distance to the centre, decreasing as the distance from it. The speed of movement of the Sun, which is located on the outskirts of the Galaxy, is 220-250 km/S.

In the center of the disk of the Galaxy has a bulge - core with a diameter of 1300 parsecs. It is in the constellation Sagittarius. In the core of a very high concentration of stars: stellar density here millions of times greater than in the vicinity of the Sun. But, despite the fact that the nucleus is concentrated so many stars, it is a long time not been able to see because around the symmetry plane of the Galaxy are a huge dark clouds of dust that absorb the light of the stars. They hide from us the core of the Galaxy. Therefore, it has become possible to study only after the creation of the receivers, infrared and radio emission that is absorbed to a lesser extent. By the way, the study of the native Galaxy is difficult for us because we are inside it, outside of any object to learn easier. In addition, the Sun is in the plane of the stellar disk: there is a high density of interstellar matter, and it makes observations due to light absorption.

Our galaxy is a view from the side
Like our Galaxy from

Besides the huge number of stars in the Central region of the Galaxy is observed EcoLeaders gas disk with a radius of more than 1000 light years, which consists primarily of molecular hydrogen. In the very center of the Galaxy suggest the existence of a black hole[?] a mass of about a million Solar masses.

The second component of the Galaxy, which, in fact, determines its external dimensions, has a spherical shape. It's called halo. The radius of the halo is significantly larger than the disk - it reaches a few hundred thousand light-years. The center of symmetry of the halo of the milky Way coincides with the center of the galactic disk.

Halo, as the disk rotates around the center of the Galaxy, but with much less speed, as the stars within the halo move quite randomly.

The Central part of the halo within a few thousand light-years from the Galactic centre is the most dense, it's called the bulge (from the English word bulge, meaning “thickening”, "swelling").

The structure of our Galaxy - side view
The structure of our Galaxy (side view)

In addition to single stars in the Galaxy are found star clusters. They are divided into scattered clusters, globular clusters and stellar associations.

Diffuse star clusters are found near the galactic plane, where dust and interstellar gas. There are now over 1200 open clusters, one of them studied in detail 500. The most famous among them is the Galaxy and Heady in the constellation of Taurus. The total number of open clusters in the Galaxy, perhaps up to a hundred thousand, each containing from a few hundred to a few thousand stars. Their mass is small, and therefore, the gravitational field can not long keep them in a small volume of space, so for billions of years scattered clusters disintegrate.

The Pleiades
Roseanne star cluster Pleiades

The globular cluster stand out against the starry background due to the significant number of stars in them and clear spherical form. The diameter of the globular clusters ranges from 20 to 100 parsecs. At the dawn of the evolution of Galaxies on it wandered thousands of globular clusters. Many of them were destroyed in the clashes with each other or with the galactic center. Today globular clusters in our Galaxy is around 200, and they are located in a spherical halo. It is the oldest formation in our Galaxy - the age of 10 to 12 billion years. The age of the stars included in globular clusters, very solid: they have come a long way of evolution and become neutron stars or white dwarfs. Stars in globular clusters move in their orbits around the center of the cluster, and the cluster is, in turn, moves in an orbit around the Galactic centre.

Globular star cluster
The globular cluster Messier 80,
located midway between α Scorpion (Antares) and β Scorpion (Acrab)
in parts of the milky Way, rich nebulae

The third type of clusters - stellar Association. This group of young stars, the so-called OB associations. They have a length of from 15 to 300 parsecs and contain from a few dozen to several hundred young stars - hot blue giants and supergiants. As the giants of early spectral classes quickly pass the path of evolution all stars formed at the same time and have a small age. There are also tons of Association, containing variable stars, which are at the very early stages of stellar evolution.

Star Association
Stellar Association LH 72 in the Large Magellanic Cloud.
Photograph obtained using a wide-angle camera of the Hubble telescope.
Photo: ESA/Hubble, NASA and D. A. Gouliermis

Along sleeveless stellar disk are the most young stars (age-a few dozen million years ago), diffuse star clusters and associations, as well as dense clouds of interstellar gas, which continue to form stars. In spiral branches often observed the supernova explosions. Older stars in the spiral galaxy, like our Sun, located in the sleeve, and between them, creating a fairly uniform distribution of stars in the disk. Unlike halo, where manifestations of stellar activity are extremely rare in the branches continue their rapid life connected with continuous transition of matter from interstellar space to the stars and back. Active star formation in spiral arms is associated with a higher density of matter in them. Because this increases the average pressure on the clouds of gas in interstellar space. When the gas cloud is more dense part of the spiral arm, because of the increased pressure, the cloud is divided into smaller clumps of matter that can condense into stars. As a result of this process within the spiral arms stars are born. Thus, the sleeves are like giant incubator space in which young stars are located near the anterior border of the sleeves. Stars of the galactic disk are called population I type.

The halo consists mostly of very old, pale small stars that emerged in the early stages of the evolution of Galaxies - their age is about 12 billion years. They are both singly and in the form of globular clusters, including more than a million stars. Star spherical component of the concentrate to the center of the Galaxy, and the density of matter halos decreases rapidly with distance from it. The halo stars are called population II type.

The space between the stars is filled with rarefied matter, radiation and magnetic field. In the disk, especially a lot of interstellar dust, with temperatures of 15-25 K, which was formed as a result of the activity of stars. The average radius of the dust is fractions of a micrometer. Now consider that dust particles are composed of a mixture of iron and silicate particles covered with a shell of organic molecules and ice. The total mass of dust only 0.03% of the total mass of the Galaxy, but its full luminosity is 30% of the luminosity of the stars and completely determines the radiation of the Galaxy in the infrared range.

Analysis of the motion of bodies in the Galaxy showed that its mass should be an order of magnitude more than that we're defined by visible objects. So, in addition to the halo, bulge and disk, with a stars and gas, there is a huge amount of invisible matter, which manifests itself only in the gravitational interaction, but not recorded by any device. It is called dark matter. The disk and halo of the Galaxy immersed in the crown of dark matter dimensions and weight which is 10 times larger than the dimensions of the disk and the mass of visible matter of the Galaxy. Dark mass exists not only in our Galaxy, but in the intergalactic space. The nature of the hidden mass in the Universe is still unclear - we still don't know what it is.


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