The stars we see at night in the sky, like the Sun, belong to our galaxy, the Milky Way System. We call the Milky Way the silver band that we can see in the sky. If we look at this band with an eye or a telescope, then we can see that it is made up of billions of stars. But in reality we see only the edge or section of the galaxy. The galaxy itself is like a huge disk that “swells” in the middle.
This formation is called nucleus, it can be found near the constellation Sagittarius, the area with the highest density of stars of the Milky Way System. It cannot be seen inside the nucleus because it is surrounded by very dense dust clouds. In the surrounding disk, the stars are grouped around certain spiral arms starting from the nucleus. Our galaxy is one of the spiral galaxies of the Universe.
Like other galaxies, it also rotates circularly in the Universe. Looked from afar, it can be seen as a flaming wheel, with many spears.
Specialists have defined several galaxy spiral arms by studying the spread and direction of motion of the stars. With the help of radio telescopes, they found the position of hydrogen gas clouds in the spiral arms. The arms close to us are the Orion arm, the Carina arm and it has been shown that there is a Centaurus arm. These arms are found in the constellations from which they received the name.
Our galaxy belongs to the group of galaxies with a slightly larger size than in general. It contains about 100 billion stars, with the diameter of about 100,000 light years.
Sun is in the disk of the galaxy, the Orion spiral arm at a distance from the center of the galaxy 30,000 light years and make one revolution around the center in 225 million years – one year cosmic.
Like stars that cluster in galaxies, galaxies also form galaxy groups. Our galaxy is part of the Local Group formation. This includes the neighboring galaxies, the Small and the Large Magellanic Clouds, small and disordered galaxies, as well as the famous Andromeda galaxy. And this is a galaxy with spiral organization, but slightly larger than ours.
There is a visible difference between the nucleus and the disk of the galaxy. The disk stars are relatively young. There are many blue, white and blue stars. Some of them were formed together, and on these we can see them as open formations; an eloquent example is the Pleiades of the Taurus Constellation (Taurus). The portion between the stars of the disk is filled with large clouds of gas. From such clouds the stars are born. According to calculations, it is assumed that 10% of the galaxy’s mass consists of such clouds.
The gas and dust clouds may contain materials that have spread in space during the explosion of supernovae. In these materials are also found metals, so the stars that are born from such clouds contain metals.
The characteristic stars of the disc are young, hot and contain a large amount of metals. Astronomers call them stars of the first population.
In the nucleus of the Milky Way System there are especially old, red stars. Most were formed at the birth of our galaxy, following explosions, 12 billion years ago. The disk stars are much younger, the Sun being only 5 billion years old.
The old, red stars of the nucleus are called stars of the second population. Their composition differs from that of the disk stars. They consisted of clouds of hydrogen and helium and before entering their composition metals resulting from the explosion of supernovae, almost no metals.
Old, red stars are also outside the convex part of the nucleus, and form spherical halos (circular luminous areas around the galaxy). In some places such stars gather (several hundred thousand) and form a group similar to a boxing glove called a spherical set.
The brightest formations of this kind are Omega Centauri and 47 Tucanae, which can also be seen with the naked eye in the southern hemisphere.
In total there are 200 known spherical sets.
Strangely enough, the stars in the spherical and halo groups do not rotate with the other parts of the galaxy. They have their own trajectory around the center of the galaxy. It is believed that their trajectory formed with the birth of the galaxy.
With the help of telescopes, astronomers can penetrate deep into the galaxy. They found that the nucleus contains gas rings that spin and expand, some of them having extremely high temperatures (10,000 degrees Celsius).
The velocity of gas cloud rings near the center of the galaxy is very high.
In order to keep the galaxy in its current formation it is necessary that at its center there is a body with an enormous mass – about 5 million times larger than the mass of the Sun in our solar system.
From the center of our galaxy, some very strong radio signals arrive: it is believed that their source is Sagittarius A. This whole area emits also Roentgen rays. According to astronomers, only a black hole can force an object to emit such an amount of energy. This is in accordance with the view that there must be a heavenly body of immense mass that can withstand the clouds of gas. Scientists believe that in most galaxies, in their center, there are black holes.