Galactic Map

StarDate: September 12, 2010

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This is a great time of year to watch the Milky Way -- the glowing band of stars that outlines the disk of our home galaxy. As the Moon sets around 9 or 10 o'clock tonight, the Milky Way arcs high overhead. But you need dark skies to see it.

Today, we know that the Milky Way is shaped like a disk with a bulge in the middle. We also know that we're a long way from the middle.

But it took a lot of work to figure all of that out. One of the first steps took place 225 years ago, when William Herschel tried to decipher the structure of the Milky Way and our place in it.

Herschel had built the world's largest telescope. He and his loyal assistant -- his sister Caroline -- used it to count the number of stars in 683 directions in the sky. Herschel assumed that all the stars were the same brightness, so those that looked faintest must also be the farthest away.

Their counts showed that the galaxy was shaped like a lens -- a big bulge in the middle and tapered at the ends. That part was pretty accurate.

Herschel also concluded that Earth was at the center of the galaxy, since the stars seemed to extend the same distance in all directions. That was a reasonable conclusion -- but wrong. The biggest problem is that the galaxy is filled with clouds of dust that absorb the light of stars behind them. That makes it impossible to see very far into the galaxy. So trying to measure the Milky Way's size and shape by counting its stars just doesn't work.


Script by Damond Benningfield, Copyright 2010


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