About the Milky Way Galaxy

Formation

• Until recently, the Milky Way Galaxy was thought to have been formed when several small galaxies came together, but that thought is changing. The original theory was that as new galaxies were absorbed into the Milky Way, the stars that existed in the disk were forced to the center and it was those stars that formed the bulge. Recently scientists have been able to determine that the chemical components of the stars in the bulge are different from those found in the Disk, and so they could not have formed in the disk but rather developed in the bulge itself. The stars were formed by the collapse of numerous gas clouds, and the already formed center's gravity held them in place.

Features

• Our Milky Way Galaxy has an estimated 400 billion stars. It is a spiral galaxy, with three parts - the disk, the central bulge and the halo. There are two types of spiral galaxies - a normal spiral and a bar spiral. The difference between the two lies in the formation of the center bulge. In a normal spiral, it is round, and in a bar spiral, it is elongated into almost a straight line. Because of the cosmic dust that exists in the disk, our view of our own galaxy only goes to about a few thousand light years, therefor it is hard to determine which type the Milky Way is, but the evidence points to the Milky Way being a bar spiral. The Milky Way is thought to be as old as the universe itself, about 13 billion years of age, with a circumference of about 250 to 300 thousand light-years.

Disk

• The disk is where the young stars -- those under 10 billion years old -- exist. The disk has 4 spiral arms. The sun and the planets we know are in one arm of the disk. The disk rotates around the center and it takes our sun somewhere in the neighborhood of 250 million years to make one revolution.

Bulge

• The bulge is at the center of the Milky Way Galaxy and has a vary high density, so dense that the planets collide with one another about once every 106 years.. The stars in this part of the galaxy are about 10 billion years old, middle-aged in the galactic scheme of things . The middle of the bulge is thought to be a giant black hole. It is the gravitational pull of the black hole that keeps the planets, stars and dust in their orbits.

Halo

• The halo is the outermost part of the Milky Way Galaxy. It has the lowest density and contains about 10,000 to 1,000,000 stars. These stars are the oldest, from 10 to 15 billion years old. The halo is so named because it contains a hot, ionized gas that gives off a halo effect. Not much is known about the exact composition of the halo due to the fact that it reaches as far as hundreds of thousands of light years from the edge of the galaxy.