Milky Way

[Jim Colyer]

The Milky Way is our own galaxy, and we are inside it. This is not apparent right away as we look at the glimmering arch in the night sky. The Greeks saw spilt milk. But the Milky Way consists of 400 billion suns 100,000 light years across and 2,000 light-years thick. If we could stand outside the Milky Way, we would see a disk with a bulge in its middle. It is shaped like a flying saucer. Our solar system revolves two-thirds of the way from the center toward the rim. It takes 200 million years for our solar system to revolve around our galaxy. This is a cosmic year. The last time the sun and planets were in this same position, dinosaurs roamed the earth. In the desert in 1979, there was an instant when I felt us revolving around the galactic center.

Since we are inside the galaxy, trying to divine its shape is like someone inside a house trying to determine the shape of the house. The structure of the Milky Way and our sun's position in it was ascertained by Harlow Shapley in 1917. We grasp it when we know what we are looking at. The Milky Way circles the sky. Its bulging center lies in the direction of Sagittarius where star clouds are thick. The thin part of the circle, visible in winter, is in the direction of the outer rim. When we look at right angles to the Milky Way, we are looking out the top or bottom of the disk where stars are sparse. As we might expect, more galaxies can be seen out the top or bottom.

Dust patches like the Coalsack and Cygnus Rift obscure parts of the Milky Way. People once thought they were holes. Because of dust, radio telescopes are used to study the center of the galaxy. The Milky Way is 10 billion years old.

ASTRONOMY http://jimcolyer.com/papers/entry?id=2