Missing Arms

• ‹ Previous program
• 232 of 1997
• Next program ›

Our home galaxy, the Milky Way, seems to have lost a couple of arms. They disappeared in a new survey of the galaxy's structure.

The problem is that we're studying the Milky Way from inside -- its stars and gas clouds completely surround us. It's like standing in the middle of a forest and trying to map the whole thing when all you can see are the trees around you.

Astronomers have been trying to map the Milky Way for centuries. They started by mapping its stars. But many stars are hidden behind big clouds of dust. So they turned to radio astronomy, which maps clouds of gas. Radio shines through the dust, so it's a good tool. The radio waves reveal that the Milky Way is a spiral galaxy -- a flat disk of stars laced with delicate spiral arms. Early maps showed four spiral arms.

Over the last few years, though, astronomers have used a new technique. They've studied the galaxy's infrared glow with Spitzer Space Telescope. The infrared not only shines through the dust, some of it comes from the dust itself, which is warmed by the stars.

The Spitzer observations show that the galaxy has a bar of stars in its middle that, seen from above, looks like a fat cigar. Two major spiral arms extend out from the bar, with some minor arms between them.

But the astronomers who worked on the Spitzer project say that even now, the exact shape of the Milky Way isn't settled. They're still trying to see the forest through the trees.

More about the Milky Way tomorrow.

Script by Damond Benningfield, Copyright 2008

For more skywatching tips, astronomy news, and much more, read StarDate magazine.