If we look at the night sky on a cloudless night, we see a streak of light stretching right down the middle of the sky or near the horizon. This glow comes from billions of stars that are too far away to be seen individually-we only see their combined light. This is the Milky Way.
The Milky Way is an ancient galaxy that appeared just a few hundred million years after the Big Bang.
Together with the Andromeda Galaxy (M31), the Triangle Galaxy (M33) and more than 40 dwarf satellite galaxies, the Milky Way forms the Local Group of Galaxies, which is part of the Local Supergroup.
The Milky Way's companion galaxies, the Magellanic Clouds (Large and Small), are visible in the South.
Diameter of the Milky Way - 100,000 light years. The Galaxy itself consists of about 200-400 billion stars, several hundred billion planets, and rarefied gas, dust, and cosmic rays.
But all this makes up less than 20% of the Milky Way's mass. The rest, like any other galaxy, is dark matter. It is not completely clear what it is, but it is known that it can exhibit gravitational interaction: it attracts other matter, but does not emit or absorb light - hence the name "dark".
The sun and our planet are not in the center of the Milky Way, but on its "quiet" outskirts - about 25 000 light-years from the galactic nucleus, on a small ledge of the Orion's spiral arm. Spiral arms are areas in the galaxy where the density of stars is slightly higher than average, and because they are areas of increased star formation, there are many more bright blue stars than usual, which is why the arms stand out so brightly.
But at the center of the Milky Way is the massive compact object Sagittarius A* (Sgr A*), which is thought to be a black hole with a mass of four million suns. That sounds impressive, but it is less than 1% of the Milky Way's total mass.
The Sun revolves around the center of the Galaxy at about 220-240 km/s on an elliptical trajectory. It makes a complete revolution about every 220-250 million years.
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