The Milky Way

 

The Milky Way as a night sky phenomenon.

The Milky Way is usually visible only when the sky is clear and moonless and there is no light pollution.  It looks like a misty (milky) ribbon of light across the night sky.  Just like the stars, it rises and sets because of the earth’s rotation.  With a telescope, one can see that the apparent mist is actually composed of dense clouds of stars.  It is a spectacular sight to see.

 

The Milky Way as a galaxy.

It is even more spectacular when we realize what we are seeing.  In essence, the Milky Way we see is the rim of the Milky Way galaxy in which we live.  To understand this, imagine a spiral of four arms slowly turning around a central density.  The spiral is about 100,000 light years across and 10,000 light years thick.  Our solar system is on an arm about halfway between the center and the outer edge.  So when we look out and see the Milky Way, we are looking through the 100,000-light-year-wide disc of stars.  Naturally we see so many stars that they look like milk spilled across the dark sky of night.