Milky Way galaxy is our home.

Astronomers have concluded for many years that at the center of our galaxy, is a bar of stars. Because of the dust surrounding the central region of the galaxy, we could only indirectly devise how this bar looks or what is at the center after all. It is also believed that at the very center, there is a supermassive black hole, called Sagittarius A, which holds this galaxy together gravitationally.

Through infrared images from the Spitzer's Space Telescope, NASA astronomers were able to determine that our galaxy has at least two major spiral arms, and at least three minor arms. The two major arms are called, Scutum-Centaurus and Perseus. Two of the minor arms (Norma and Sagittarius) are less distinct and are located between the major arms. Major arms consist of much denser regions of stars, while the minor arms are primarily gas. What's interesting, is that the stars on the inner part of the arms rotate faster than on the outer part. This would lead to gradual "twisting-up" of the spiral galaxies with time. The fact that the arms of spiral galaxies like ours are still not twisted-up, means the universe is much younger than previously believed.

Huge rings were discovered racing around our galaxy perpendicular to its disk. These rings are between 13,000 and 130,000 light years above the Earth, and extend over much of the Northern Sky. These rings are streams of stars which are seperated by distances of several light-years from each other. These rings are believed to be remnants of globular star clusters. Of course, we could only guess what they really were.

The galactic disk is surrounded by globular clusters which move at peculiar orbits. 40% of these clusters rotate in the direction opposite to the rotation of the Milky Way's galactic disk, which poses a serious challenge to the scenario of the Big Bang explosion where everything should rotate in the same direction.

Our galaxy shares space with about 20 dwarf galaxies (smaller galaxies). Those are part of a group of galaxies called the Local Group. Milky Way and Andromeda galaxies are the largest members of this group. There are more than 30 galaxies in total in the Local Group. This group itself is part of a Local Supercluster of galaxies, called Virgo Supercluster.

The Milky Way contains 200-400 billion stars. It is a galaxy of many planets, at least 50 billion of them. And those are just the planets that orbit stars. New studies show that there could be twice as many free-floating planets as there are stars in our galaxy.

Our Solar System is located about two-thirds of the distance away from the Milky Way's center, on the inner edge of a smaller Orion–Cygnus Arm. Our Sun orbits the center of the galaxy. It would take a galactic year for it to make one revolution, which is 225-250 million Earth years.

With our eyes, we can distinguish only the brightest parts of the galaxy. That is the band of stars we see across a big portion of the night sky. If we could see all the stars, they would stretch across the entire night sky viewable from Earth. The fact that this band of stars seperates the night sky into nearly equal hemispheres indicates that our Solar System lies inside the galactic plane.

The disk of the Milky Way is approximately 9 X 10^17 km in diameter, and 100 times smaller in thickness. The disk of the galaxy does not have a definite edge, and the density of stars drops gradually as we move further away from the center of the galaxy. Extending beyond this disk of stars, is a much larger disk of gas, which is 12 times thicker than the disk of stars. The Milky Way has two satellites, Large and Small Magellanic Clouds, which orbit the galaxy at a distance 180 times larger than the diameter of the disk of stars in our galaxy.

The Milky Way galaxy is estimated to weigh between 6 × 10^11 and 1 x 10^12 solar masses.



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