Arms Race

StarDate: July 9, 2009

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Mapping a forest when you're standing in the middle of it is a tough job. And so is mapping a galaxy -- our home galaxy, the Milky Way. In fact, two teams of astronomers have come out with two different maps in the last year. One of them shows that the galaxy has two major spiral arms, while the other shows four.

The problem is that the Milky Way forms a broad, flat disk, and we're inside it. From that vantage point, we have a clear view of the stars and gas clouds that are nearby. But as you look farther away, the view becomes fuzzier -- blurred by the great distances, or by intervening clouds of gas and dust.

We do know that the galaxy consists of a long, fat "bar" of stars in its middle, with spiral arms wrapping around the bar. The arms are outlined by multitudes of hot, bright, young stars. But mapping the arms is a challenge.

One team of astronomers did the job with an infrared telescope in space, and found just two full spiral arms, with several partial arms. Another team did it with an array of radio telescopes on the ground, and found four full arms. But few astronomers think that either map is the complete picture of the galaxy.

The band of stars that outlines the galaxy's disk arcs across the eastern sky this evening. You need to get away from city lights to see it. The Moon rises in late evening, overpowering the faint glow. But it rises later each night after this, providing extra time to view our galactic home.

Script by Damond Benningfield, Copyright 2009

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

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