We live inside a giant spiral galaxy known as the Milky Way. It contains lots of stars and big clouds of gas and dust. A supermassive black hole lurks in its center. And we're in the galactic suburbs, more than halfway out from the center to the rim.
Other than that bare outline, just about everything else about the Milky Way is up for debate -- the number of spiral arms, the nature of its center, its size, and even the number of stars.
The problem is that we're inside the Milky Way, so we can't see its overall structure. It's like trying to map the Sahara from a single dune in its middle.
Even with centuries of studying the Milky Way, there are lots of uncertainties. Early maps, for example, showed four spiral arms wrapping around the galaxy. But recent observations found only two, with some minor arms between them. And the arms are more well defined on the southern side of the galaxy than the northern side.
The nature of the galaxy's center wasn't well understood until the last few decades, and even now there are questions. It's a dense clustering of stars. But instead of a ball, as early observations suggested, it's probably shaped like two thick wedges of cake stuck back to back.
And what about the size of the galaxy? Its disk spans around a hundred thousand light-years, but even here the numbers are uncertain. The range of estimates varies by about 50,000 light-years.
More about the Milky Way tomorrow.
Script by Damond Benningfield, Copyright 2008
For more skywatching tips, astronomy news, and much more, read StarDate magazine.
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